A threshold is the exposure level or dose of an agent above which toxicity or adverse health effects can occur, and below which toxicity or adverse health effects are unlikely. For example, taking aspirin is therapeutic and not dangerous up to a contain dose, but above that dose it can cause nausea, brain damage, bleeding, and, eventually, death. Sulfuric acid is not dangerous when only small amounts of it get on a person's skin, but if the amount gets too high, it burns. Thresholds for toxicity exist because, up to a certain point, the body can repair damage and detoxify chemicals to which it is exposed. If the exposures get too high, however, the detoxification and repair mechanisms are overwhelmed and toxicity starts to occur.
Thresholds for toxicity can be different in different people, with some people likely to be sensitive to smaller levels of exposures than others. In other words, toxicity thresholds are distributed differently within a population. For example, some people can breathe a lot of paint stripper without feeling ill, while others get sick from it quite easily. So while it may be easy to demonstrate a chemical's threshold for toxicity in identical laboratory animals, a threshold for toxicity in a diverse human population may be very difficult to determine.
The concept of a threshold for toxicity has played an important role in chemical regulation. Until recently, chemicals that cause cancer were assumed to have no threshold for their effects, while chemicals that cause other kinds of health effects were assumed to have thresholds. It is now known that some cancer-causing chemicals have thresholds and some other toxic agents do not, and this knowledge is slowly making its way into regulatory guidelines.
An example of a nonregulatory guideline that is based on toxicity thresholds is the threshold limit value (TLV). TLVs were derived as chemical exposure levels that are permissible in the workplaces—if workplace exposures stay below the TLVs, workers are unlikely to be adversely affected. TLVs were established first in 1968 by a nongovernmental organization known as the American Conference of Governmental Industrial Hygienists (ACGIH) based on available scientific information and best professional judgment. The ACGIH TLV Committee periodically reevaluates and updates the TLVs, based on professional judgment and new scientific information, but it uses no explicit risk-based or feasibility-based methodology. When the Occupational Safety and Health Act was enacted in 1970, the new Occupational Safety and Health Administration adopted existing TLVs as workplace permissible exposure limits (PELs).
(SEE ALSO: Carcinogen; Exposure Assessment; Herbicides; National Institute for Occupational Safety and Health; Occupational Safety and Health Administration; Pesticides; Regulatory Authority; Risk Assessment, Risk Management; Toxicology)
Aldridge, W. N. (1986). "The Biological Basis and Measurement of Thresholds." Annual Review of Pharmacology and Toxicology 26:39–58.
—— (1995). "Defining Thresholds in Occupational and Environmental Toxicology." Toxicology Letters 77:109–118.
American Conference of Governmental Industrial Hygienists (ACGIH) (2001). Documentation of the Threshold Limit Values and Biological Exposure Indices, 7th edition. Cincinnati, OH: Author.
Ottoboni, M. A. (1997). The Dose Makes the Poison: A Plain Language Guide to Toxicology, 2nd edition. New York: Van Nostrand Reinhold Co.