Disorders due to chemical agents
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Hazardous chemicals can act directly on the skin, resulting in local irritation or an allergic reaction, or they may be absorbed through the skin, ingested, or inhaled. In the workplace ingestion of toxic chemicals is usually accidental and most commonly results from handling contaminated food, drink, or cigarettes. Substances that occur as gases, vapours, aerosols, and dusts are the most difficult to control, and most hazardous chemicals are therefore absorbed through the respiratory tract. If inhaled, airborne contaminants act as irritants to the respiratory tract or as systemic poisons. Toxicity in such cases depends on the contaminant’s concentration, particle size, and physicochemical properties, particularly its solubility in body fluids. An individual’s reaction to any hazard depends primarily on the length, pattern, and concentration of exposure but is also affected by such factors as age, sex, ethnic group, genetic background, nutritional status, coexistent disease, concomitant exposure to other toxic agents, life-style, and history of previous exposure to the agent in question. The wide range of both naturally occurring and synthetic chemical compounds that can give rise to adverse health effects can be roughly organized into four major categories: gases, metals, organic compounds, and dusts.
Gases
Gases may act as local irritants to inflame mucous surfaces. Common examples include sulfur dioxide, chlorine, and fluorine, which have pungent odours and can severely irritate the eyes and the respiratory tract. Some gases, such as nitrogen oxides and phosgene, are much more insidious. Victims may be unaware of the danger of exposure because the immediate effects of these gases may be mild and overlooked. Several hours after exposure, however, breathlessness and fatal cardiorespiratory failure due to pulmonary edema (collection of fluid in the lungs) may develop.
Gases that interfere with oxygen supply to the tissues are known as asphyxiants and are of two principal types. Simple asphyxiants are physiologically inert gases that act by diluting atmospheric oxygen. If the concentration of such gases is high enough, hypoxia (deficiency of oxygen reaching the tissues of the body) results. Victims of mild hypoxia may appear to be intoxicated and may even resist rescue attempts. Common examples of simple asphyxiants are methane and carbon dioxide.
In contrast to simple asphyxiants, chemical asphyxiants, such as carbon monoxide and hydrogen sulfide, are highly reactive. They cause a chemical action that either prevents the blood from transporting oxygen to the tissues or interferes with oxygenation in the tissues. For example, carbon monoxide, a frequently encountered gas produced by incomplete combustion, combines with hemoglobin in the blood and reduces its oxygen-carrying capacity. In low concentration carbon monoxide poisoning can cause symptoms of fatigue, headache, nausea, and vomiting, but heavy exposure leads to coma and death. It is especially dangerous because it is both colourless and odourless. Hydrogen sulfide, however, can be recognized by its characteristic smell, suggestive of rotten eggs. It is produced when sulfur compounds decompose and acts by inhibiting the respiratory enzyme cytochrome oxidase, thus giving rise to severe tissue hypoxia. In addition to its asphyxiant properties, hydrogen sulfide also acts as an irritant to the eyes and mucous membranes.
Preventing gas poisoning involves preventing exposure. Workers should never enter enclosed spaces that have suspect atmospheres alone; workplaces should provide adequate ventilation, and air should be regularly tested for contamination. If exposure does occur, treatment involves the removal of the victim from the contaminated atmosphere, artificial respiration, and administration of oxygen or recommended antidotes. Victims exposed to gases with insidious delayed effects should be kept under medical observation for an appropriate period.
Metals
Metals and their compounds are among the poisons most commonly encountered in the home and workplace. Even metals essential for life can be toxic if they are present in excessive amounts. Iron, for example, is an essential element and is sometimes given therapeutically; if taken in overdose, however, it can be lethal.
Mercury poisoning, one of the classic occupational diseases, is a representative example of metal poisoning. Exposure to mercury can occur in many situations, including the manufacture of thermometers, explosives, fungicides, drugs, paints, batteries, and various electrical products. The disorders it can cause vary depending on the type of mercury compound and the method of exposure.
