Basic and Clinical Pharmacology, 13th Ed.

Introduction to Toxicology: Occupational & Environmental

Daniel T. Teitelbaum, MD*

Humans live in a chemical world. They inhale, ingest, and absorb through the skin many of these chemicals. The occupational-environmental toxicologist is primarily concerned with adverse effects in humans resulting from exposure to chemicals encountered at work or in the general environment. In clinical practice, the occupational-environmental toxicologist must identify and treat the adverse health effects of these exposures. In addition, the trained occupational-environmental toxicologist will be called upon to assess and identify hazards associated with chemicals used in the workplace or introduced into the human environment.

Occupational and environmental toxicology cases present unusually complex problems. Occupational and environmental exposure is rarely limited to a single type of molecule. Most workplace or environmental materials are compounds or mixtures, and the ingredients are often poorly described in the documentation that is available for physician review. Moreover, although regulatory agencies in many countries have requirements for disclosure of hazardous materials and their health impacts, proprietary information exclusions often make it difficult for those who treat occupationally and environmentally poisoned patients to understand the nature and scope of the presenting illness. Because many of these illnesses have long latency periods before they become manifest, it is often a matter of detective work, when patients finally present with disease, to ascertain exposure and relate it to clinical effect. Monitoring of exposure concentrations both in the workplace and in the general environment has become more common, but it is far from widespread, and so it is often very difficult to establish the extent of exposure, its duration, and its dose rate when this information is critical to the identification of the toxic disorder and its management.

Occupational Toxicology

Occupational toxicology deals with the chemicals found in the workplace. The major emphasis of occupational toxicology is to identify the agents of concern, identify the acute and chronic diseases that they cause, define the conditions under which they may be used safely, and prevent absorption of harmful amounts of these chemicals. The occupational toxicologist will also be called upon to treat the diseases caused by these chemicals if he or she is a physician. Occupational toxicologists may also define and carry out programs for the surveillance of exposed workers and the environment in which they work. They frequently work hand in hand with occupational hygienists, certified safety professionals, and occupational health nurses in their activities.

Regulatory limits and voluntary guidelines have been elaborated to establish safe ambient air concentrations for many chemicals found in the workplace. Governmental and supragovernmental bodies throughout the world have generated workplace health and safety rules, including short- and long-term exposure limits for workers. These permissible exposure limits (PELs) have the power of law in the United States. Copies of the U.S. Occupational Safety and Health Administration (OSHA) standards may be found on OSHA’s website at http://www.osha.gov. Copies of the U.S. Mine Safety and Health Administration (MSHA) standards may be found at http://www.msha.gov. In addition to the PELs that appear in the OSHA publications and on the website, OSHA promulgates standards for specific materials of particularly serious toxicity. These standards are developed following extensive scientific study, stakeholder input at hearings, public comment, and other steps such as publication in the Federal Register. Such standards have the force of law and employers who use these materials are obligated to comply with the standards. OSHA standards may be found in full on the OSHA website at http://www.osha.gov.

Voluntary organizations, such as the American Conference of Governmental Industrial Hygienists (ACGIH), periodically prepare lists of recommended threshold limit values (TLVs) for many chemicals. These guidelines are periodically updated. Regulatory imperatives in the United States may also be updated from time to time when new information about toxicity becomes available. However, this process is slow and requires input from many sources except under certain extraordinary circumstances. In those cases, emergency alterations to standards may be made and an emergency temporary standard may be promulgated after appropriate regulatory procedures. The ACGIH TLV guidelines are useful as reference points in the evaluation of potential workplace exposures. Compliance with these voluntary guidelines is not a substitute for compliance with the OSHA requirements in the United States. TLVs do not have the force of law. Current TLV lists may be obtained from the ACGIH at http://www.acgih.org.

Environmental Toxicology

Environmental toxicology deals with the potentially deleterious impact of chemicals, present as pollutants of the environment, on living organisms. The term environment includes all the surroundings of an individual organism, but particularly the air, soil, and water. Although humans are considered a target species of particular interest, other species are of considerable importance as potential biologic targets. Scientific study of signal occurrences in animals often provides early warning of impending human events as a result of ecotoxic impacts.

Air pollution is usually a product of industrialization, technologic development, and increased urbanization. On rare occasions, natural phenomena such as volcanic eruptions may result in air pollution with gases, vapors, or particulates that are harmful to humans. Humans may also be exposed to chemicals used in the agricultural environment as pesticides or in food processing that may persist as residues or ingredients in food products. Air contaminants are regulated in the United States by the Environmental Protection Agency (EPA) based on both health and esthetic considerations. Tables of primary and secondary regulated air contaminants and other regulatory issues that relate to air contaminants in the United States may be found at http://www.epa.gov. Many states within the USA also have individual air contaminant regulations that may be more rigorous than those of the EPA. Many other nations and some supragovernmental organizations regulate air contaminants. In the case of adjoining countries, transborder air and water pollution problems have been of concern in recent years. Particulates, radionuclides, acid rain, and similar problems have resulted in cross-contamination of air and water in different countries. Maritime contamination, too, has raised concern about oceanic pollution and has had an impact on the fisheries of some countries. This type of pollution is now the subject of much research and of new international treaties.

The United Nations Food and Agriculture Organization and the World Health Organization (FAO/WHO) Joint Expert Commission on Food Additives adopted the term acceptable daily intake (ADI) to denote the daily intake of a chemical from food that, during an entire lifetime, appears to be without appreciable risk. These guidelines are reevaluated as new information becomes available. In the United States, the FDA and the Department of Agriculture are responsible for the regulation of contaminants such as pesticides, drugs, and chemicals in foods. Major international problems have occurred because of traffic among nations in contaminated or adulterated foods from countries whose regulations and enforcement of pure food and drug laws are lax or nonexistent. Recently, for example, both human and animal illnesses have resulted from ingestion of products imported from China that contained melamine.

Ecotoxicology

Ecotoxicology is concerned with the toxic effects of chemical and physical agents on populations and communities of living organisms within defined ecosystems; it includes the transfer pathways of those agents and their interactions with the environment. Traditional toxicology is concerned with toxic effects on individual organisms; ecotoxicology is concerned with the impact on populations of living organisms or on ecosystems. Ecotoxicology research has become one of the foremost areas of study for toxicologists.

TOXICOLOGIC TERMS & DEFINITIONS

Hazard & Risk

Hazard is the ability of a chemical agent to cause injury in a given situation or setting; the conditions of use and exposure are primary considerations. To assess hazard, one needs to have knowledge about both the inherent toxicity of the substance and the amounts to which individuals are liable to be exposed. Hazard is often a description based on subjective estimates rather than objective evaluation.

Risk is defined as the expected frequency of the occurrence of an undesirable effect arising from exposure to a chemical or physical agent. Estimation of risk makes use of dose-response data and extrapolation from the observed relationships to the expected responses at doses occurring in actual exposure situations. The quality and suitability of the biologic data used in such estimates are major limiting factors. Risk assessment has become an integral part of the regulatory process in most countries. However, many of the assumptions of risk assessment scientists remain unproven, and only long-term observation of population causes and outcomes will provide the basis for validation of newer risk assessment technologies.

Routes of Exposure

The route of entry for chemicals into the body differs in different exposure situations. In the industrial setting, inhalation is the major route of entry. The transdermal route is also quite important, but oral ingestion is a relatively minor route. Consequently, primary prevention should be designed to reduce or eliminate absorption by inhalation or by topical contact. Atmospheric pollutants gain entry by inhalation and by dermal contact. Water and soil pollutants are absorbed through inhalation, ingestion, and dermal contact.

Quantity, Duration, & Intensity of Exposure

Toxic reactions may differ depending on the quantity of exposure, its duration, and the rate at which the exposure takes place. An exposure to a toxic substance that is absorbed by the target human or animal results in a dose. A single exposure, or multiple exposures that occur over a brief period from seconds to 1-2 days, represents acute exposure. Intense, rapidly absorbed acute doses of substances that may ordinarily be detoxified by enzymatic mechanisms in small doses may overwhelm the body’s ability to detoxify the substance and may result in serious or even fatal toxicity. The same amount of the substance, absorbed slowly, may result in little or no toxicity. This is the case with cyanide exposure. Rhodanese, a mitochondrial enzyme present in humans, effectively detoxifies cyanide to relatively nontoxic thiocyanate when cyanide is presented in small amounts, but the enzyme is overwhelmed by large, rapidly encountered cyanide doses, with lethal effect.

