Minimal Risk Levels (MRLs)

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 .

ATSDR serves the public by using the best science, taking responsive public health actions, and providing trusted health information to prevent harmful exposures and diseases related to toxic substances.

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ATSDR and Minimal Risk Levels (MRLs )

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.

What is the process for developing MRLs at ATSDR ?

There are several steps to develop MRLs.

  1. ATSDR’s process for developing MRLs begins with a thorough review of the relevant scientific  literature to understand what organs in the body seem to be affected by the smallest dose of this chemical.
  2. The findings and conclusions from Step 1 are then reviewed by an internal ATSDR Health Effects/MRL workgroup. This workgroup includes scientific experts from many disciplines (toxicologists, epidemiologists, veterinarians, clinicians, and others) who examine the studies and agree on whether or not the data are sufficient to develop MRLs.
  3. After the internal ATSDR health effects MRL workgroup makes its recommendations, an Interagency MRL Workgroup meets to discuss them.  Scientists from ATSDR, Centers for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH), EPA and the National Institute of Environmental Health Sciences (NIEHS) /National Toxicology Program (NTP) are all on this workgroup. They carefully examine the data and discuss the proposed MRLs. They may accept the proposed values, revise the numbers, propose new numbers, or decide that the data are not sufficient to propose MRLs.
  4. The proposed MRLs then go through an external peer review. The peer reviewers selected are experts in subjects related to content of Toxicological Profile.  ATSDR will then respond to any comments from the peer reviewers and make any revisions that are needed. These responses to peer reviewer comments can be viewed here:
  5. After these revisions are made, the MRL, as part of a Draft for Public Comment Toxicological Profile is posted online for public comment for a set period of time, usually 90 days. Comments are submitted through regulations.govexternal icon. The announcement that these are now available for public comment is posted in the Federal Register Noticeexternal icon. The new or revised MRLs are now draft MRLs.
  6. After the public comment period, all comments are addressed, and revisions are made. Once this is completed, the MRLs and the Toxicological profile they are found in are considered final and publicly posted on and the ATSDR website.
Page last reviewed: June 4, 2018