Environmental toxicology

(medicine) A broad field of study encompassing the production, fate, and effects of natural and synthetic pollutants in the environment.

A broad field of study encompassing the production, fate, and effects of natural and synthetic pollutants in the environment. The breadth of this field depends on the definition of environment. It can be defined as narrowly as the home and workplace or as broadly as the entire Earth and its biosphere. Environmental toxicology is truly an interdisciplinary science. The effects of a pollutant on the environment depend on the amount released (the dose) and its chemical and physical properties. Pollutants can be grouped according to their origin and effects.

Pollution from nutrients is generally a problem of aquatic systems. Carbon, nitrogen, and phosphorus are essential nutrients and, when present in excess, can result in an overstimulation of microbial and plant growth. Nutrients enter the environment in runoff from fertilized agricultural areas, and in effluents from municipal and industrial waste and decaying plant material. See also Eutrophication.

Pathogenic bacteria and protozoa can be a major source of pollution in areas that receive untreated sewage, items from ocean dumping, and improperly discarded hospital waste. Toxic metabolites of fungal origin (mycotoxins) are also potential pollutants. See also Aflatoxin.

Forest fires, volcanic eruptions, and dust storms can be major sources of suspended materials. These materials can also originate in runoff from agricultural areas, construction and mining sites, and roads and other paved areas. Truck and automobile exhaust and industrial discharge to the atmosphere are also sources of suspended solids.

Metabolic processes and natural combustion and thermal activity (such as forest fires and volcanoes) can release large amounts of gaseous by-products to the atmosphere. However, natural inputs are minor compared to atmospheric pollutants due to human activity. Although most anthropogenic air pollution is produced by the various forms of transportation, emission from stationary sources of fuel combustion (for example, factories and power plants) are responsible for the greatest amount of hazardous materials released. See also Air pollution; Greenhouse effect.

All living organisms require certain metals for physiological processes. These elements, when present at concentrations above the level of homeostatic regulation, can be toxic. In addition, there are metals that are chemically similar to, but higher in molecular weight than, the essential metals (heavy metals).

Organic solvents are used widely and in large amounts in industries, laboratories, and homes. They are released to the atmosphere as vapor and can pose a significant inhalation hazard. Improper storage, use, and disposal have resulted in the contamination of surface and ground waters and drinking water. See also Water pollution.

The pesticides represent an important group of materials that can enter the environment as pollutants. They are highly toxic, and many nontarget organisms can suffer harmful effects if misuse or unintended release occurs. See also Pesticide.

Coal and petroleum-derived materials and by-products are major environmental pollutants. Widespread use has led to enormous releases to the environment of distillate fuels, crude oils, runoff from coal piles, exhaust from internal combustion-fired power plants, industrial emissions, and emissions from municipal incinerators. The toxicity of polycyclic aromatic hydrocarbons is perhaps one of the most serious long-term problems associated with the use of petroleum. They accumulate in soil, sediment, and biota, and at high concentrations can be acutely toxic. See also Fossil fuel.

Polychlorinated biphenyls (PCBs) are produced by the chlorination of biphenyl, giving rise to mixtures of up to 210 possible products. They have been used worldwide in electrical equipment, vacuum pumps, hydraulic fluids, heat-transfer systems, lubricants, and inks. The related polybrominated biphenyls (PBBs) have been used as fire retardants. Major sources of polychlorinated biphenyls have included leaks from waste disposal facilities, vaporization during combustion, and disposal of industrial fluids. Their use has been largely restricted or eliminated. Environmental concentrations are decreasing, but with their persistence they remain significant pollutants. Chlorinated dibenzo-p-dioxins and dibenzofurans are formed during the heating of chlorophenols, and have been identified as potential contaminants in the herbicide 2,4,5-T. They can be formed during the incineration of municipal wastes, polychlorinated biphenyls, or plant materials treated with chlorophenols. See also Environmental engineering; Polychlorinated biphenyls; Toxicology.

Most toxic agents in the environment are present at very low levels. In exceptional cases, however, the levels of environmental pollutants become so high that the effects on health are readily apparent, and severe enough to precipitate remedial action. The persistent London fog of 1952 led to so many cases of death due to respiratory problems that within a few years an Act of Parliament was passed which laid down limits on urban air pollution. Most large conurbations around the world have experienced episodes of dramatic increases in pollution that have led to legally enforced changes in lifestyle, such as the introduction of restrictions in the use of private cars in Athens on days when air pollution rises above a certain level.

