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 adverse effects on human health. Although exposure assessment methods will undoubtedly improve, there remains significant uncertainty 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 confidence placed in the conclusions from the test results are also different.
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.