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Air Toxics Program

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Environment, Great Lakes, and Energy

Air Toxics Program


Brian Hughes, Air Quality Division

For Screening Lists and Justifications:

Doreen Lehner, Air Quality Division

The Air Quality Division (AQD) regulates sources of air pollutants to protect human health. AQD's Toxicologists develop health-based screening levels for toxic air contaminants (TACs). Health-based screening levels are used in air permitting under the Permits to Install regulatory program for the assessment of TAC emissions. Under Michigan's air rules, the amount of TAC emissions cannot result in impacts more than the screening levels.

What are Air Toxic Contaminants (TACs)?

A TAC is any air contaminant for which there is no national ambient air quality standard and is not exempt from the TAC definition in the air rules as defined in Part 1 Rule 120(f). During air permitting, permit engineers and toxicologists work together to ensure releases of TACs meet any screening levels that apply.

Looking at Exposure Risk

Normally, people are exposed to many chemicals from different sources every day in food, water, soil, and air -- both human-made and naturally occurring.  For example:


  • Ingestion of naturally occurring chemicals in food and food additives
  • Inhalation of emissions from cars, wildfires, and industrial sources
  • Drinking water containing natural minerals and by-products from disinfection
  • Chemicals from soil that volatilize (become vapors)
  • Dermal or skin exposure to pesticides via application or residues on vegetation
  • Inhalation from chemicals in soil that can become airborne

Toxic air contaminants or TACs, are one part of looking at risk. Exposure to chemicals is linked with some level of risk of adverse effects on health, which can include problems such as irritation and lung disease, as well as cancer. The body can generally remove harmful chemicals, but sometimes the body may not be able to get rid of a chemical before it causes harm. As the amount of chemicals a person is exposed to increases, so does the likelihood of harm.

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For the Public

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For Industry

  • Michigan’s Air Pollution Control Part 2 Rules contains the air toxics rules (Rule 224 - Rule 233). These rules only apply when a company is applying for an air permit. Learn more about the rules and how they apply below in the Michigan Air Toxics Rules Overview.

  • Although TACs are released from a variety of sources, we only have the authority to regulate emissions from industrial sources, such as power plants and manufacturing facilities. The federal government has authority over most mobile sources like cars, trucks, buses, and planes. We currently use screening levels (SLs) to evaluate exposures to airborne TACs through air permitting and to interpret air monitoring results.

    To protect the public, our toxicologists develop or adopt health-based screening levels for the air permitting process to determine the maximum an industrial process may release. The emission rates, or limits on the emission rates are then written into enforceable air permits. Screening levels are scientifically sound assessments of a chemical's potential for adverse health effects. Screening levels are designed to limit a person’s risk of adverse health effects, including cancer.

    Two screening levels may be developed for a TAC. For carcinogens, these are called risk levels. For noncarcinogens, we look at hazard levels. The risk levels are called the initial risk screening level (IRSL) and the secondary risk screening level (SRSL). We refer to the hazard level as the initial threshold screening level (ITSL). AQD’s rules were developed so the initial risk screening level and secondary risk screening levels could be used together for risk assessment of aggregate exposures. Exposure to a chemical below these screening levels are not expected to result in a public health concern.

    To apply these screening levels, we employ staff that use computer models to look at potential outdoor air concentrations of TACs based on a worst-case operating scenario from information provided in a permit application.  Comparing to the screening levels, the computer model is used to evaluate the acceptability of what a company wants to release into the air at its full operating capacity.   If the modeled concentrations cannot meet the screening levels, the permit cannot be granted because it would not be meeting air quality rules and regulations.

  • We evaluate measurements of TACs in ambient or outdoor air through air monitoring sites located throughout the state. Not every monitoring site measures air toxics. These measured chemical concentrations are evaluated and are designed to help us focus our resources. Although the evaluations may present theoretical estimates of health risks, they are not designed as comprehensive assessments of individual health risks. This information is also shared with the United States Environmental Protection Agency (EPA) for use in their Air Toxics Screening Assessment (AirToxScreen).

    AirToxScreen is EPA's screening tool to provide communities with information about health risks from air toxics. AirToxScreen is part of EPA's approach to air toxics and provides data and risk analyses on an annual basis, helping state, local and tribal air agencies, EPA, and the public more easily identify existing and emerging air toxics issues.

  • Screening Levels are designed to protect against health problems like developing cancer or breathing problems.

    What are the health-based screening levels and how are they determined?

    The health-based screening level for the non-carcinogenic effects of a toxic air contaminant is called the Initial Threshold Screening Level (ITSL). It is determined by several different methods, depending upon the available toxicological data. The rules detail a specific list 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.

    Chemicals Under Review

    AQD’s toxicologists often review existing screening levels and develop new ones for TACS to ensure the screening levels are protective.  The chemicals under review are available and updated regularly.  An example of the process is included below.

    Update Review Identify Develop Evaluate
    Update screening levels with emerging science Review scientific literature Identify key studies Develop screening levels from study results Evaluate exposures using screening levels

    Lists of Existing TACs

    The four bulleted links below are different ways you can use and access the list of air toxic screening levels.

    You can also use the query tools below to search for different TACs and footnote information.


    • Screening Level Justifications Open for Public Comment: Information on how these health-based screening levels were derived is contained in the AQD justification memos that can be accessed in the full list in the "Justification & Responses" column.
    • Use the "Send Comment" links or send an e-mail to with the subject line "Air Toxics Screening Level Justifications for" with the chemical in question.
    • Documents that end in "RTC.pdf" are the Response to Comments.

Michigan Air Toxics Rules Overview

  • 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.

  • 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
  • There is no list of all toxic air contaminants. Part 1 of the rules defines 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.

  • 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 rules allow exemption from the Best Available Control Technology for Toxics (T-BACT) requirement for processes with limited emissions. Also exempted from T-BACT are processes that meet BACT, Lowest Achievable Emission Rate(LAER), or Maximum Achievable Control Technology(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.
    • TACs and associated emission units 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.
  • Yes, the rules allow for accounting for additive effects of chemicals and other considerations like multi-pathway risk assessment.

    • With the use of screening levels under the authority of the air toxics rules, cumulative risk assessments have also been performed on chemical mixtures, like gasoline.
    • Regarding sensitivity indicators, Michigan air toxics rules can consider sensitive subpopulations in the derivation of health-based screening levels for single chemicals or chemical mixtures.
    • Under Rule 225(6), cumulative risk assessments have been performed on TACs that have the same toxicological health risks (e.g., dioxin and dioxin-like PCBs, polycyclic aromatic hydrocarbons, and certain classes of petroleum distillates).
    • Under the authority of Rule 226(d), background levels have been incorporated into aggregate exposure risk analysis for a single pollutant.
    • 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. Under the authority of Rule 228, for some persistent, bioaccumulative, and toxic substances, such as mercury, background levels are also incorporated into the risk analysis for multi-media exposure pathways.
    • Under Rule 229(2)(b), when “based on toxicological grounds and supported by the scientific data,” TACs are evaluated using cumulative risk assessment. To this end, the TAC list used in NSR permitting contains some health-based screening levels for TACs based on chemical mixtures, potency factors, and hazard indexes.

Links of Interest

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Other Air Toxics Websites