The Department of Environmental Protection (MassDEP) Office of Research and Standards (ORS) develops health-based  Air Guideline Values - Ambient Air Limits (AALs) and Threshold Effect Exposure Limits (TELs) - that are used to evaluate potential human health risks from exposures to chemicals in air.

These guidelines are set at concentrations intended to protect the general population, including sensitive populations such as children, from adverse health effects over a lifetime of continuous exposure.


To determine the AALs and TELs for individual chemicals, MassDEP first develops:

  • Non-Threshold Effects Exposure Limits (NTELs) based on known or suspected carcinogenic health effects. The NTEL is a concentration associated with a one in a million excess lifetime cancer risk over a lifetime of continuous exposure; and
  • Threshold Effects Exposure Limits (TELs) based on non-cancer health effects. The TEL is a concentration intended to protect the general population, including sensitive populations such as children, from adverse health effects over a lifetime of continuous exposure. TELs take into account the fact that people may be exposed to a chemical from other sources, including indoor air, food, soil and water.

MassDEP compares the NTEL and the TEL and designates the lower concentration as the AAL. Since, in general, NTELs are lower than TELs, most AALs are based on the NTEL, or risk of excess cancer. For chemicals that do not pose cancer risks, the AAL is based on the TEL, and in this case the published AAL and TEL values are the same.

Use of AALs & TELs

MassDEP uses AALs and TELs primarily in its air pollution control permitting program. The agency also uses AALs and TELs to evaluate the potential for health effects from chemicals present in ambient and indoor air.

It should be noted that exposure above an AAL or TEL does not automatically mean an individual will develop cancer or experience non-cancer health effects. However, the risk or probability of developing adverse effects increases with intensity and frequency of exposure.

MassDEP Methods for Deriving AALs & TELs

MassDEP updated its methods for deriving AALs and TELs in 2011 following a scientific peer review of the new updating methodology. MassDEP's new method makes use of existing peer reviewed air guidance levels developed by the U.S. Environmental Protection Agency (EPA), California Environmental Protection Agency (CalEPA) and other agencies. The Air Guidelines continue to consider cancer and non-cancer health effects. AALs and TELs for chemicals listed with evaluation dates of 1990 and 1995 were developed using the 1990 and 1995 methods respectively. AALs and TELs derived by earlier methods and published on this web site remain the state air guidelines until MassDEP updates them.

See Risk Assessment for additional information.

Availability of New Air Guidelines

MassDEP releases AALs and TELs to the public by adding or updating guidelines and supporting information on its Air Guideline Values web page. You may sign up to be notified when new values are published.

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