What sources are subject to the air toxics rules?

Any new or modified process for which an application for a permit to install is required and which emits a toxic air contaminant (TAC).

Which rules are considered the "air toxics rules"?

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. Some of these revisions became effective on November 10, 1998, and other revisions became effective in 2016. With these revisions, the air toxics rules now include Rules 224 through 233. These rules contain the requirements for regulated sources that emit TACs. Part 2 Rules R 336.1224 : Best available control technology (T-BACT); requirements for new and modified sources 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 : Lists for toxic air contaminants health-based screening levels, emissions rate reviews, 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

Is there a list of all toxic air contaminants?

There is no list of all TACs. Part 1 of the rules defines a TAC 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 a TAC, including such things as inert gases, nuisance particulates, and substances that have relatively low toxicity.

What do the rules require?

Each source must apply the best available control technology for toxics (T-BACT). After the application of T-BACT, the emissions of the TAC cannot result in a concentration that exceeds the applicable health-based screening level.

Are there any exemptions from the T-BACT requirement?

The rules allow exemption from the 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.

Are there any exemptions from the health based screening level requirement?

There are several exemptions from the health-based screening level requirement. These include the following: Emissions of toxic air contaminants (TACs) that are less than 10 pounds per month and 0.14 pound per hour, provided that the TAC 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 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 TACs 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.

Do the rules take into account additive effects of chemicals, effects on the environment, or exposure through indirect routes of exposure such as atmospheric deposition of pollutants and then uptake through the food chain?

Yes, the rules allow for accounting for additive effects of chemicals and other considerations like multi-pathway risk assessment.

Toxics

Contact

Brian Hughes, Air Quality Division
HughesB8@Michigan.gov
517-648-7353

For Screening Lists and Justifications:

Doreen Lehner, Air Quality Division
LehnerD@Michigan.gov
517-582-3779

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 Toxic Air Contaminants (TACs)?

A TAC is any air contaminant except pollutants with National Ambient Air Quality Standards and pollutants exempt from the TAC definition 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 man-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 from pesticides via application or residues on vegetation

TACs are one part of looking at risk. Exposure to chemicals is often linked with some level of risk of adverse effects on health, including problems such as lung 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.

For the Public

For Industry

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

The health-based screening levels are developed from research studies and set at levels to protect against health problems from breathing toxic air contaminants (TACs). The screening level for the non-carcinogenic effects of a toxic air contaminant (TAC) is called the Initial Threshold Screening Level (ITSL).

It is determined by several different methods, depending upon the available toxicological data. Michigan’s Air Pollution Control 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, and the Secondary Risk Screening Level (SRSL), which is defined as an increased cancer risk of one in one hundred thousand.
How are health-based screening levels used?

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.

The health-based screening levels are used in Michigan’s Air Pollution Control Rules. Part 2 Rules contain the air toxics rules (Rule 224 - Rule 233). These rules only apply when a company applies for an air permit. Learn more about the rules and how they apply below in the Michigan Air Toxics Rules Overview.
How do screening levels protect the public?

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 limits on the emission rates are then written into enforceable air permits. Screening levels are based on 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.

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.
How does the Air Quality Division (AQD) monitor for toxic air contaminants (TACs) in the ambient (outdoor) air?
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 to

ensure standards are met

understand pollution trends

assess community exposure

Screening Levels

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

Chemicals Under Review

AQD’s toxicologists often review existing screening levels and develop new ones to ensure the screening levels are protective. 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

Toxic Air Contaminant (TAC Lists) - Current and Historical

Visit MiEnviro Portal for the most up-to-date toxics information. You can search and filter available information through MiEnviro Portal.

To use MiEnviro Portal:

Open the MiEnviro Portal Air Toxic Chemical Search
Click in the search bar at the top of the screen to display the dropdown menu and search criteria.
Enter the chemical name or Chemical Abstracts Service (CAS) Number.
After entering the chemical name or CAS Number, click the blue "Search" button to the right of the search bar.
For more information, click the "Download Documents" button that corresponds to the chemical.

The list below will not be updated after December 10, 2024, but are available for reference:

Screening Levels Listed by CAS Number (current through 12/10/2024)
Full List of Screening Levels and Justifications Sorted by CAS Number (current through 12/10/2024)

Open for Public Comment
Screening Level Justifications Open for Public Comment: Information on how these health-based screening levels were derived is contained in the justification memos. Memos can be accessed by downloading documents in the MiEnviro Portal Air Toxic Chemical Search.