Single or multiple exposures over a longer period of time represent chronic exposure. In the occupational setting, both acute (eg, accidental discharge) and chronic (eg, repetitive handling of a chemical) exposures occur. Exposures to chemicals found in the environment such as air and water pollutants are often chronic, resulting in chronic disease, as in the Minamata Bay, Japan, methyl mercury disaster. Sudden large chemical releases may result in acute massive population exposure with serious or lethal consequences. The tragedy in Bhopal, India, was such an event, in which methyl isocyanate was released into a crowded population area, resulting in almost 4000 deaths and more than half a million injuries. The release of dioxin in Seveso, Italy, contaminated a populated area with a persistent organic chemical having both acute and long-term chronic effects. More recently, the massive oil spill caused by the explosion of BP’s Deepwater Horizon drilling rig in the Gulf of Mexico highlighted the potential for long-term ecotoxic impacts involving widespread geographic areas.

ENVIRONMENTAL CONSIDERATIONS

Certain chemical and physical characteristics are important for the estimation of the potential hazard of environmental toxicants. Data on toxic effects of different organisms, along with knowledge about degradability, bioaccumulation, and transport and biomagnification through food chains, help in this estimation. (See Box: Bioaccumulation & Biomagnification, for a classic example involving the Great Lakes.) Poorly degraded chemicals (by abiotic or biotic pathways) exhibit environmental persistence and can accumulate. Such chemicals include the persistent organic pollutants (POPs), polychlorinated biphenyls, dioxins and furans, and similar substances. Lipophilic substances such as the largely banned or abandoned organochlorine pesticides tend to bioaccumulate in body fat. This results in tissue residues that are slowly released over time. These residues and their metabolites may have chronic adverse effects such as endocrine disruption. When the toxicant is incorporated into the food chain, biomagnification occurs as one species feeds on others. This concentrates the chemical in organisms higher on the food chain. Humans stand at the apex of the food chain. They may be exposed to highly concentrated pollutant loads as bioaccumulation and biomagnification occur. The pollutants that have the widest environmental impact are poorly degradable; are relatively mobile in air, water, and soil; exhibit bioaccumulation; and also exhibit biomagnification.

image SPECIFIC CHEMICALS

AIR POLLUTANTS

Air pollution may result from vapors, aerosols, smokes, particulates, and individual chemicals. Five major substances have been said to account for about 98% of air pollution: carbon monoxide (about 52%); sulfur oxides (about 14%); hydrocarbons (about 14%); nitrogen oxides (about 14%) and ozone, their breakdown product; and particulate matter (about 4%). Agriculture, especially industrial-scale farming, contributes a variety of air pollutants: dusts as particulates, pesticidal chemicals, hydrogen sulfide, and others. Sources of pollutants include fossil fuel burning, transportation, manufacturing, other industrial activities, generation of electric power, space heating, refuse disposal, and others. Studies in Helsinki and other cities have shown that uncatalyzed automobile traffic emissions are larger contributors to ground-level air pollution than any other source. The introduction of catalytic converters on automobiles and their mandatory use in many countries has greatly reduced automobile-released air pollution. In addition, the ban on tetraethyl lead in gasoline has eliminated a major source of lead contamination and childhood lead poisoning in urban environments. In emerging economies, the use of transport based on two-cycle engines creates heavy ground-level air pollution in very crowded cities. The introduction of “clean, low-sulfur” diesel fuels is helping to reduce urban and highway pollutants such as sulfur oxides.

Bioaccumulation & Biomagnification

If the intake of a long-lasting contaminant by an organism exceeds the latter’s ability to metabolize or excrete the substance, the chemical accumulates within the tissues of the organism. This is called bioaccumulation.

Although the concentration of a contaminant may be virtually undetectable in water, it may be magnified hundreds or thousands of times as the contaminant passes up the food chain. This is called biomagnification.

The biomagnification of polychlorinated biphenyls (PCBs) in the Great Lakes of North America is illustrated by the following residue values available from a classic Environment Canada report published by the Canadian government, and elsewhere.

The biomagnification for this substance in the food chain, beginning with phytoplankton and ending with the herring gull, is nearly 50,000-fold. Domestic animals and humans may eat fish from the Great Lakes, resulting in PCB residues in these species as well.

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Sulfur dioxide and smoke from incomplete combustion of coal have been associated with acute adverse effects among children, the elderly, and individuals with preexisting cardiac or respiratory disease. Ambient air pollution has been implicated as a cause of cardiac disease, bronchitis, obstructive ventilatory disease, pulmonary emphysema, bronchial asthma, and airway or lung cancer. Extensive basic science and clinical epidemiologic literature on air pollutant toxicology has been published and has led to modifications of regulatory standards for air pollutants. EPA standards for these substances apply to the general environment, and OSHA standards apply to workplace exposure. Ambient air standards for carbon monoxide and five other harmful pollutants—particulate matter, nitrogen dioxide, ozone, sulfur dioxide, and lead—may be found at http://www.epa.gov/air/criteria.html.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, tasteless, odorless, and non-irritating gas, a byproduct of incomplete combustion. The average concentration of CO in the atmosphere is about 0.1 ppm; in heavy traffic, the concentration may exceed 100 ppm. Current recommended permissible exposure limit (PEL) values are shown in Table 56–1 (see also http://www.osha.gov, 1910.1000, Table Z-1).

TABLE 56–1 Examples of permissible exposure limit values (PELs) of some common air pollutants and solvents in parts per million (ppm).1

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1. Mechanism of actionCO combines tightly but reversibly with the oxygen-binding sites of hemoglobin and has an affinity for hemoglobin that is about 220 times that of oxygen. The product formed—carboxyhemoglobin—cannot transport oxygen. Furthermore, the presence of carboxyhemoglobin interferes with the dissociation of oxygen from the remaining oxyhemoglobin as a result of the Bohr effect. This reduces the transfer of oxygen to tissues. Organs with the highest oxygen demand (the brain, heart, and kidneys) are most seriously affected. Normal nonsmoking adults have carboxyhemoglobin levels of less than 1% saturation (1% of total hemoglobin is in the form of carboxyhemoglobin); this has been attributed to the endogenous formation of CO from heme catabolism. Smokers may exhibit 5–10% CO saturation. The level depends on their smoking habits. A person who breathes air that contains 0.1% CO (1000 ppm) would have a carboxyhemoglobin level of about 50% in a short period of time.

2. Clinical effectsThe principal signs of CO intoxication are those of hypoxia. They progress in the following sequence: (1) psychomotor impairment; (2) headache and tightness in the temporal area; (3) confusion and loss of visual acuity; (4) tachycardia, tachypnea, syncope, and coma; and (5) deep coma, convulsions, shock, and respiratory failure. There is great variability in individual responses to carboxyhemoglobin concentration. Carboxyhemoglobin levels below 15% may produce headache and malaise; at 25% many workers complain of headache, fatigue, decreased attention span, and loss of fine motor coordination. Collapse and syncope may appear at around 40%; and with levels above 60%, death may ensue as a result of irreversible damage to the brain and myocardium. The clinical effects may be aggravated by heavy labor, high altitudes, and high ambient temperatures. CO intoxication is usually thought of as a form of acute toxicity. There is evidence that chronic exposure to low CO levels may lead to adverse cardiac effects, neurologic disturbance, and emotional disorders. The developing fetus is quite susceptible to the effects of CO exposure. Exposure of a pregnant woman to elevated CO levels at critical periods of fetal development may cause fetal death or serious and irreversible but survivable birth defects.

3. TreatmentPatients who have been exposed to CO must be removed from the exposure source immediately. Respiration must be maintained and high flow and concentration of oxygen—the specific antagonist to CO—should be administered promptly. If respiratory failure is present, mechanical ventilation is required, High concentrations of oxygen may be toxic and may contribute to the development of acute respiratory distress syndrome. Therefore, patients should be treated with high concentrations only for a short period. With room air at 1 atm, the elimination half-time of CO is about 320 minutes; with 100% oxygen, the half-time is about 80 minutes; and with hyperbaric oxygen (2–3 atm), the half-time can be reduced to about 20 minutes. Although some controversy exists about hyperbaric oxygen for CO poisoning, it may be used if it is readily available. It is particularly recommended for the management of pregnant women exposed to CO. Hypothermic therapy to reduce metabolic demand of the brain has also been useful. Cerebral edema that results from CO poisoning does not seem to respond to either mannitol or steroid therapy and may be persistent. Progressive recovery from treated CO poisoning, even of a severe degree can be complete but some patients manifest neuropsychological and motor dysfunction for a long time after recovery from acute CO poisoning.

