Scientists in the Office of Research and Standards at MassDEP develop health- and science-based air guidelines - known as Ambient Air Limits (AALs) and Threshold Effect Exposure Limits (TELs) - to evaluate potential human health risks from exposures to chemicals in air. (Revised Sept 2024)
How MassDEP Develops & Uses Air Guideline Values
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.
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.
Current AALs & TELs
MassDEP updated the AALs and TELs in September 2024 to incorporate new information about the toxicity of the chemicals as described in the Methodology (see Additional Resources below). In this update MassDEP also added 141 chemicals to reflect a more complete list of the toxic chemicals potentially emitted by facilities in the Commonwealth.
MassDEP Allowable Ambient Limits (AALs) and Threshold Effect Exposure Limits (TELs) Updated 2024 (English, XLSX 66 KB)
MassDEP releases AALs and TELs to the public by adding or updating guidelines and supporting information on this web page. You may sign up to be notified when new values are published.