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Air Toxics and Mercury

Air toxics, also called hazardous air pollutants (HAPs), are substances either known or suspected to cause cancer or other serious health problems, including damage to the immune, neurological, reproductive and respiratory systems. These pollutants are emitted by sources such as vehicles, factories and power plants. They may also be found in building materials and cleaning solvents.

Mercury is a persistent, bioaccumulative toxic pollutant that affects human health and wildlife. Although it is found naturally in the environment, problems arise from its release from man-made products such as thermometers, lamps and switches, and through burning coal. Once mercury is released into the environment, it can convert to a toxic compound called methylmercury that can contaminate fish and harm people who eat fish. To control mercury emissions from coal combustion, Wisconsin promulgated a state mercury rule for coal-fired utility plants in 2004 and revised the rule in 2008.

Other hazardous or toxic air pollutants are regulated both for their short-term and long-term health effects. Emissions standards have been developed at both the federal and state levels. The federal Clean Air Act regulates air toxics through National Emission Standards for Hazardous Air Pollutants (NESHAP) – unlike criteria pollutants such as sulfur dioxide and ozone, HAPs are not regulated through National Ambient Air Quality Standards (NAAQS). In addition, Wisconsin regulates several hundred toxic air pollutants under ch. NR 445, Wis. Adm. Code [PDF exit DNR].

NESHAPs are rules that apply to specific types of equipment or industries that emit hazardous air pollutants. NESHAPs control HAPs through maximum achievable control technology (MACT) standards, which include control requirements for sources at facilities that have a potential to emit HAPs over the major source thresholds. The U.S. Environmental Protection Agency issued a guidance memo in 1995, referred to as the "Once-In-Always-In" policy, which offers guidance on MACT standard applicability. This guidance was reversed in an EPA guidance memo dated Jan. 25, 2018 [PDF exit DNR].

The following fact sheet provides more information on EPA's "Once-In-Always-In" policy and EPA's recent guidance memo.

Federal standards

National Emission Standards for Hazardous Air Pollutants (NESHAPs)

The Clean Air Act requires the U.S. Environmental Protection Agency to establish national emissions standards for all categories and subcategories of major sources of federally listed Hazardous Air Pollutants (HAPs) [exit DNR]. These standards are called Maximum Achievable Control Technology (MACT) standards and are developed based on the emissions levels achieved by the facilities with the lowest emissions in a source category. More stringent standards may also be established to address residual economic, environmental or health risks.

In addition, the Clean Air Act requires the EPA to establish national standards for smaller sources of emissions, called area sources. These standards could be MACT or Generally Available Control Technology (GACT) standards.

The EPA publishes NESHAPs in the Code of Federal Regulations:

You can find more information through our Frequently asked questions.

State rule

Wisconsin air toxics rule

Wisconsin's Air Toxics rule, Ch. NR 445, Wis. Adm. Code [PDF exit DNR], was first promulgated in 1988 and revised in 2004.

For general information about the rule, including information about applicability, control requirements and compliance, see Wisconsin Air Toxics Rule (AM-405) [PDF].

For additional information about specific provisions of the air toxics rule, see these fact sheets:


Air toxics emissions standards and limits for stationary sources are outlined in facilities' air permits. For more information about permitting, see our Air Permits pages.

Annual Air Emissions Inventory

In order to support a determination of compliance with s. NR 445.07, Wis. Adm. Code, the DNR’s system for annual air emissions inventory submittal, the Air Reporting System (ARS), has been updated to perform a quality assurance review between emissions and s. NR 445.08(2)(a), Wis. Adm. Code. Starting with the 2024 annual air emissions inventory reporting season for calendar year 2023, the following stack parameters are required to submit an annual air emissions inventory to the DNR:

  • Stack height
  • Exhaust gas discharge direction (vertical or non-vertical)
    • Stacks oriented within 10 degrees of upward vertical are considered to have vertical discharge. Stacks oriented > 10 degrees of upward vertical are considered horizontal (non-vertical).
  • Stack exhaust obstruction status (yes, the stack is obstructed; no, the stack is not obstructed)
    • If no portion of the vertical flow out of the exhaust stream (stack) is impeded while the process is in operation, the stack is considered unobstructed.

Stack height, exhaust gas discharge direction and stack exhaust obstruction status values entered in ARS will carry forward from year to year. As long as stack parameters do not change, this data entry is only required once.