Sulfur Dioxide

Sulfur dioxide (SO2) is a colorless irritant gas generated primarily by the combustion of sulfur-containing fossil fuels. The current OSHA PEL (Table 56–1) is given on the OSHA website (see http://www.osha.gov, 1910.1000, Table Z-1).

1. Mechanism of actionAt room temperature, the solubility of SO2 is approximately 200 g SO2/L of water. Because of its high solubility, when SO2 contacts moist membranes, it transiently forms sulfurous acid. This acid has severe irritant effects on the eyes, mucous membranes, and skin. Approximately 90% of inhaled SO2 is absorbed in the upper respiratory tract, the site of its principal effect. The inhalation of SO2 causes bronchial constriction and produces profuse bronchorrhea; parasympathetic reflexes and altered smooth muscle tone appear to be involved. The clinical outcome is an acute irritant asthma. Exposure to 5 ppm SO2 for 10 minutes leads to increased resistance to airflow in most humans. Exposures of 5–10 ppm are reported to cause severe bronchospasm; 10–20% of the healthy young adult population is estimated to be reactive to even lower concentrations. The phenomenon of adaptation to irritating concentrations has been reported in workers. However, current studies have not confirmed this phenomenon. Asthmatic individuals are especially sensitive to SO2.

2. Clinical effects and treatmentThe signs and symptoms of intoxication include irritation of the eyes, nose, and throat, reflex bronchoconstriction, and increased bronchial secretions. In asthmatic subjects, exposure to SO2 may result in an acute asthmatic episode. If severe exposure has occurred, delayed-onset pulmonary edema may be observed. Cumulative effects from chronic low-level exposure to SO2 are not striking, particularly in humans, but these effects have been associated with aggravation of chronic cardiopulmonary disease. When combined exposure to high respirable particulate loads and SO2 occurs, the mixed irritant load may increase the toxic respiratory response. Treatment is not specific for SO2 but depends on therapeutic maneuvers used to treat irritation of the respiratory tract and asthma. In some severely polluted urban air basins, elevated SO2 concentrations combined with elevated particulate loads has led to air pollution emergencies and a marked increase in cases of acute asthmatic bronchitis. Children and the elderly seem to be at greatest risk. The principal source of urban SO2 is the burning of coal, both for domestic heating and in coal-fired power plants. High-sulfur transportation fuels also contribute. Both also contribute to the respirable fine particulate load and to increased urban cardiorespiratory morbidity and mortality.

Nitrogen Oxides

Nitrogen dioxide (NO2) is a brownish irritant gas sometimes associated with fires. It is formed also from fresh silage; exposure of farmers to NO2 in the confines of a silo can lead to silo-filler’s disease, a severe and potentially lethal form of acute respiratory distress syndrome. The disorder is uncommon today. Miners who are regularly exposed to diesel equipment exhaust have been particularly affected by nitrogen oxide emissions with serious respiratory effects. Today, the most common source of human exposure to oxides of nitrogen, including NO2, is automobile and truck traffic emissions. Recent air pollution inventories in cities with high traffic congestion have demonstrated the important role that internal combustion engines have in the increasing NO2 urban air pollution. A variety of disorders of the respiratory system, cardiovascular system, and other problems have been linked to NO2 exposure.

1. Mechanism of actionNO2 is a relatively insoluble deep lung irritant. It is capable of producing pulmonary edema and acute adult respiratory distress syndrome (ARDS). Inhalation damages the lung infrastructure that produces the surfactant necessary to allow smooth and low effort lung alveolar expansion. The type I cells of the alveoli appear to be the cells chiefly affected by acute low to moderate inhalation exposure. At higher exposure, both type I and type II alveolar cells are damaged. If only type I cells are damaged, after an acute period of severe distress, it is likely that treatment with modern ventilation equipment and medications will result in recovery. Some patients develop nonallergic asthma, or “twitchy airway” disease, after such a respiratory insult. If severe damage to the type I and type II alveolar cells occurs, replacement of the type I cells may be impaired; progressive fibrosis may ensue that eventually leads to bronchial ablation and alveolar collapse. This can result in permanent restrictive respiratory disease. In addition to the direct deep lung effect, long-term exposure to lower concentrations of nitrogen dioxide has been linked to cardiovascular disease, increased incidence of stroke, and other chronic disease.

The current PEL for NO2 is given in Table 56–1. Exposure to 25 ppm of NO2 is irritating to some individuals; 50 ppm is moderately irritating to the eyes and nose. Exposure for 1 hour to 50 ppm can cause pulmonary edema and perhaps subacute or chronic pulmonary lesions; 100 ppm can cause pulmonary edema and death.

2. Clinical effectsThe signs and symptoms of acute exposure to NO2 include irritation of the eyes and nose, cough, mucoid or frothy sputum production, dyspnea, and chest pain. Pulmonary edema may appear within 1–2 hours. In some individuals, the clinical signs may subside in about 2 weeks; the patient may then pass into a second stage of abruptly increasing severity, including recurring pulmonary edema and fibrotic destruction of terminal bronchioles (bronchiolitis obliterans). Chronic exposure of laboratory animals to 10–25 ppm NO2 has resulted in emphysematous changes; thus, chronic effects in humans are of concern.

3. TreatmentThere is no specific treatment for acute intoxication by NO2; therapeutic measures for the management of deep lung irritation and noncardiogenic pulmonary edema are used. These measures include maintenance of gas exchange with adequate oxygenation and alveolar ventilation. Drug therapy may include bronchodilators, sedatives, and antibiotics. New approaches to the management of NO2-induced ARDS have been developed and considerable controversy now exists about the precise respiratory protocol to use in any given patient.

Ozone & Other Oxides

Ozone (O3) is a bluish irritant gas found in the earth’s atmosphere, where it is an important absorbent of ultraviolet light at high altitude. At ground level, ozone is an important pollutant. Atmospheric ozone pollution is derived from photolysis of oxides of nitrogen, volatile organic compounds, and CO. These compounds are produced primarily when fossil fuels such as gasoline, oil, or coal are burned or when some chemicals (eg, solvents) evaporate. Nitrogen oxides are emitted from power plants, motor vehicles, and other sources of high-heat combustion. Volatile organic compounds are emitted from motor vehicles, chemical plants, refineries, factories, gas stations, paint, and other sources. An EPA fact sheet on ground-level ozone, its sources, and consequences may be found at http://www.epa.gov/glo/.

Ozone can be generated in the workplace by high-voltage electrical equipment, and around ozone-producing devices used for air and water purification. Agricultural sources of ozone are also important. There is a near-linear gradient between exposure to ozone (1-hour level, 20–100 ppb) and bronchial smooth muscle response. See Table 56–1 for the current PEL for ozone.

1. Mechanism of action and clinical effectsOzone is an irritant of mucous membranes. Mild exposure produces upper respiratory tract irritation. Severe exposure can cause deep lung irritation, with pulmonary edema when inhaled at sufficient concentrations. Ozone penetration in the lung depends on tidal volume; consequently, exercise can increase the amount of ozone reaching the distal lung. Some of the effects of O3 resemble those seen with radiation, suggesting that O3 toxicity may result from the formation of reactive free radicals. The gas causes shallow, rapid breathing and a decrease in pulmonary compliance. Enhanced sensitivity of the lung to bronchoconstrictors is also observed. Exposure around 0.1 ppm O3 for 10–30 minutes causes irritation and dryness of the throat; above 0.1 ppm, one finds changes in visual acuity, substernal pain, and dyspnea. Pulmonary function is impaired at concentrations exceeding 0.8 ppm.

Airway hyperresponsiveness and airway inflammation have been observed in humans. The response of the lung to O3 is a dynamic one. The morphologic and biochemical changes are the result of both direct injury and secondary responses to the initial damage. Long-term exposure in animals results in morphologic and functional pulmonary changes. Chronic bronchitis, bronchiolitis, fibrosis, and emphysematous changes have been reported in a variety of species, including humans, exposed to concentrations above 1 ppm. Increased visits to hospital emergency departments for cardiopulmonary disease during ozone alerts have been reported. A study of the basic physiologic responses of humans to ozone exposure and the biomarkers evoked provides useful insight into the fundamental toxicologic impacts of ozone.

2. TreatmentThere is no specific treatment for acute O3 intoxication. Management depends on therapeutic measures used for deep lung irritation and noncardiogenic pulmonary edema that have resulted in ARDS. Current national ambient air quality standards for ozone are listed at http://www.epa.gov/air/criteria.html.