Use the "Add Comment" tab in MiEnviro or send an e-mail to EGLE-AQD-AirToxicsPublicComments@Michigan.gov 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.

When does an air permit need to include an air toxics evaluation?
Michigan’s Air Pollution Control Part 2 Rules contains the air toxics rules (Rule 224 - Rule 233) which only apply when a company is modifying an air permit or applying for a new air permit. Learn more about the rules and how they apply below in the Michigan Air Toxics Rules Overview.

Learn more about how to complete your air permit application by engaging with a series of short videos that walk you through the process. This series has a variety of resources, including:

Module 5.3: Additional Supporting Information - Air Toxics Overview (short version)

Module 7: Air Toxics Evaluation in Detail (long version)

Demonstrating Compliance with Rule 225 - Rule 227 Spreadsheet

Guidelines for Conducting a Rule 224 Best available control technology for toxics (T-BACT) Analysis

Guidance for Toxic Air Contaminants-Demonstrating Compliance with Rule 225

Screening Levels

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

Toxic Air Contaminant (TAC) Lists - Current and Historical

Visit MiEnviro Portal for the most up-to-date toxics information. You can search and filter available information through MiEnviro Portal.

To use MiEnviro Portal:

Open the MiEnviro Portal Air Toxic Chemical Search
Click the search bar at the top of the screen to display the dropdown menu and search criteria.
Enter the chemical name or Chemical Abstracts Service (CAS) Number.
After entering the chemical name or CAS Number, click the blue "Search" button to the right of the search bar.
Click the "Download Documents" button that corresponds to the chemical.

The lists below will not be updated after December 10, 2024, but are available for reference:

Screening Levels Listed by CAS Number (current through 12/10/2024)
Full List of Screening Levels and Justifications Sorted by CAS Number (current through 12/10/2024)

Chemicals Under Review

Our toxicologists often review existing screening levels and develop new ones for TACs to ensure the screening levels are protective. 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

Open for Public Comment
Screening Level Justifications Open for Public Comment: Information on how these health-based screening levels were derived is contained in the justification memos that can be accessed by downloading documents in the MiEnviro Portal Air Toxic Chemical Search.

Use the "Add Comment" in MiEnviro or send an e-mail to EGLE-AQD-AirToxicsPublicComments@Michigan.gov 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

What sources are subject to the air toxics rules?

Any new or modified process for which an application for a permit to install is required and which emits a toxic air contaminant (TAC).
Which rules are considered the "air toxics rules"?
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. Some of these revisions became effective on November 10, 1998, and other revisions became effective in 2016. With these revisions, the air toxics rules now include Rules 224 through 233. These rules contain the requirements for regulated sources that emit TACs.

Part 2 Rules

R 336.1224: Best available control technology (T-BACT); requirements for new and modified sources 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: Lists for toxic air contaminants health-based screening levels, emissions rate reviews, 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
Is there a list of all toxic air contaminants?

There is no list of all TACs. Part 1 of the rules defines a TAC 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 a TAC, including such things as inert gases, nuisance particulates, and substances that have relatively low toxicity.
What do the rules require?

Each source must apply the best available control technology for toxics (T-BACT). After the application of T-BACT, the emissions of the TAC cannot result in a concentration that exceeds the applicable health-based screening level.
Are there any exemptions from the T-BACT requirement?

The rules allow exemption from the 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.
Are there any exemptions from the health based screening level requirement?
There are several exemptions from the health-based screening level requirement. These include the following:

Emissions of toxic air contaminants (TACs) that are less than 10 pounds per month and 0.14 pound per hour, provided that the TAC 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 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 TACs 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.
Do the rules take into account additive effects of chemicals, effects on the environment, or exposure through indirect routes of exposure such as atmospheric deposition of pollutants and then uptake through the food chain?
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 some chemical mixtures, like gasoline.

Regarding sensitivity indicators, Michigan air toxics rules can consider sensitive subpopulations in the 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 polychlorinated biphenyls, 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 TACs, 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 New Source Review permitting contains health-based screening levels for some TACs based on chemical mixtures, potency factors, and hazard indexes.

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

Chemicals Under Review

Toxic Air Contaminant (TAC Lists) - Current and Historical

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

Toxic Air Contaminant (TAC) Lists - Current and Historical

Chemicals Under Review

Michigan Air Toxics Rules Overview

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