Ingestion of mercury salts such as mercuric chloride (corrosive sublimate) leads to nausea, vomiting, and bloody diarrhea. Kidney damage resulting in death may follow in extreme cases. Inhalation or absorption through the skin of mercury vapour causes salivation, loosening of the teeth, and tremor; it also affects the higher centres of the brain, resulting in irritability, loss of memory, depression, anxiety, and other personality changes. This mental deterioration, known as erethism, led to the well-known saying “mad as a hatter,” because, in the past, hatters commonly became ill when they used mercury salts to make felt out of rabbit fur. Poisoning with organic mercury compounds (used in fungicides and pesticides) results in permanent neurological damage and can be fatal.
Other hazardous metals commonly encountered in industry include arsenic, beryllium, cadmium, chromium, lead, manganese, nickel, and thallium. Some have been shown to be carcinogenic, including certain compounds of nickel (linked to lung and nasal cancer), chromium (lung cancer), and arsenic (lung and skin cancer).
Organic compounds
The organic compounds that pose the greatest occupational hazards are various aromatic, aliphatic, and halogenated hydrocarbons and the organophosphates, carbamates, organochlorine compounds, and bipyridylium compounds used as pesticides.
Pesticides are used the world over; and, even though precautionary measures (such as using protective clothing and respirators, monitoring contamination of equipment and clothing, keeping workers out of recently sprayed areas, and requiring workers to wash thoroughly after exposure) can be instituted, poisoning not infrequently occurs in agricultural communities. The organophosphates and the generally less toxic carbamates exert their effects by inhibiting cholinesterase, an enzyme that prevents stimulation from becoming too intense or prolonged by destroying the acetylcholine involved in the transmission of impulses in the autonomic nervous system. Cholinesterase inhibitors allow the accumulation of acetylcholine, causing symptoms related to parasympathetic overactivity, such as chest tightness, wheezing, blurring of vision, vomiting, diarrhea, abdominal pain, and in severe cases respiratory paralysis. Atropine and certain oximes counteract their effects.
Paraquat and diquat, the bipyridylium compounds, are deadly if ingested. Skin contact or inhalation of a concentrate of paraquat can cause fatal lung damage. Because no specific antidote is known, treatment consists of minimizing the body’s absorption of the poison.
The organochlorine compounds, such as DDT, are being progressively phased out of use. Because they are fat-soluble and very stable, they accumulate and remain in the fatty tissues of the body for prolonged periods. Symptoms of poisoning include nausea, irritability, weakness, muscle tremors, and convulsions. There is no specific antidote.
Other groups of pesticides that are used less frequently or are less hazardous include the organomercury compounds (see above Metals); the dinitro and arsenic compounds; and nicotine.
Hydrocarbons are used industrially in the derivation of other compounds and in solvents, degreasing agents, refrigerants, fire extinguishers, dry cleaning agents, paint removers, and other products. Many are volatile and can be absorbed by inhalation; some are fat-soluble and can be readily absorbed following spills on the skin.
Gasoline, fuel oils, and other petroleum products are common examples of aliphatic hydrocarbons. If they are ingested or inhaled, dizziness, weakness, nausea, or irritation of the lungs may follow. In very severe cases victims may become unconscious or experience convulsions. Direct contact causes skin irritation and dryness. Prolonged exposure to certain petroleum oils may result in skin cancer.
The aromatic hydrocarbon benzene provides the basis for the synthesis of many other organic compounds. It is rapidly absorbed following inhalation or skin contact. Symptoms from mild exposure include dizziness, headache, euphoria, confusion, and nausea. Long-term exposure may be followed by bone marrow depression, anemia, spontaneous bleeding, and leukemia. Several aromatic hydrocarbons are known to be carcinogens. Particularly hazardous are naphthylamine, benzidine, and 4-amino diphenyl, which cause bladder cancer. Previously used in the synthetic dye, synthetic rubber, cable-making, and chemical industries, they have been banned in a number of countries.
