Understanding Air Toxics Evaluation

For most regulated entities, air toxics are first encountered under the Kentucky Division for Air Quality’s (DAQ) permitting program; however, air toxics play an important role in statewide air quality and can significantly impact local air quality. Air toxics are those air pollutants that are known or suspected to cause cancer or other serious health effects. Air toxics can come from natural sources (e.g., radon gas coming from the ground) or man-made sources such as motor vehicles and industrial processes.

Many regulated entities are subject to federal air toxic standards through the National Emission Standards for Hazardous Air Pollutants(NESHAP) regulations, where control technologies are specified based on the industry category and type of air toxic emitted. In the absence of a NESHAP for a specific pollutant or sector, Kentucky DAQ regulates air toxics under state regulations 401 KAR 63:020 and 401 KAR 63:021.

Questions regarding air toxics usually arise when a source is applying for or modifying a permit and is faced with choosing additional controls or limits as a result of air toxics evaluations during the application review period. Often sources don’t understand the air toxics evaluation process and perceive it as a “black box”; however the underlying process is relatively simple to understand.

  1. Air toxics used for evaluation are determined from  emission estimates in a source’s air permit application
  2. A screening model is run for the air toxics to assess the human health impacts of the emissions
  3. Modeling results are compared to regional screening levels (RSL).
  4. If a source’s modeling results exceeds the threshold concentrations, a more refined model is conducted.
  5. After further modeling, the source may “pass” the air toxic evaluation or be required to demonstrate how emissions will be reduced so as to not pose an unacceptable risk.
  6. DAQ may require a full risk assessment as part of this demonstration.

When faced with an air toxic evaluation, a source may expect questions from the assigned permit reviewer that will help determine inputs for the models such as exact stack or exhaust locations, nearest receptors such as houses, schools, businesses, actual operating hours and information on control devices.

Understanding and being prepared for the air toxic evaluation process can help a source work cooperatively with the permit review staff as the application is processing. For more information, please visit the DAQ Air Toxics Program page.