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Exposure to chemical agents in the environment – in air, water, food, and soil – has been implicated in numerous adverse effects, from cancer to lung disease to brain damage to birth defects. Some evidence is ironclad; some is suggestive at best. Although the acute effects such as poisonings are the best understood, it seems clear that hazardous pollutants contribute to the large and growing toll of chronic conditions, such as cancers and heart disease. Chemical pollutants can also play a role in infectious diseases, perhaps by rendering the body less able to ward off infections. The exact magnitude of the risk they pose, however, is difficult to quantify. This has fueled an intense debate over what constitutes safe use and disposal of toxic substances. This debate has occurred mostly in the affluent countries but is increasingly occurring in the developing world as well.
Although cause-and-effect relationships for most infectious diseases are well known, the links between chemical pollutants and disease are murkier. A person contracting cholera, for instance, clearly has been exposed to the cholera vibrio. The circumstances of that exposure may remain mysterious – and other contributing factors may be involved – but exposure to the bacterium is an established causal factor. The same pathway cannot be traced for a disease such as lung cancer. Cancers may take 10 to 40 years to develop, and many factors may contribute to the appearance of the disease in a particular person. Accordingly, chemical risks tend to be described in terms of the numbers of people exposed – for instance, 1.4 billion urban dwellers exposed to air quality that exceeds health guidelines, as WHO estimates (129). (See Urban Air: Health Effects of Particulates, Sulfur Dioxide, and Ozone.) Such estimates reveal little about how many people will actually suffer the adverse effects from these exposures. (See From Exposure to Effect.)
On a global scale, the health risk from chemical agents in the environment is considerably smaller than that from biological pathogens described in the previous section. In some heavily polluted areas, however, chemical risks can be quite high. Risks are likely to be higher in less developed as opposed to more developed countries, because fewer safeguards are typically in place there, either to reduce emissions or to protect people from exposure. Equally important, regardless of their magnitude, the risks from chemical pollutants are preventable.
Body of Evidence
Environmental pollutants arise from many sources, and exposures may occur through many different routes. (See Chemicals in the Environment.) Many occur on the job, where workers are exposed to doses typically much higher than those encountered in the ambient environment. Although occupational exposures are a major source of ill health globally, this subject is outside the purview of this section, which focuses on chemicals in the ambient environment.
Some pollutants of particular concern are airborne substances – primarily suspended particulates, sulfur dioxide, nitrogen oxides, ozone, and carbon monoxide – emitted as byproducts of energy production, industry, and transportation (130). While most scientific attention and policy debate has focused on outdoor air pollutants, indoor air pollutants such as smoke and soot, arising from the burning of biomass fuels such as dung and wood, appear to pose an even greater risk to human health, because indoor exposures tend to be many times higher than those encountered outdoors. Hundreds of millions of people in the developing world are regularly exposed to potentially harmful indoor emissions, which are believed to contribute to chronic lung disease, cancer, and ARI.
Exposure to pesticides can occur directly, typically to agricultural workers and their families and those who live near farms where pesticides are heavily used. Exposures can also be indirect, when pesticides contaminate surface waters or groundwater or soil, or when pesticide residues on food or contaminated fish or wildlife are ingested. Some of the most problematic pesticides are organic compounds that are slow to degrade and thus persist in the environment for years, accumulating in animal and human tissues.
Some industrial chemicals, such as PCBs, also persist in the environment and have caused widespread environmental contamination. Exposure to industrial chemicals can result from intentional releases to either air or water, from accidents, or from the leaching of these substances from disposal sites into surrounding soil or water.
Heavy metals are used for a variety of agricultural and industrial purposes and are frequent contaminants in industrial wastes. Many are toxic: mercury, for instance, is a known neurotoxin; arsenic can cause skin and other cancers. One of the most important environmental pollutants is lead, which at even very low doses can cause significant neurological impairment and loss of intelligence. Major sources of lead in the environment include exhaust from vehicles using leaded gasoline, lead-based paints, and some types of water pipes.
129. David Satterthwaite et al., The Environment for Children: Understanding and Acting on the Environmental Hazards that Threaten Children and their Parents (Earthscan Publications, Ltd., London, 1996), p. 44.
130. David Satterthwaite et al., The Environment for Children: Understanding and Acting on the Environmental Hazards that Threaten Children and their Parents (Earthscan Publications, Ltd., London, 1996), p. 44.