Risk assessment helps estimate the probability that an event will occur and lets us set priorities and manage risks in an appropriate way. As an example, consider a person who smokes a pack of cigarettes a day and drinks well water containing traces of the cancer-causing chemical trichloroethylene (in acceptable amounts, as established by the Environmental Protection Agency, or EPA). Without a knowledge of risk assessment, this person might buy bottled water in an attempt to reduce his or her chances of getting cancer. Based on risk assessment calculations, the annual risk from smoking is 0.00059, or 5.9 × 10-4, whereas the annual risk from drinking water with EPA-accepted levels of trichloroethylene is 0.000000002, or 2.0 × 10-9. This means that this person is almost 300,000 times more likely to get cancer from smoking than to get it from ingesting such low levels of trichloroethylene. Knowing this, the person in our example would, we hope, stop smoking.
One of the most perplexing dilemmas of risk assessment is that people often ignore substantial risks but get extremely upset about minor risks. The average life expectancy of smokers is more than 8 years shorter than that of nonsmokers, and almost one-third of all smokers die from diseases that the habit causes or exacerbates. Yet many people get more upset over a one-in-a-million chance of getting cancer from pesticide residues on food than they do over the relationship between smoking and cancer. Perhaps part of the reason for this attitude is that behaviors such as diet, smoking, and exercise are parts of our lives that we can change if we choose to (Figure 4-2 and Table 4-2). Risks over which most of us have no control, such as pesticide residues or nuclear wastes, tend to evoke more fearful responses.