Overall, exposure assessments can be performed most reliably for recent events and much less reliably for past exposures. The difficulties in exposure assessment often make it the weak link in trying to determine the connection between an environmental contaminant and its adverse effects on human health. Although exposure assessment methods will undoubtedly improve, significant uncertainty will remain in the foreseeable future. Acute effects occur within minutes, hours or days while chronic effects appear only after weeks, months or years. The quality and quantity of scientific evidence gathered is different for each type of effect and, as a result, the conclusions from the test results are also different.
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.