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Air Toxics Program

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Environment, Great Lakes, and Energy

Air Toxics Program

The Air Quality Division (AQD) regulates sources of air pollutants to protect human health. Heath-based screening levels are utilized in AQD's Permits to Install (PTI) regulatory program for the assessment of toxic air contaminant (TAC) emissions.


Brian Hughes, Air Quality Division

For Screening Lists and Justifications::

Doreen Lehner, Air Quality Division

Screening Levels

Michigan Air Toxics Rules Overview

  • The original "air toxics rules" were promulgated on April 17, 1992, and included Rules 230, 231, and 232. After several years of implementation of these rules, revisions were made to the air toxics rules. These revisions became effective on November 10, 1998. With these revisions, the air toxics rules now include Rules 224 through 233. These rules contain the requirements for sources that emit toxic air contaminants.

    Part 2 Rules

    • R 336.1224: T-BACT requirement for new and modified source of air toxics; exemptions
    • R 336.1225: Health-based screening level requirement for new or modified sources of air toxics
    • R 336.1226: Exemptions from the health-based screening level requirement
    • R 336.1227: Demonstration of compliance with health-based screening level
    • R 336.1228: Requirement for lower emission rate than required by T-BACT and health-based screening levels
    • R 336.1229: Methodology for determining health-based screening levels
    • R 336.1230: Informational list for health-based screening levels and T-BACT determinations
    • R 336.1231: Cancer risk assessment screening methodology
    • R 336.1232: Methodology for determining initial threshold screening level
    • R 336.1233: Methodology for determining initial threshold screening levels based on acute data
  • Any new or modified emission unit or units for which an application for a permit to install is required and which emits a toxic air contaminant. The air toxics rules do not apply to existing sources.

  • There is no list of all toxic air contaminants. The rules define toxic air contaminant as any air contaminant for which there is no national ambient air quality standard and which is or may become harmful to public health or the environment when present in the outdoor atmosphere in sufficient quantities and duration. Forty-one substances are specifically exempt from the definition of toxic air contaminant, including such things as inert gases, nuisance particulates, and substances that have relatively low toxicity.

  • There are two basic requirements of the rules. First, each source must apply the best available control technology for toxics (T-BACT). After the application of T-BACT, the emissions of the toxic air contaminant cannot result in a maximum ambient concentration that exceeds the applicable health based screening level.

  • The health based screening level for non-carcinogenic effects of a toxic air contaminant is called the Initial Threshold Screening Level (ITSL). It is determined by a number of different methods, depending upon the available toxicological data. The rules specify a hierarchy of methods for determining the ITSL. There are two health based screening levels for carcinogenic effects. These include the Initial Risk Screening Level (IRSL), which is defined as an increased cancer risk of one in one million (10-6), and the Secondary Risk Screening Level (SRSL), which is defined as an increased cancer risk of one in one hundred thousand (10-5). The IRSL applies only to the new or modified source subject to the permit application. If the applicant cannot demonstrate that the emissions of the toxic air contaminant meet the IRSL, they may choose to demonstrate compliance with the SRSL, however in this case they must include all sources of that toxic air contaminant emitted from the plant, not just the emission unit being permitted.

  • The rules allow exemption from the T-BACT requirement for processes emitting small amounts of low potency carcinogens or non-carcinogens that have relatively low toxicity. Also exempted from T-BACT are processes that meet BACT, LAER, or MACT requirements.

  • There are several exemptions from the health based screening level requirement. These include the following:

    • Emissions of toxic air contaminants that are less than 10 pounds per month and 0.14 pound per hour, provided that the toxic air contaminant is not a carcinogen or a high concern compound. The high concern toxic air contaminants include 38 chemical substances or classes of compounds specifically listed in Table 20 of the rules.
    • Processes that are regulated by a National Emission Standard for Hazardous Air Pollutants (NESHAP) as listed in Rule 226(c).
    • Emissions of hazardous air pollutants listed in Section 112(b) of the Federal Clean Air Act for which a standard has been promulgated under Section 112(f) of this act.
    • Rule 226(d) exempts emissions of toxic air contaminants from the health based screening level requirement if it can be demonstrated that the emissions will not cause or contribute to a violation of the provisions of Rule 901. Rule 901 prohibits emissions of air contaminants that alone or in reaction with other air contaminants, cause injurious effects to human health or safety, animal life, plant life or significant economic value or property. The demonstration under Rule 226(d) must be made on a case-by-case basis and include consideration of all relevant scientific information.
  • Rule 228 allows the Department to require a lower emission rate than that specified by T-BACT or the health based screening level, on a case-by-case basis if it is determined that these requirements may not provide adequate protection of human health or the environment. In making this case-by-case determination, all relevant scientific information is considered, including such things as exposure from routes of exposure other than direct inhalation, synergistic or additive effects of toxic air contaminants, and effects on the environment.