SOLVENTS

Halogenated Aliphatic Hydrocarbons

These “halohydrocarbon” agents once found wide use as industrial solvents, degreasing agents, and cleaning agents. The substances include carbon tetrachloride, chloroform, trichloroethylene, tetrachloroethylene (perchloroethylene), and 1,1,1-trichloroethane (methyl chloroform). Many halogenated aliphatic hydrocarbons are classified as known or probable human carcinogens. Carbon tetrachloride and trichloroethylene have largely been removed from the workplace. Perchloroethylene and trichloroethane are still in use for dry cleaning and solvent degreasing, but it is likely that their use will be very limited in the future. The EPA now considers perchloroethylene a likely human carcinogen. The EPA data sheet may be found at http://www.epa.gov/ttnatw01/hlthef/tet-ethy.html. Dry cleaning as an occupation is listed as a class 2B carcinogenic activity by the International Agency for Research on Cancer (IARC). The Canadian Center for Occupational Health and Safety lists occupations and exposures to occupational carcinogens at http://www.ccohs.ca/oshanswers/diseases/carcinogen_occupation.html.

Fluorinated aliphatics such as the freons and closely related compounds have also been used in the workplace, in consumer goods, and in stationary and mobile air conditioning systems. Because of the severe damage they cause to the ozone layer in the troposphere, their use has been limited or eliminated by international treaty agreements. The common halogenated aliphatic solvents also create serious problems as persistent water pollutants. They are widely found in both groundwater and drinking water as a result of poor disposal practices.

Table 56–1 includes recommended OSHA PELs for several of these compounds (see also http://www.osha.gov, Table Z-1).

1. Mechanism of action and clinical effectsIn laboratory animals, the halogenated hydrocarbons cause central nervous system (CNS) depression, liver injury, kidney injury, and some degree of cardiotoxicity. Several are also carcinogenic in animals and are considered probable human carcinogens. Trichloroethylene and tetrachloroethylene are listed as “reasonably anticipated to be a human carcinogen” by the U.S. National Toxicology Program, and as class 2A probable human carcinogens by IARC. These substances are depressants of the CNS in humans. Chronic workplace exposure to halogenated hydrocarbon solvents can cause significant neurotoxicity with impaired memory and peripheral neuropathy. All halohydrocarbon solvents can cause cardiac arrhythmias in humans, particularly in situations involving sympathetic excitation and norepinephrine release.

Hepatotoxicity is also a common toxic effect that can occur in humans after acute or chronic halohydrocarbon exposures. Nephrotoxicity can occur in humans exposed to carbon tetrachloride, chloroform, and trichloroethylene. Chloroform, carbon tetrachloride, trichloroethylene, and tetrachloroethylene carcinogenicity have been observed in lifetime exposure studies performed in rats and mice and in some human epidemiologic studies. Dichloromethane (methylene chloride) is a potent neurotoxin, a generator of CO in humans, and a probable human carcinogen. It has been widely used as a paint stripper, plastic glue, and for other purposes. Epidemiologic studies of workers who have been exposed to aliphatic hydrocarbon solvents that include dichloromethane, trichloroethylene, and tetrachloroethylene have found significant associations between the agents and renal, prostate, and testicular cancer. Trichloroethylene is now considered a class 1, known human carcinogen by IARC; renal cancers and non-Hodgkin’s lymphoma have been reported. Other cancers are increased but their incidence has not reached statistical significance.

2. TreatmentThere is no specific treatment for acute intoxication resulting from exposure to halogenated hydrocarbons. Management depends on the organ system involved.

Aromatic Hydrocarbons

Benzene is used for its solvent properties and as an intermediate in the synthesis of other chemicals. It remains an important component of gasoline. Benzene may be found in premium gasolines at concentrations of about 1.5%. In cold climates such as Alaska, benzene concentrations in gasoline may reach 5% in order to provide an octane boost. It is one of the most widely used industrial chemicals in the world. The current PEL is 1.0 ppm in the air (see Table 56–1 and http://www.osha.gov, Table Z-1), and a 5 ppm limit is recommended for skin exposure. The National Institute for Occupational Safety and Health (NIOSH) and others have recommended that the exposure limits for benzene be further reduced to 0.1 ppm because excess blood cancers occur at the current PEL.

The acute toxic effect of benzene is depression of the CNS. Exposure to 7500 ppm for 30 minutes can be fatal. Exposure to concentrations larger than 3000 ppm may cause euphoria, nausea, locomotor problems, and coma. Vertigo, drowsiness, headache, and nausea may occur at concentrations ranging from 250 to 500 ppm. No specific treatment exists for the acute toxic effect of benzene.

Chronic exposure to benzene can result in very serious toxic effects, the most significant of which is bone marrow injury. Aplastic anemia, leukopenia, pancytopenia, and thrombocytopenia occur, as does leukemia. Chronic exposure to low levels of benzene has been associated with leukemia of several types as well as lymphomas, myeloma, and myelodysplastic syndrome. Recent studies have shown the occurrence of leukemia following exposures as low as 2 ppm-years. The pluri-potent bone marrow stem cells appear to be targets of benzene or its metabolites and other stem cells may also be targets.

Benzene has long been known to be a potent clastogen, ie, a mutagen that acts by causing chromosomal breakage. Recent studies have suggested specific chromosome reorganization and genomic patterns that are associated with benzene-induced leukemia. Epidemiologic data confirm a causal association between benzene exposure and leukemia and other bone marrow cancers in workers. IARC classifies benzene as a class 1, known human carcinogen. Most national and international organizations classify benzene as a known human carcinogen.

Toluene (methylbenzene) does not possess the myelotoxic properties of benzene, nor has it been associated with leukemia. It is not carcinogenic and is listed as class 3 by IARC. It is, however, a CNS depressant and a skin and eye irritant. It is also fetotoxic. See Table 56–1 and OSHA Tables Z-1, and Z-2 (http://www.osha.gov) for the PELs. Exposure to 800 ppm can lead to severe fatigue and ataxia; 10,000 ppm can produce rapid loss of consciousness. Chronic effects of long-term toluene exposure are unclear because human studies indicating behavioral effects usually concern exposures to several solvents. In limited occupational studies, however, metabolic interactions and modification of toluene’s effects have not been observed in workers also exposed to other solvents. Less refined grades of toluene contain benzene. If technical grade toluene is to be used where there is human contact or exposure, analysis of the material for benzene content is advisable.

Xylene (dimethylbenzene) has been substituted for benzene in many solvent degreasing operations. Like toluene, the three xylenes do not possess the myelotoxic properties of benzene, nor have they been associated with leukemia. Xylene is a CNS depressant and a skin irritant. Less refined grades of xylene contain benzene. Estimated TLV-TWA and TLV-STEL are 100 and 150 ppm, respectively. The current OSHA PELs may be found at http://www.osha.gov, Table Z-1.

PESTICIDES

Organochlorine Pesticides

These agents are usually classified into four groups: DDT (chlorophenothane) and its analogs, benzene hexachlorides, cyclodienes, and toxaphenes (Table 56–2). They are aryl, carbocyclic, or heterocyclic compounds containing chlorine substituents. The individual compounds differ widely in their biotransformation and capacity for storage in tissues; toxicity and storage are not always correlated. They can be absorbed through the skin as well as by inhalation or oral ingestion. There are, however, important quantitative differences between the various derivatives; DDT in solution is poorly absorbed through the skin, whereas dieldrin absorption from the skin is very efficient. Organochlorine pesticides have largely been abandoned because they cause severe environmental damage. They are now known to be endocrine disrupters in animals and humans. DDT continues to have very restricted use for domestic mosquito elimination in malaria-infested areas of Africa. This use is controversial, but it is very effective and is likely to remain in place for the foreseeable future. Organochlorine pesticide residues in humans, animals, and the environment present long-term problems that are not yet fully understood.

TABLE 56–2 Organochlorine pesticides.

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1. Human toxicologyThe acute toxic properties of all the organochlorine pesticides in humans are qualitatively similar. These agents interfere with inactivation of the sodium channel in excitable membranes and cause rapid repetitive firing in most neurons. Calcium ion transport is inhibited. These events affect repolarization and enhance the excitability of neurons. The major effect is CNS stimulation. With DDT, tremor may be the first manifestation, possibly continuing to convulsions, whereas with the other compounds convulsions often appear as the first sign of intoxication. There is no specific treatment for the acute intoxicated state, and management is symptomatic.

