Minimal Risk Levels

Highlights

  • Minimal Risk Levels (MRLs) are a prediction of how much someone can be exposed to a chemical without detectable health risks.
  • Exposure to a chemical above the MRL values does not mean a person will have health problems.
  • Sometimes, the data on human health effects is incomplete. That affects the MRL values.
Scientist wearing lab coat and gloves pouring a dark blue chemical from one beaker into another. Other beakers sitting on the table containing chemicals, including amber colored beakers.

Background

ATSDR 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). This is in response to congressional mandate under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA).

An MRL is an estimate of the amount of a chemical a person can eat, drink, or breathe daily 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 occur. Health assessors may want to look more closely at a site where they find exposures higher than the MRLs. ATSDR works closely with the Environmental Protection Agency (EPA) at both a national and regional level. Together, they monitor sites where exposures are estimated to possibly become a health risk.

MRLs measure the length of time people are exposed to a chemical. The measurements are captured for 3 different time periods:

  • Acute (about 1 to 14 days)
  • Intermediate (from 15-364 days)
  • Chronic (exposure for more than 364 days)

Our approach

Scientists review data about the chemical or substance. This includes the following:

  • The ways that people could be exposed to the chemical.
  • How long people are exposed to the chemical.
  • The amount of the chemical and its potential health effects.
  • How old the person is when they are exposed.
  • Whether the data are from animal studies or based on human exposures.
  • The quality of the human and animal data found in the scientific literature.
    • Particularly if those studies consistently find similar health effects.

Sometimes, information about the health effects in humans may not be complete. In these cases, ATSDR may use animal studies when the health effects in animals may be similar to those in humans.

Uncertainty factors

The way the MRL is calculated can change depending on type and quality of data available. The scientific data on a hazardous substance may not be complete. Because of this, 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.
  • When we don’t know certain things about how a chemical may affect a sensitive population
  • When we do not have complete information about the chemical levels that may be associated with health effects.

Sensitive populations can include the very young or people who may have other health problems.

Process for developing MRLs at ATSDR

There are several steps to develop MRLs.

  1. 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 are reviewed by an internal ATSDR Health Effects/MRL workgroup.
    1. This includes scientific experts from many disciplines 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 findings and conclusions.
    1. They carefully examine the data and discuss the proposed MRLs.
    2. 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.
    1. The peer reviewers selected are experts in subjects related to content of Toxicological Profile.
  5. ATSDR will then respond to any comments from the peer reviewers and make any revisions that are needed.
    1. These responses to peer reviewer comments can be viewed on the Peer Review Agenda.
  6. 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.
    1. Comments are submitted through regulations.gov.
    2. The announcement that these are now available for public comment is posted in the Federal Register Notice.
    3. The new or revised MRLs are now draft MRLs.
  7. All comments are addressed, and revisions are made after the public comment period.
  8. The MRLs and the Toxicological profile they are found in are considered final and publicly posted on regulations.gov and the ATSDR website.

Resources

Print
Content Source:
Agency for Toxic Substances and Disease Registry