environmental toxicology

environmental 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.