The potential carcinogenic properties of organochlorine pesticides have been extensively studied, and results indicate that chronic administration to laboratory animals over long periods results in enhanced carcinogenesis. Endocrine pathway disruption is the postulated mechanism. Numerous mechanisms for xenoestrogen (estrogen-like) carcinogenesis have been postulated. To date, however, several large epidemiologic studies in humans have not found a significant association between the risk of cancer and specific compounds or serum levels of organochlorine pesticide metabolites. The results of a case-control study conducted to investigate the relation between dichlorodiphenyldichloroethylene (DDE, the primary metabolite of DDT) and DDT breast adipose tissue levels and breast cancer risk did not confirm a positive association. In contrast, recent work supports an association between prepubertal exposure to DDT and brain cancer. Recent studies also suggest that the risk of testicular cancer and non-Hodgkin’s lymphoma is increased in persons with elevated organochlorine levels. Noncancer end points are also of concern. Recent work associates cryptorchidism and hypospadias in newborns with maternal adipose levels of chlordane metabolites. These residues are also linked to testicular cancer.

2. Environmental toxicologyThe organochlorine pesticides are considered persistent chemicals. Degradation is quite slow when compared with other pesticides, and bioaccumulation, particularly in aquatic ecosystems, is well documented. Their mobility in soil depends on the composition of the soil; the presence of organic matter favors the adsorption of these chemicals onto the soil particles, whereas adsorption is poor in sandy soils. Once adsorbed, they do not readily desorb. These compounds induce significant abnormalities in the endocrine balance of sensitive animal and bird species, in addition to their adverse impact on humans. Since the early 1960s, when Rachel Carson’s work and subsequent book, Silent Spring, brought attention to the issue, the organochlorine pesticides have been recognized as pernicious environmental toxins. Their use is banned in most jurisdictions.

Organophosphorus Pesticides

These agents, some of which are listed in Table 56–3, are used to combat a large variety of pests. They are useful pesticides when in direct contact with insects or when used as plant systemics, where the agent is translocated within the plant and exerts its effects on insects that feed on the plant. The many varieties currently in use are applied by spray techniques including hand, tractor, and aerial methods. They are often spread widely by wind and weather and are subject to widespread drift. The organophosphate pesticides are based on compounds such as soman, sarin, and tabun, which were developed for use as war gases. Some of the less toxic organophosphorus compounds are used in human and veterinary medicine as local or systemic antiparasitics (see Chapters 7 and 53). The compounds are absorbed by the skin as well as by the respiratory and gastrointestinal tracts. Biotransformation is rapid, particularly when compared with the rates observed with the chlorinated hydrocarbon pesticides. Storm and collaborators reviewed current and suggested human inhalation occupational exposure limits for 30 organophosphate pesticides (see References).

TABLE 56–3 Organophosphorus pesticides.

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1. Human toxicologyIn mammals as well as insects, the major effect of these agents is inhibition of acetylcholinesterase through phosphorylation of the esteratic site. The signs and symptoms that characterize acute intoxication are due to inhibition of this enzyme and accumulation of acetylcholine; some of the agents also possess direct cholinergic activity. Specific treatment with antidotes and useful antagonists is available. In addition, pretreatment with physostigmine and other short-acting compounds may provide protection against these pesticides or their war gas analogs if used in timely fashion. These effects and their treatment are described in Chapters 7 and 8 of this book. Altered neurologic and cognitive functions, as well as psychological symptoms of variable duration, have been associated with exposure to these pesticides. Furthermore, there is some indication of an association of low arylesterase activity with neurologic symptom complexes in Gulf War veterans.

In addition to—and independently of—inhibition of acetylcholinesterase, some of these agents are capable of phosphorylating another enzyme present in neural tissue, the so-called neuropathy target esterase (NTE). This results in progressive demyelination of the longest nerves. Associated with paralysis and axonal degeneration, this lesion is sometimes called organophosphorus ester-induced delayed polyneuropathy (OPIDP). Delayed central and autonomic neuropathy may occur in some poisoned patients. Hens are particularly sensitive to these properties and have proved very useful for studying the pathogenesis of the lesion and for identifying potentially neurotoxic organophosphorus derivatives. There is no specific treatment for NTE toxicity.

In humans, progressive chronic axonal neurotoxicity has been observed with triorthocresyl phosphate (TOCP), a noninsecticidal organophosphorus compound. It is also thought to occur with the pesticides dichlorvos, trichlorfon, leptophos, methamidophos, mipafox, trichloronat, and others. The polyneuropathy usually begins as burning and tingling sensations, particularly in the feet, with motor weakness occurring a few days later. Sensory and motor difficulties may extend to the legs and hands. Gait is affected, and ataxia may be present. Central nervous system and autonomic changes may develop still later. There is no specific treatment for this form of delayed neurotoxicity. The long-term prognosis of NTE inhibition is highly variable. Reports of this type of neuropathy (and other toxicities) in pesticide manufacturing workers and in agricultural pesticide applicators have been published (see References).

Recent clinical observation has also defined an intermediate syndrome in severely organophosphate-poisoned patients. This syndrome is characterized by neuromuscular transmission failure, and cardiac failure more typical of nicotinic than muscarinic poisoning. Progressive neuromuscular failure leads to weakness of the respiratory muscles and eventually to death. The physiologic abnormalities are complex but involve a progressive decrement in neuromuscular junction transmission efficiency. Patients who develop this intermediate syndrome are at great risk of cardiorespiratory failure and may require mechanical ventilation. Because organophosphorus poisoning frequently occurs in less developed parts of the world where medical resources are very limited, the development of the intermediate syndrome is frequently a lethal complication. It is not effectively treated with the usual management protocol for organophosphate pesticide poisoning.

2. Environmental toxicologyOrganophosphorus pesticides are not considered to be persistent pesticides. They are relatively unstable and break down in the environment as a result of hydrolysis and photolysis. As a class they are considered to have a small permanent impact on the environment, in spite of their acute effects on organisms.

Carbamate Pesticides

These compounds (Table 56–4) inhibit acetylcholinesterase by carbamoylation of the esteratic site. Thus, they possess the toxic properties associated with inhibition of this enzyme as described for the organophosphorus pesticides. However, as described in Chapters 7 and 8, the binding is relatively weak, dissociation occurs after minutes to hours, and clinical effects are of shorter duration than those observed with organophosphorus compounds. Spontaneous reactivation of cholinesterase is more rapid after inhibition by the carbamates. The therapeutic index, the ratio of the doses that cause severe toxicity or death to those that result in minor intoxication, is larger with carbamates than with the organophosphorus agents. Although the clinical approach to carbamate poisoning is similar to that for organophosphates, the use of pralidoxime is not recommended.

TABLE 56–4 Carbamate pesticides.

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The carbamates are considered to be nonpersistent pesticides. They exert only a small impact on the environment.

Botanical Pesticides

Pesticides derived from natural sources include nicotine, rotenone, and pyrethrum. Nicotine is obtained from the dried leaves of Nicotiana tabacum and N rustica. It is rapidly absorbed from mucosal surfaces; the free alkaloid, but not the salt, is readily absorbed from the skin. Nicotine reacts with the acetylcholine receptor of the postsynaptic membrane (sympathetic and parasympathetic ganglia, neuromuscular junction), resulting in depolarization of the membrane. Toxic doses cause stimulation rapidly followed by blockade of transmission. These actions are described in Chapter 7. Treatment is directed toward maintenance of vital signs and suppression of convulsions. Nicotine analogs (neonicotinoids) have been developed for use as agricultural pesticides and have been accused of a role in bee colony collapse.

Rotenone (Figure 56–1) is obtained from Derris ellipticaD mallaccensisLonchocarpus utilis, and L urucu. The oral ingestion of rotenone produces gastrointestinal irritation. Conjunctivitis, dermatitis, pharyngitis, and rhinitis can also occur. Treatment is symptomatic.

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FIGURE 56–1 Chemical structures of selected herbicides and pesticides.

