Threshold

The dose or exposure below which an adverse effect is not expected. Common approaches to assessing the risks associated with noncancer toxicity are generally different from that used to assess the potential risks associated with carcinogenesis. Scientists often assume that a small number of molecular events can evoke carcinogenic and/or mutagenic changes in a single cell, which can lead to self-replicating damage. Often, this is considered a nonthreshold effect since there is presumably no level of exposure that does not pose a small, but finite, probability of generating a response. It is most often assumed that noncancer effects have a threshold, that is, a dose level below which a response is unlikely, because a compensatory effect or adaptive effect in the cell protects against an adverse effect.

This threshold concept is important in many regulatory contexts. The individual threshold hypothesis holds that some exposures can be tolerated by an organism with essentially no chance for expression of a adverse effect. Further, risk management decisions frequently focus on protecting the more sensitive members of a population. In these cases efforts are made to keep exposures below the more sensitive subpopulation threshold, although it is recognized that hypersensitivity and chemical idiosyncrasy may exist at yet lower doses.