The memorandum dated October 20, 2005 (AM19-0029) provides details of a method of demonstrating compliance when a facility has obstructed or non-vertical stacks or non-exempt potential fugitive emission sources (such as those from a bulk gasoline terminal) of HAPs. The method requires calculation of total non-exempt potential emissions of a HAP (or air toxic) for the entire facility and comparing the result to the corresponding value listed in Table A of NR 445 for the pollutant and the particular stack height category. For additional information, review the NR 445 fact sheet.

Coal-fired utility rule

Wisconsin coal-fired utility rule

Wisconsin's rule to limit mercury emissions from coal-fired electric utility plants took effect in 2004 and was revised in 2008.

  • 40% reduction by 2010: By Jan. 1, 2010, large coal-fired power plants must reduce mercury emissions by 40%.
  • 90% reduction by 2015: By Jan. 1, 2015, large coal-fired power plants must reduce mercury emissions by 90% or limit the concentration of mercury emissions to 0.0080 pounds of mercury per gigawatt-hour.
  • Multi-pollutant option: Large coal-fired power plants can choose a multi-pollutant alternative. This option requires affected power plants to achieve nitrogen oxides (NOx) and sulfur dioxide (SO2) reductions beyond current federal and state regulations. Under the multi-pollutant option, an additional six years is allowed to achieve the 90% mercury emission reduction standard. Affected power plants must meet 70% emission reduction or emission concentration of 0.0190 pounds mercury per gigawatt-hour by Jan. 1, 2015, 80% emission reduction or emission concentration of 0.0130 pounds of mercury per gigawatt-hour by Jan. 1, 2018, and 90% mercury emission reduction or emission concentration limit of 0.0080 pounds of mercury per gigawatt-hour by Jan. 1, 2021.

Chemical spreadsheet tool

Combined chemical table spreadsheet tool for Wisconsin air toxics rule

The DNR has prepared an Excel spreadsheet [XLS] that provides information about the pollutants covered by Wisconsin's Air Toxics rule (ch. NR 445, Wis. Adm. Code), permit rules (ch. NR 407), and emissions inventory reporting rules (ch. NR 438). The table contains reporting levels for these pollutants, as well as emissions thresholds, standards and control requirements. In addition to combining information from several rules, the spreadsheet offers useful search and sorting functions.


In case of conflict, the tables in the published rule (available at Wisconsin Legislative Documents [exit DNR]) supersede the table presented here. Every effort has been made to correct errors, but it is possible that some remain. 

Guidance for using the Combined Chemical Table

The table uses filters located at the headings of each field to retrieve data of interest. The filters can be used as simple query tools. Click on the black downward pointing arrows at the bottom-right of the column headers to see a list of choices.

When you click on one of the choices from the list, only that choice is shown in the field and the list is filtered. You can use the filter on more than one field at a time. Caution: If you cannot find a chemical that you know is there, look for any filter arrows that are blue, then click on the arrow and choose "all". This will turn the filter off for that field (the filter arrow will turn black).

Column A: Using this column you can "check" for air pollutants of interest. For example, you could put X's in this column to mark which chemicals are emitted by a facility. You could also use different letters or numbers to represent parts of a facility or organization subdivisions that have those chemicals. You would then use the filtering feature of Excel to filter the list for the chemicals of interest.

Column B: Chemical name (note: common synonyms are also listed in some cases, so the same CAS number may appear more than once).

Column C: Alphabetical sort column (use this column to sort chemicals alphabetically).

Column D: Chemical Abstracts Service number (CAS number). Some CAS numbers represent multiple compounds and there may be more than one CAS number that fits the category. Many metals are not listed individually, for example: "Cobalt, elemental, and inorganic compounds, as Co". For other chemicals there is no CAS number listed because the mixture is too complex to attribute a single CAS number to the listing, for example, see "glycol ethers", "coke oven emissions" or "fluorides").

Column E: CAS number with dashes removed. Used for sorting and searching purposes.

Columns F, G, and H tell you what general types of health effects (non-cancer and cancer) are regulated for that chemical.

Column I: Use this column to filter out chemicals that are not listed in NR 445. This is done by clicking on the black downward-pointing arrow (list box button) at the bottom-right side of the column header that says "Chems not in NR 445, but in permits and inventory" then choose "blanks". Any chemical that is not in NR 445 is filtered out and only the NR 445 chemicals remain. Note the list box button changes to the color blue and this lets you know that this field is filtered. Remember to turn this filter off when you are done by clicking on the black downward-pointing arrow (list box button) again and choosing "all", which is at the top of the list box. If the list box button turns back to black, then the filter is off.