Pyrethrum consists of six known insecticidal esters: pyrethrin I (Figure 56–1), pyrethrin II, cinerin I, cinerin II, jasmolin I, and jasmolin II. Synthetic pyrethroids account for an increasing percentage of worldwide pesticide usage. Pyrethrum may be absorbed after inhalation or ingestion. When absorbed in sufficient quantities, the major site of toxic action is the CNS; excitation, convulsions, and tetanic paralysis can occur. Voltage-gated sodium, calcium, and chloride channels are considered targets, as well as peripheral-type benzodiazepine receptors. Treatment of exposure is usually directed at management of symptoms. Anticonvulsants are not consistently effective. The chloride channel agonist, ivermectin, is of use, as are pentobarbital and mephenesin. The pyrethroids are highly irritating to the eyes, skin, and respiratory tree. They may cause irritant asthma and, potentially, reactive airways dysfunction syndrome (RADS) and even anaphylaxis. The most common injuries reported in humans result from their allergenic and irritant effects on the airways and skin. Cutaneous paresthesias have been observed in workers spraying synthetic pyrethroids. The use of persistent synthetic pyrethroids to exterminate insects on aircraft has caused respiratory and skin problems as well as some neurologic complaints in flight attendants and other aircraft workers. Severe occupational exposures to synthetic pyrethroids in China resulted in marked effects on the CNS, including convulsions. Other previously unreported toxic manifestations have been manifest in pyrethrin-exposed individuals.

HERBICIDES

Chlorophenoxy Herbicides

2,4-Dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), and their salts and esters have been used as herbicides for the destruction of weeds (Figure 56–1). These compounds are of relatively low acute human toxicity. However, despite their low acute hazard, they cause serious long-term human and environmental toxicity. 2,4-D remains in wide commercial and domestic use for lawn weed control. 2,4,5-T had similar uses but was infamously incorporated into Agent Orange, used as a defoliant during the Vietnam conflict. Agent Orange was contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (a potent animal carcinogen and likely human carcinogen) and other toxic, persistent, and undesirable polychlorinated compounds. When this toxicity was discovered, the U.S. Department of Agriculture canceled the domestic pesticide registrations for trichlorophenoxy herbicides, and these compounds are no longer used. However, other, less thoroughly studied compounds, eg, chlorinated xanthenes, are present in both the dichlorophenoxy and trichlorophenoxy herbicides (see below).

In humans, 2,4-D in large doses can cause coma and generalized muscle hypotonia. Rarely, muscle weakness and marked hypotonia may persist for several weeks. In laboratory animals, signs of liver and kidney dysfunction have also been reported with chlorphenoxy herbicides. Several epidemiologic studies performed by the U.S. National Cancer Institute confirmed the causal link between 2,4-D and non-Hodgkin’s lymphoma. Evidence for a causal link to soft tissue sarcoma, however, is considered equivocal.

The dichlorophenoxy and related herbicides have been found to contain and to generate dimethylnitrosamine (N-nitrosodimethylamine; NDMA), a potent human carcinogen, during environmental transformation as well as non-chlorine water disinfection. Studies by Environment Canada and others have questioned the use of this compound because of water contamination. Studies of related nitrosamine-forming herbicidal compounds have raised questions about the suitability of these compounds for general weed control. Because of the extremely high economic value of herbicides to the agricultural community, however, long-term decisions on their use have been delayed.

Glyphosate

Glyphosate (N-[phosphonomethyl] glycine, Figure 56–1), the principle ingredient in Roundup, is now the most widely used herbicide in the world. It functions as a contact herbicide and is absorbed through the leaves and roots of plants. It is generally formulated with surfactant to enhance its intended effect on noxious plants. Because it is nonselective, it may damage important crops and desirable ornamental plants even when used as directed. Therefore, genetically modified plants such as soybean, corn, and cotton that are glyphosate-resistant have been developed and patented. They are widely grown throughout the world. Almost all soybean crops and many corn crops grown today are of the glyphosate-resistant type. These genetically modified (GMO) crops are grown from patented seeds and have great economic value to growers, contributing to the food supply in a meaningful way. However, in some jurisdictions their use is highly controversial. While there is no evidence that the modified crops are toxic or dangerous to humans or animals, the long-term agricultural impact of widespread use of glyphosate herbicides on resistant crops remains to be determined. Additionally, the impact of effective weed elimination on the food supply and habitat of critical insect species, eg, bees, and some migrating birds has been a source of increasing concern.

Because of the widespread availability and use of this herbicide, glyphosate-surfactant poisonings are common. Many of the observed ingestions and reports of poisoning are from developing countries, where suicide by pesticide is common. Many injuries are minor, but some serious and lethal poisonings have been reported. Glyphosate is a significant eye and skin irritant. When ingested it can cause mild to moderate esophageal erosion. It also causes aspiration pneumonia and renal failure. There have been some reports of teratogenic outcomes in workers who handle and apply glyphosate, but the epidemiologic evidence is not clear. There is a growing literature on management of acute glyphosate poisoning. Treatment is symptomatic and no specific protocol is indicated. Hemodialysis has been used with success in cases of renal failure.

Although glyphosate seems to have little persistence and lower toxicity than other herbicides, the commercial formulations often contain surfactants and other active compounds that complicate the toxicity of the product. Some of the toxic effects are related to the surfactant material.

Bipyridyl Herbicides

Paraquat is the most important agent of this class (Figure 56–1). Its mechanism of action is said to be similar in plants and animals and involves single-electron reduction of the herbicide to free radical species. Ingestion (accidental or suicidal) is among the most serious and potentially lethal pesticide poisonings. Many serious exposures take place in developing countries where limited treatment resources are available. Paraquat accumulates slowly in the lung by an active process and causes lung edema, alveolitis, and progressive fibrosis. It probably inhibits superoxide dismutase, resulting in intracellular free-radical oxygen toxicity.

In humans, the first signs and symptoms after oral exposure are hematemesis and bloody stools. Within a few days, however, delayed toxicity occurs, with respiratory distress and the development of congestive hemorrhagic pulmonary edema accompanied by widespread cellular proliferation. During the acute period, oxygen should be used cautiously to combat dyspnea or cyanosis, because it may aggravate the pulmonary lesions. Hepatic, renal, or myocardial involvement may develop. The interval between ingestion and death may be several weeks.

Because of the delayed pulmonary toxicity, prompt immobilization of the paraquat to prevent absorption is important. Adsorbents (eg, activated charcoal, Fuller’s earth) are routinely given to bind the paraquat and minimize its absorption. Gastric lavage is not recommended as it may promote aspiration from the stomach into the lungs. Once the paraquat is absorbed, treatment is successful in fewer than 50% of cases. Monitoring of plasma and urine paraquat concentrations is useful for prognostic assessment. CT scanning has also been used to follow the pulmonary lesions as they develop and to help with prognosis. The pulmonary proliferative phase begins 1–2 weeks after paraquat ingestion. Although a few reports indicate some success with dialysis, hemodialysis and hemoperfusion rarely change the clinical course. Many approaches have been used to slow or stop the progressive pulmonary fibrosis. Immunosuppression using corticosteroids and cyclophosphamide is widely practiced, but evidence for efficacy is weak. Antioxidants such as acetylcysteine and salicylate might be beneficial through free radical-scavenging, anti-inflammatory, and nuclear factor kappa-B inhibitory actions. However, there are no published human trials. The case fatality rate is high in all centers despite large variations in treatment. Patients require prolonged observation and treatment for respiratory and renal insufficiency if they survive the acute stage of poisoning.

ENVIRONMENTAL POLLUTANTS

Polychlorinated & Polybrominated Biphenyls

Highly halogenated biphenyl compounds, which have desirable properties for insulation, fire retardancy, and many other uses, were manufactured in large quantities during the mid-20th century. The quantities produced and the almost universal dispersion of the materials in which they were incorporated have produced an enormous environmental problem. Both chlorinated and brominated biphenyls are environmentally dangerous and significantly toxic, and are now banned from use.

The polychlorinated biphenyls (PCBs, coplanar biphenyls) were used as dielectric and heat transfer fluids, lubricating oils, plasticizers, wax extenders, and flame retardants. Their industrial use and manufacture in the USA were terminated by 1977. The chlorinated products used commercially were actually mixtures of PCB isomers and homologs containing 12–68% chlorine. These chemicals are very stable, highly lipophilic, poorly metabolized, and very resistant to environmental degradation; thus they bioaccumulate in food chains.

Food is the major source of PCB residues in humans. Accumulation of PCB in fish species led Canada and the USA to restrict commercial fishing and to limit consumption of fish from the Great Lakes of North America (see Box: Bioaccumulation & Biomagnification, earlier). In addition, large industrial site contamination, illegal dumping, migration from hazardous waste sites and other large-scale sources, and widespread use of PCBs in electrical transformers has led to multiple localized areas of contamination and human exposure. Leakage of transformer dielectric fluids in neighborhoods and backyards has caused significant numbers of serious but highly localized PCB exposure events.

