The Agency for Toxic Substances and Disease Registry (ATSDR) develops health-based values to protect the health of the general population. Those charged with protecting public health, including ATSDR, as well as state and local health departments, use this information to help identify populations, communities, and individuals that could potentially develop health problems from exposures in the environment.

The ATSDR, in response to congressional mandate under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), develops comparison values to help identify chemicals that may be of concern at hazardous waste sites. One type of these values is called minimal risk levels (MRLs).

An MRL is an estimate of the amount of a chemical a person can eat, drink, or breathe each day without a detectable risk to health. MRLs are developed for health effects other than cancer.

If someone is exposed to an amount above the MRLs, it does not mean that health problems will happen. When health assessors find exposures higher than the MRLs, it means that they may want to look more closely at a site. ATSDR works closely with the Environmental Protection Agency (EPA) at both a national and regional level at sites where exposures are estimated to exceed health-based values such as MRLs.

MRLs can be made for 3 different time periods [the length of time people are exposed to the chemical: acute (about 1 to 14 days), intermediate (from 15-364 days), and chronic (exposure for more than 364 days)].

How are MRLs calculated ?

Scientists review data about the chemical or substance, including:

  • The ways that people could be exposed to the chemical (eating? drinking? breathing? touching?);
  • How long people are exposed to the chemical;
  • The concentration of the chemical and its potential health effects;
  • How old the person is when they are exposed (an infant or an adult?);
  • Whether the data are from animal studies or based on human exposures. Sometimes, information about the health effects in humans may not be complete. In these cases, scientists at ATSDR may use animal studies when the health effects in animals may be similar to the health effects in humans;
  • The quality of the human and animal data found in the scientific literature, and if those studies consistently find similar health effects.

The way the MRL is calculated can change depending on type and quality of data available. Because the scientific data on a hazardous substance may not be complete, uncertainty factors can be applied as part of the MRL calculations.

Uncertainty factors help us account for differences between health effects in humans and animals; or when we don’t know certain things about how a chemical may affect a sensitive population (for example, the very young, or people who may have other health problems); or when we do not have complete information about the chemical levels that may be associated with health effects.

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