Toxicology

The toxicologist employs the tools and methods of science to better understand the consequences of exposure to toxic chemicals. Toxicologists typically assess the relationship between toxic chemicals and environmental health by evaluating such factors as:

• Risk. To assess the risk associated with exposure to a toxic substance, the toxicologist first measures the exposure characteristics and then computes the doses that enter the human body. He or she then compares these numbers to derive an estimate of risk, sometimes based on animal studies. In cases where human data exist for a toxic substance, such as benzene, more straightforward correlations with the humans risk of illness or death are possible.
• Precautionary strategies. Given recommendations from toxicologists, government agencies sometimes decide to regulate a chemical based on limited evidence from animal and human epidemiological studies that the chemical is toxic. Such decisions may have both ethical and political ramifications; to fail to issue warnings about a "suspect" chemical could leave vulnerable members of the population at risk of contracting an avoidable illness; on the other hand, any restrictions placed on the use of the chemical could place burdensome cleanup costs on private industry.
• Clinical data. Some toxicologists devise new techniques and develop new applications of existing methods to monitor changes in the health of individuals exposed to toxic substances. For example, one academic research group in the United States has spent many years developing new methods for monitoring the effects of exposure to oxidants (e.g., free radicals) in healthy and diseased humans.
• Epidemiological evidence. Another way to understand the environmental factors contributing to human illness is to study large populations that have been exposed to substances suspected of being toxic. Scientists then attempt to tie these observations to clinical data. Ecologic studies seek to correlate exposure patterns with a specific outcome. Case-control studies compare groups of persons with a particular illness with similar healthy groups, and seek to identify the degree of exposure required to bring about the illness. Other studies may refine the scope of environmental factor studies, or examine a small group of individuals in which there is a high incidence of a rare disease and a history of exposure to a particular chemical.
• Evidence of bio-accumulation. When a chemical is nonbiodegradable, it may accumulate in biosystems, with the result that very high concentrations may accumulate in animals at the top of food chains. Chlorinated pesticides such as dieldrin and DDT, for example, have been found in fish in much greater concentrations than in the seawater where they swim.