There are numerous reports of large population exposures to PCBs. A serious exposure to PCBs—lasting several months—occurred in Japan in 1968 as a result of cooking oil contamination with PCB-containing transfer medium (Yusho disease). A similar outbreak called Yucheng disease occurred at about the same time in Taiwan. Effects on the fetus and on the development of the offspring of poisoned women were reported. It is now known that the contaminated cooking oil contained not only PCBs but also polychlorinated dibenzofurans (PCDFs) and polychlorinated quaterphenyls (PCQs). It is likely that the effects initially attributed to the PCBs were actually caused by a mixture of contaminants. Workers occupationally exposed to PCBs develop dermatologic problems that include chloracne, folliculitis, erythema, dryness, rash, hyperkeratosis, and hyperpigmentation. Some hepatic abnormalities have been found in PCB poisoning, and plasma triglycerides are elevated.

Information about the effects of PCBs on reproduction and development is accumulating. The halogenated pesticides are potent endocrine disrupters and there is widespread concern about the persistent estrogenic effect of these chemicals. Adverse reproductive impacts of PCBs have been found in many animal studies. Direct teratogenic effects in humans have yet to be established: studies in workers and in the general population exposed to moderate or to very high levels of PCBs have not been conclusive. Some adverse behavioral effects in infants have been reported. An association between prenatal exposure to PCBs and deficits in childhood intellectual function was described for children born to mothers who had eaten large quantities of contaminated fish. Epidemiologic studies have established increases in various cancers including melanoma, breast, pancreatic, and thyroid cancers. These findings and animal studies provided a sufficient basis for the IARC to classify some co-planar PCBs as class 1, carcinogenic to humans, in volume 100 of the IARC monographs. A comprehensive EPA fact sheet on PCBs may be found at http://www.epa.gov/epawaste/hazard/tsd/pcbs/index.htm.

The polybrominated biphenyls (PBBs) and their esters (PBDEs) share many of the toxic and environmentally damaging persistent qualities of PCBs. They were introduced as fire retardants in the 1950s and have been used in massive quantities since that time. The biphenyls are no longer produced and may no longer be used, but the biphenyl esters remain in use as fire retardants in plastics for bedding and in automobile upholstery. PBB fire retardant contamination has been extensive in the Great Lakes region, resulting in large exposure to the population. PBBs are considered IARC class 2a: probable human carcinogens. PDBEs are not classified. An EPA technical fact sheet on PBB and PBDEs may be found at http://www2.epa.gov/fedfac/technical-fact-sheet-polybrominated-diphenyl-ethers-pbdes-and-polybrominated-biphenyls-pbbs.

The polychlorinated dibenzo-p-dioxins (PCDDs), or dioxins, are a group of halogenated congeners of which tetrachlorodibenzodioxin (TCDD) has been the most carefully studied. There is a large group of dioxin-like compounds, including polychlorinated dibenzofurans (PCDFs) and coplanar biphenyls. While PCBs were used commercially, PCDDs and PCDFs are unwanted byproducts that appear in the environment and in manufactured products as contaminants because of improperly controlled combustion processes. They are also produced when unexpected heating to temperatures over 600° C occurs as in lightning strikes or electrical fires in PCB-containing transformers. Like PCBs, these chemicals are very stable and highly lipophilic. They are poorly metabolized and very resistant to environmental degradation. Several significant environmental contamination episodes involving dioxins and furans from industrial sites have occurred. Recent publications have demonstrated an elevated incidence of subsequent chronic diseases (eg, diabetes, metabolic syndrome, and obesity) in exposed persons. Laboratory studies of the blood concentrations of TCDD and its metabolites have provided insight into the persistence and metabolism of the contaminants.

In laboratory animals, TCDD has produced a variety of toxic effects. Wasting syndrome (severe weight loss accompanied by reduction of muscle mass and adipose tissue), thymic atrophy, epidermal changes, hepatotoxicity, immunotoxicity, effects on reproduction and development, teratogenicity, and carcinogenicity have been produced. The effects observed in workers involved in the manufacture of 2,4,5-T (and therefore presumably exposed to TCDD) consisted of contact dermatitis and chloracne. In severely TCDD-intoxicated patients, discrete chloracne may be the only manifestation.

The presence of TCDD in 2,4,5-T, commercially known as Silvex, was believed to be responsible for other human toxicities associated with the herbicide. There is epidemiologic evidence for an association between occupational exposure to the phenoxy herbicides and an excess incidence of non-Hodgkin’s lymphoma. The TCDD contaminant in these herbicides seems to play a role in a number of cancers such as soft tissue sarcomas, lung cancer, Hodgkin’s lymphomas, and others. TCDD is considered an IARC class 1, known human carcinogen. Other halogenated compounds of this type are not currently classifiable as to carcinogenicity; they are listed as IARC class 3.

Perfluorinated Compounds (PFCs)

Fluorinated hydrocarbon chemicals have been of commercial interest since the mid-20th century. Their uses have included coolant materials in air conditioning systems; artificial oxygen-carrying substances in experimental clinical studies; and heat-, stain-, and stick-resistant coatings for cookware, fabrics, and other materials. The fluorocarbons were produced in very large quantities and have become widespread in the environment. When it later became apparent that migration of lower molecular weight fluorocarbons to the troposphere had a deleterious effect on the protective ozone layer, they were banned from use. The higher molecular weight, more highly fluorinated compounds, now called perfluorinated substances (eg, Teflon), have remained in broad use. Like the heavily chlorinated and brominated hydrocarbons, their commercial usefulness has been complicated by a recognition of adverse environmental and suspected human toxic impacts that resemble some of the adverse qualities of the other halogenated hydrocarbons. A useful reference is the Centers for Disease Control (CDC) fact sheet on PFCs. It is found at http://www.cdc.gov/biomonitoring/pdf/PFCs_FactSheet.pdf.

1. Human toxicologyConcerns about the toxicology of PFCs have centered on their estrogenic properties and accumulation and persistence in humans. Human exposure to perfluoro compounds takes place through ingestion and inhalation. Since these compounds enter the food chain and water sources and are persistent, ingestion of contaminated food and water products is a major source of human accumulation. The human half-life of PFOA is estimated to be about 3 years. As a persistent chemical and an endocrine disrupter, it is likely that it has some long-term adverse impact on reproductive function, cellular proliferation, and other cellular homeostatic mechanisms. Several PFCs (but not perfluoro compounds derived from PFOA) have been found to act as proliferators of breast cancer cells. However, a large epidemiologic study recently demonstrated a statistically significant association between high and very high serum PFOA levels in workers and kidney cancer, and possibly prostate cancer, ovarian cancer, and non-Hodgkin’s lymphoma. There also may be modest associations with cholesterol elevation and uric acid abnormalities. Finally, an acute pulmonary disorder, polymer fume fever, is caused by the pyrolysis of PFOA. Like metal fume fever, seen in welders as a result of cadmium vaporization, polymer fume fever has an acute onset several hours after exposure to the vaporized PFOA and may cause severe respiratory distress. The onset of constitutional symptoms, malaise, chills and fever, and respiratory distress is characteristic of fume fevers. While polymer fume fever is usually mild and self-limited, noncardiogenic pulmonary edema has occurred. Whenever PFOA is heated above 350–400° C, toxic fumes capable of causing polymer fume fever are emitted. Overheated household cookware or burning of coated fabrics present this risk.

Other human effects are not clearly defined, although animal studies have shown toxic effects on immune, liver, and endocrine function, and some increase in tumors and neonatal deaths. A useful American Cancer Society fact sheet on the subject may be found at http://www.cancer.org/cancer/cancercauses/othercarcinogens/athome/teflon-and-perfluorooctanoic-acid--pfoa.

2. Environmental toxicologyPerfluoro compounds are persistent environmental chemicals having a broad environmental impact. PFOA and related compounds are now found widely in water, soil, and many terrestrial and avian species. Aquatic organisms have accumulated significant loads of PFCs. An extensive risk assessment of the perfluoro chemicals has been carried out by Environment Canada, and guidelines have been developed for the management of PFOA and related compounds. These may be found at http://www.ec.gc.ca/ese-ees/default.asp?lang=En&n=451C95ED-1.