When aliphatic and aromatic hydrocarbons have hydrogen atoms in their structure replaced by halogens (often chlorine), they are known as halogenated hydrocarbons. In general, increasing the chlorination of aliphatic hydrocarbons increases their toxicity, while the reverse is true of the aromatic series. Many chlorinated hydrocarbons, including chloroform and trichloroethylene, act as depressants on the central nervous system, producing anesthetic or narcotic effects that may be abused. Occupational exposure to many solvents may act synergistically with alcohol, resulting in more damage than either agent could produce on its own. Some halogenated hydrocarbons cause extensive disorders in addition to their common narcotic effect. Inhaling or ingesting the solvent carbon tetrachloride, for example, leads to liver damage; and exposure to vinyl chloride causes Raynaud’s phenomenon (spasms in the small arteries that cause the extremities to become pale and cold, as well as painful), necrosis of the small bones of the hand, liver damage, and a rare, highly malignant tumour of the liver.
Workers exposed to hydrocarbons should wear protective clothing or masks when appropriate, moderate alcohol consumption, and verify that work areas are well ventilated and that recommended exposure levels are not exceeded.
Dusts
The inhalation of a variety of dusts is responsible for a number of lung and respiratory disorders, whose symptoms and severity depend on the composition and size of the dust particle, the amount of dust inhaled, and the length of exposure. The lung diseases known as the pneumoconioses result when certain inhaled mineral dusts are deposited in the lungs, where they cause a chronic fibrotic reaction that leads to decreasing capacity for exercise and increasing breathlessness, cough, and respiratory difficulty. No specific treatment is known, but as with all respiratory disorders patients are urged to quit smoking, which aggravates the condition. Suggested measures for limiting exposure include using water and exhaust ventilation to lower dust levels and requiring workers to wear respirators or protective clothing, but such procedures are not always feasible. Coal worker’s pneumoconiosis, silicosis, and asbestosis are the most common pneumoconioses.
As its name suggests, coal worker’s pneumoconiosis (also known as black lung) occurs most frequently among coal miners and workers involved in the transporting or processing of coal. It is generally benign in its early stages, but after a variable number of years of exposure to coal dust, progressive massive fibrosis may develop, ending in cardiorespiratory failure. Miners and quarry workers are the people most likely to suffer from silicosis. Because silica is found in many rocks and is used in a variety of industries, workers involved in stonecutting, grinding, drilling, foundry work, sandblasting, pottery making, and the manufacture of abrasives are also at risk. Silicosis is an aggressive form of pulmonary fibrosis that speeds the progress of tuberculosis. Routine chest X rays can aid early diagnosis by revealing abnormal shadowing. Asbestosis is more difficult to detect in the early stages because chest X rays usually reveal little until the disease is advanced. From onset asbestosis progresses more rapidly than the other pneumoconioses and can result from relatively low exposure. Asbestos is the general term for a number of fibrous silicates that are used primarily in various fireproofing, insulation, and cement products. In addition to pulmonary fibrosis, inhaling asbestos fibres has also been shown to cause lung and other cancers.
Prolonged exposure to certain plant and animal dusts can cause asthma, even in people without a predisposition for allergies. Specific hazards include dusts from flour, grains, and wood and wood products. Cotton workers and others handling hemp or flax may develop a condition known as byssinosis, similar to asthma. The group of diseases known as farmer’s lung, malt worker’s lung, bird fancier’s lung, and so forth are caused by an allergic inflammatory reaction to the fungal spores present in moldy hay or barley, bird droppings, feathers, and a variety of other organic materials. Symptoms initially resemble those of influenza or pneumonia, but repeated episodes eventually lead to pulmonary fibrosis with chronic respiratory impairment. The only treatment for these disorders is avoiding exposure to the dusts.