Contamination of the atmosphere by the products of combustion has been a problem ever since the beneficial effects of fire were discovered. The blackened roofs of caves known to have been inhabited in prehistoric times are a silent reminder that environmental pollution is by no means a modern phenomenon. Any combustion in the atmosphere inevitably leads to the production of oxides of nitrogen (nitrogen makes up 80% of the atmosphere) and oxides of sulphur and carbon (by combustion of organic matter). The nitrogen oxides are potent pulmonary irritants and concentrations as low as 0.5 parts per million can lead to respiratory problems. Incomplete combustion of complex organic molecules leads to the formation of polyaromatic hydrocarbons (PAHs), which are known to be potent carcinogens. The interaction between nitrogen oxides and PAHs leads to formation of nitrated PAHs, which are in turn even more potent carcinogens. However, ascribing increased risk of certain chronic diseases, such as lung cancer, to atmospheric pollution in general, or to individual components in particular, has proved rather difficult. For example, levels of many atmospheric pollutants are so low that active or passive exposure to cigarette smoke is the major contributor to exposure. Indoor air pollution is a particular case where exposures can be high simply because of the confined space and slow rates of exchange of air with the outside. Most combustion processes also lead to the production of particulate matter of such small dimensions (less than 10 microns in diameter) that they penetrate deeply into the lungs, where they lodge in the bronchioles or in the alveoli and cause local inflammation. Automobile emissions share many of these properties of all combustion processes, but can also contain particular toxic agents such as lead, benzene, and 1, 3-butadiene.

Water is, in principle, the easiest of the environmental matrices to obtain in a pure state. Even the most contaminated water supply is likely to be greater than 99% H₂O. However, our requirement for drinkable water of 1-2 litres per day lays quite stringent limits on the levels of contaminants which are compatible with a healthy lifestyle. The provision of clean water supplies has been, and continues to be for many parts of the world, one of the greatest contributions to improved public health. Contamination of water by industrial processes can contribute to episodes of human poisoning either directly or indirectly, for example through the food chain. Perhaps one of the most dramatic examples of the latter in recent times was an incident of mercury poisoning caused by consumption of contaminated fish by families in a fishing community in Minimata, Japan. A proper balance has to be found between various risks from various toxic agents in the environment. For example, chlorination of public water supplies has been found to lead to the production of low levels of organochlorine compounds via the reaction of chlorine with natural humic acids; as with many organochlorine compounds there have been concerns about possible toxicity from these substances. However, the hazards of not chlorinating water supplies were highlighted most recently by epidemics of cholera in Peru, when flooding led to contamination of water supplies and public health officials were hesitant about the use of chlorine.

Environmental pollutants can also be found in foods through contamination of soil. Metals (such as mercury, cadmium, and arsenic) and organohalogen compounds (containing chlorine and bromine) are examples of the kinds of toxic compounds that are absorbed through roots or leaves of plants.

Much attention has been focused on the effects of environmental toxins on non-human species (ecotoxicology). A high incidence of tumours in the livers of bottom feeding fish, such as flounder in Boston Harbour, has been attributed to the effects of carcinogenic constituents of heavy marine motor oils. Toxic effects in species which share our environment can sometimes act as an early warning of effects in humans in an analogous manner to the miner's canary, which at one time acted as a marker of poisonous gases in coalmines. In 1962 Rachel Carson published a highly influential book, Silent spring, which graphically described the deleterious consequences of the pollution of our environment by manmade chemicals, particularly pesticides. The widespread use, in the postwar years, of organochlorine and organophosphate pesticides for the eradication of agricultural pests and control of health hazards such as malaria began to be seen as a mitigated success. The persistence of some pesticides in the environment led to unforeseen side-effects, such as the thinning of eggshells resulting in the dramatic reduction of bird populations in agricultural areas. However, it is perhaps as a consequence of the heightened public awareness of environmental toxicology as a result of Silent spring, that the dramatic ‘Fable for Tomorrow’, describing the widespread loss of wildlife due to contamination with pesticides, which opens the book, has not (yet) come to pass.

— David Shuker

Bibliography

• Carson, R. (1962). Silent spring. Penguin, London.
• Klassen, C. D. (1996). Casarett and Doull's toxicology: the basic science of poisons. McGraw Hill, New York

See also poisons.