Endocrine Disruptors

As described above, the potential hazardous effects of some chemicals in the environment are receiving considerable attention because of their estrogen-like or antiandrogenic properties. Compounds that affect thyroid function are also of concern. Since 1998, the process of prioritization, screening, and testing of chemicals for such actions has been undergoing worldwide development. These chemicals mimic, enhance, or inhibit a hormonal action. They include a number of plant constituents (phytoestrogens) and some mycoestrogens as well as industrial chemicals, persistent organochlorine agents (eg, DDT), PCBs, and brominated flame retardants. Concerns exist because of their increasing contamination of the environment, the appearance of bioaccumulation, and their potential for toxicity. In vitro assays alone are unreliable for regulatory purposes, and animal studies are considered indispensable. Modified endocrine responses in reptiles and marine invertebrates have been observed. In humans, however, a causal relation between exposure to a specific environmental agent and an adverse health effect due to endocrine modulation has not been fully established. Epidemiologic studies of populations exposed to higher concentrations of endocrine-disrupting environmental chemicals are underway. There are indications that breast and other reproductive cancers are increased in these patients. Prudence dictates that exposure to environmental chemicals that disrupt endocrine function should be reduced.

Asbestos

Asbestos in many of its forms has been widely used in industry for over 100 years. All forms of asbestos have been shown to cause progressive fibrotic lung disease (asbestosis), lung cancer, and mesothelioma. Every form of asbestos, including chrysotile asbestos, causes an increase in lung cancer and mesothelioma. Lung cancer occurs in people exposed at fiber concentrations well below concentrations that produce asbestosis. Very large scale studies of insulation workers have shown that cigarette smoking and exposure to radon daughters increase the incidence of asbestos-caused lung cancer in a synergistic fashion. Asbestos exposure and smoking is a very hazardous combination.

All forms of asbestos cause mesothelioma of the pleura or peritoneum at very low doses. Other cancers (colon, laryngeal, stomach, and perhaps even lymphoma) are increased in asbestos-exposed patients. The mechanism for asbestos-caused cancer is not yet delineated. Arguments that chrysotile asbestos does not cause mesothelioma are contradicted by many epidemiologic studies of worker populations. Recognition that all forms of asbestos are dangerous and carcinogenic has led many countries to ban all uses of asbestos. Countries such as Canada, Zimbabwe, Russia, Brazil, and others that still produce asbestos argue that asbestos can be used safely with careful workplace environmental controls. However, studies of industrial practice make the “safe use” of asbestos highly improbable. Recent attempts to limit international trade in asbestos have been thwarted by heavy pressure from the asbestos industry and the producing countries. Information on countries that currently ban asbestos and the International Ban Asbestos movement may be found at http://ibasecretariat.org/alpha_ban_list.php.

METALS

Occupational and environmental poisoning with metals, metalloids, and metal compounds is a major health problem. Toxic metal exposure occurs in many industries, in the home, and elsewhere in the nonoccupational environment. The classic metal poisons (arsenic, lead, and mercury) continue to be widely used. (Treatment of their toxicities is discussed in Chapter 57.) Occupational exposure and poisoning due to beryllium, cadmium, manganese, and uranium are relatively new occupational problems.

Beryllium

Beryllium (Be) is a light alkaline metal that confers special properties on the alloys and ceramics in which it is incorporated. Beryllium-copper alloys find use as components of computers, in the encasement of the first stage of nuclear weapons, in devices that require hardening such as missile ceramic nose cones, and in heat shield tiles used in space vehicles. Because of the use of beryllium in dental appliances, dentists and dental appliance makers are often exposed to beryllium dust in toxic concentrations and may develop beryllium disease.

Beryllium is highly toxic by inhalation and is classified by the IARC as a class 1, known human carcinogen. Inhalation of beryllium particles produces both acute beryllium disease and chronic disease characterized by progressive pulmonary fibrosis. Skin disease also develops in workers exposed to beryllium. The pulmonary disease is called chronic beryllium disease (CBD) and is a chronic granulomatous pulmonary fibrosis. In the 5–15% of the population that is immunologically sensitive to beryllium, CBD is the result of activation of an autoimmune attack on the skin and lungs. The disease is progressive and may lead to severe disability, cancer, and death. Although some treatment approaches to CBD show promise, the prognosis is poor in most cases.

The current permissible exposure levels for beryllium of 0.01 mcg/m3 averaged over a 30-day period or 2 mcg/m3 over an 8-hour period are insufficiently protective to prevent CBD. Both NIOSH and the ACGIH have recommended that the 8-hour PEL and TLV be reduced to 0.05 mcg/m3. These recommendations have not yet been implemented. Current OSHA information on beryllium appears at https://www.osha.gov/SLTC/beryllium/index.html.

Environmental beryllium exposure is not generally thought to be a hazard to human health except in the vicinity of industrial sites where air, water, and soil pollution have occurred.

Cadmium

Cadmium (Cd) is a transition metal widely used in industry. Workers are exposed to cadmium in the manufacture of nickel cadmium batteries, pigments, low-melting-point eutectic materials; in solder; in television phosphors; and in plating operations. It is also used extensively in semiconductors and in plastics as a stabilizer. Cadmium smelting is often done from residual dust from lead smelting operations, and cadmium smelter workers often face both lead and cadmium toxicity.

Cadmium is toxic by inhalation and by ingestion. When metals that have been plated with cadmium or welded with cadmium-containing materials are vaporized by the heat of torches or cutting implements, the fine dust and fumes released produce an acute respiratory disorder called cadmium fume fever. This disorder, common in welders, is usually characterized by shaking chills, cough, fever, and malaise. Although it may produce pneumonia, it is usually transient. However, chronic exposure to cadmium dust produces a far more serious progressive pulmonary fibrosis. Cadmium also causes severe kidney damage, including renal failure if exposure continues. Cadmium is a human carcinogen and is listed as a class 1, known human carcinogen by the IARC.

The current OSHA PEL for cadmium is 5 mcg/m3 but is insufficiently protective of worker health. The OSHA cadmium standard may be found at https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=standards&p_id=10035.

Nanomaterials

Nanomaterials are defined as any material, natural or manufactured, that has at least one dimension that lies between 1 and 100 nanometers (nm) in size. The Stanford University Health and Safety Department gives a more precise definition at http://www.stanford.edu/dept/EHS/prod/researchlab/IH/nano/what_are_nanomaterials.html.

Nanomaterials have been of increasing commercial interest and are now used for an extraordinary range of purposes. In the pharmaceutical manufacturing industry, nanoparticles are being tested and used to deliver cancer chemotherapeutic and other drugs. Currently produced nanomaterials include gold, silver, cadmium, germanium, ceramic, and aluminum oxide nanowires; carbon, silicon, and germanium nanotubes; zinc oxide nanocrystals; gold nanowafers; and copper oxide nanocubes. The increasing use of nanomaterials has led to release of these nanoscale substances into the workplace and the general environment. Because nanomaterials behave in unique patterns of chemical and physical reactivity, their toxicology is often novel and there is insufficient information on the likely human or environmental impact of dispersal of these manufactured products in the environment. The University of North Carolina laboratory safety and health manual outlines the problems of working with nanomaterials in the laboratory and their safe use at http://ehs.unc.edu/manuals/laboratory/docs/lsm18.pdf.

1. Human toxicologyInhalation, oral ingestion, dermal absorption, and parenteral administration of nanomaterials have been the sources of human exposure. Because of the unique physicochemical properties of nanomaterials, their toxicity may be similar to or very different from the larger, bulk materials encountered in traditional toxicology studies. The nature of the exposure will impact the likelihood that nanomaterials will reach target organs or cells. Nanomaterials can cross cellular membranes, penetrate nuclear material and genetic information, and may impact cellular response at a nanoscale. Silica nanoparticles have been demonstrated to produce kidney toxicity in humans, and zinc oxide nanoparticles are toxic to human liver cells. Multiwalled carbon nanotubes have been found to be cytotoxic in human lung cells. Titanium dioxide nanoparticles that are widely used in sunscreens, other cosmetics, pharmaceuticals, and many other products have been noted to be toxic in the lungs and elsewhere.

2. Environmental toxicologyNanomaterials can enter the environment at all stages of their industrial life cycle, including manufacturing, delivery, use, and disposal. When nanomaterials are placed into waste streams they may enter water systems, or be carried by wind or soils, and enter the food chain. An EPA fact sheet on nanomaterials in the environment is available at http://www.epa.gov/athens/research/nano.html.

The increasing production of nanomaterials and their multiple uses has led to environmental contamination. Many species, including bacteria, small mammals, and fish and other aquatic organisms have been studied in laboratory assessments of nanomaterial toxicity. The ecotoxicology of nanomaterials remains an area of deep concern and ongoing research.

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*The author thanks the late Gabriel L. Plaa, PhD, previous author of this chapter, for his enduring contributions.