Acute toxicity is easier to test or observe. Short-term animal studies provide evidence of which effects are linked with which chemicals and the levels at which these adverse effects occur. Often, some human exposure data are 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 level at which a specific toxicant will cause a particular acute adverse effect in humans. This approach is the basis for the current regulation of toxic substances, especially in the occupational setting.
Chronic toxicity is much more difficult to assess.
There are a variety of specific tests to ascertain 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 utmost concern to the general public. In cancer assessment, it is not only the chronic nature of the disease but also the low incidence which causes difficulty. Society has decided that no more than one additional cancer in 100,000 or one million exposed people is acceptable, so assessment measures must be able to detect this small number. Two types of evidence are utilized to determine the chemical dose that will result in an adverse health effect.