Acute toxicity is easiest to deal with. Short-term studies with animals provide evidence as to which effects are linked with which chemicals and the levels at which these adverse effects occur. Often, some human experience is available as a result of accidental exposures. When these two types of evidence are available, it is usually possible to make a good estimate of the levels of a particular toxicant that will lead to a particular acute adverse effect in humans. This approach is the basis for much of the current regulation of toxic substances, especially in occupational situations.
Chronic toxicity is much more difficult to assess.
There are a variety of specific tests for adverse effects such as reproductive damage, behavioral effects, cancer, etc. It is not possible to discuss all of these, but a look at cancer assessment will reveal some of the problems inherent in long-term toxicity assessments and also focus on the health effect which seems to be of most public concern. In cancer assessment, it is not only the chronic nature of the disease but also the low incidence which is of concern, that causes difficulty. Society has decided that no more than one additional cancer in 100,000 or one million people is acceptable, so assessment measures must be able to detect this small increase. Two types of evidence are utilized to determine the dose of chemical that will result in this change.