Column J shows how many health effects are regulated for each chemical. For example, some chemicals have a 24-hour acute non-cancer health limit, are also carcinogenic, and have a U.S. EPA reference concentration limit (chronic non-cancer) as well. Thus, they have "3" in this column. For chemicals not in NR 445, they will have a "0" in this column.

Column K provides the NR 407 permit inclusion level in pounds per year, as revised in the NR 445 rule.

Column L provides the NR 438 emissions inventory reporting level in pounds per year.

Columns M-W are columns that are used for quality assurance purposes and are not important for the general user.

Column X is a column for filtering the list for new chemicals to the NR 445 list.

Column Y is for filtering chemicals that are in listed in the Clean Air Act Amendments of 1990, Section 112(b).

Column Z lists the NR 445 table where each chemical is listed. If it says NR 407 and NR 438 only, it means that the chemical is not listed in NR 445.

Column AA to column BM: The remaining columns provide the thresholds and standards for NR 445. If there is no threshold or standard listed under a given heading, the threshold or standard is not applicable. The standards are in units of micrograms per cubic meter of air (ug/m3). The thresholds are either in units of pounds per hour (for non-carcinogens with acute health effects), or in pounds per year (for non-carcinogens with chronic health effects and for carcinogens). Columns BC to BF are for carcinogens and list the EPA and IARC weight of evidence, the unit risk factors (a measure of carcinogenic potency) and the source of the potency estimate.

Ag waste BMPs

Agricultural waste best management practices

This report identifies and recommends suitable best management practices (BMPs) for the reduction of emissions of hazardous air pollutants from various types of livestock operations in Wisconsin.

All links are PDFs.

  • Cover Letter
  • DNR response to Ag Waste BMP Final Report
  • Final Report
  • Appendices
    • Appendix A - BMPs for Animal Nutrition and Feed Management
    • Appendix B - BMPs for Animal Housing
    • Appendix C - BMPs for Manure Storage
    • Appendix D - BMPs for Open Lots and Corrals
    • Appendix E - BMPs for Pastures
    • Appendix F - BMPs for Land Applications
    • Appendix G - Letter from Air and Waste Division Administrator Al Shea to Advisory Group and Ground Rules
    • Appendix H:
      • File 1 = Agriculture Counts - Bob Battaglia, Wisconsin Agricultural Statistics Service
      • File 2 = Conservation Innovation Grant Livestock Air Monitoring and Odor Project - Steve Struss, DATCP
      • File 3 = CIG Air Monitoring Project - David Grande, DNR
      • File 4 = National Air Emissions Monitoring Study - Bill Schrock, USEPA
      • File 5 = Discussion of Potential Rules Approaches - Andy Stewart, DNR
      • File 6 = Livestock Siting Odor Background and BMPs - Steve Struss, DATCP
      • File 7 = Ambient Air Quality and Feedlots - George Schwint, MPCA
      • File 8 = Air Quality and Livestock: An Iowa Perspective - Jay Harmon, Iowa State University
      • File 9 = Air Control Technologies for Animal Production Systems - Larry Jacobson, University of Minnesota
      • File 10 = Toxicity and Exposures Associated with Air Emissions from Concentrated Animal Feedlot Operations - Robert Thiboldeaux, Wisconsin Bureau of Environmental and Occupational Health
      • File 11 = Atmospheric Dispersion Modeling, John Roth, DNR
    • Appendix I - Summary of the Process for Evaluating Practices
    • Appendix J - Cover Memo:
      • Best Management Practices to Mitigate Air Quality
      • Impacts from Animal Agriculture
      • Air Mitigation Measures and Effectiveness Summary Table
      • Air Mitigation Measures and Effectiveness Detailed Table
    • Appendix K - Manure Gas Safety: Review of Practices and Recommendations for Wisconsin Livestock Farms
    • Appendix L - Final Report on Wisconsin's Dairy and Livestock Odor and Air Emission Project
    • Appendix M - 2010 Letter from Secretary Nilsestuen to the DATCP Board regarding ATCP 51 Four-Year Review
    • Appendix N - Poultry Best Management Practices Document
    • Appendix O - Sources Cited, Reviewed or Consulted
    • Appendix P - Biographies of Agricultural Waste Beneficial Management Practice Advisory Group Members
    • Appendix Q - Glossary of Terms
    • Appendix R - Meeting Agendas and Summary Notes
    • Appendix S - Draft Report Comment Summary/Response