5 CCR 1001-34
DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT REGULATION NUMBER 30 TOXIC AIR CONTAMINANTS 5 CCR 1001-34 [Editor’s Notes follow the text of the rules at the end of this CCR Document.] _________________________________________________________________________ Outline of Regulation PART A General Provisions for Toxic Air Contaminants PART B Priority Toxic Air Contaminants PART C Statements of Basis, Specific Statutory Authority and Purpose Appendices, by Part _________________________________________________________________________ Pursuant to Colorado Revised Statutes section 24-4-103(12.5), any materials incorporated by reference are available for public inspection during normal business hours, or copies may be obtained at a reasonable cost from the Air Quality Control Commission, 4300 Cherry Creek Drive South, Denver, Colorado 80246-1530. Any federal material incorporated by reference is also available through the United States Government Printing Office, online at www.govinfo.gov. Materials incorporated by reference are those editions in existence as of the date indicated and do not include any later amendments. PART A General Provisions for Toxic Air Contaminants I. General Provisions I.A. Definitions I.A.1. “Hazardous air pollutant” (HAP) has the meaning provided in section 25-7- 103(13), C.R.S.
I.A.5. “Priority toxic air contaminant” (PTAC) has the meaning provided in section 25-7- 109.5(1)(f), C.R.S.
I.A.6. “Reference concentration” (RfC) means an estimate (with uncertainty spanning perhaps an order of magnitude) of a continuous inhalation exposure concentration to a specific toxic air contaminant that is likely to be without risk of deleterious effects to people (including sensitive subgroups) during a lifetime of exposure to that individual air contaminant.
PART B Priority Toxic Air Contaminants I.A. As of January 17, 2025, the Commission identifies the following five priority toxic air contaminants, as also reflected in Appendix A.
II. Chronic Health-Protective Benchmarks II.A. The Commission proposes the chronic health-protective benchmarks in Appendix B for each PTAC identified in Appendix A. The chronic health-protective benchmarks will take effect upon General Assembly approval.
PART C Statements of Basis, Specific Statutory Authority and Purpose I. Adopted: January 17, 2025 This Statement of Basis, Specific Statutory Authority, and Purpose complied with the requirements of the Colorado Administrative Procedure Act § 24-4-103(4), C.R.S., the Colorado Air Pollution Prevention and Control Act §§ 25-7-110, -110.5, and -110.8 C.R.S. (the State Air Act), and the Air Quality Control Commission’s (Commission) Procedural Rules, 5 Code Colo. Reg. § 1001-1. Basis In 2022, the General Assembly adopted House Bill (HB) 22-1244 (Public Protections from Toxic Air Contaminants), now codified primarily in § 25-7-109.5, C.R.S. § 25-7-109.5(6)(a)(I), C.R.S. directs the Commission to adopt rules that identify up to five priority toxic air contaminants (PTACs) by no later than April 30, 2025.
The Commission adopted a new Regulation Number 30 to meet the statutory directive at § 25-7- 109.5(6)(a)(I), C.R.S.
Specific Statutory Authority The State Air Act, specifically § 25-7-105(1), C.R.S., directs the Commission to promulgate such rules and regulations as are consistent with the legislative declaration set forth in § 25-7-102, C.R.S. and that are necessary for the proper implementation and administration of the State Air Act. § 25-7-109.5(6)(a)(I), C.R.S. directs the Commission to, by no later than April 30, 2025, identify up to five PTACs, considering:
I. Existing data concerning toxic air contaminants gathered through the Air Pollution Control Division (Division) Monitoring programs;
II. Data reported to the Division concerning emissions of toxic air pollutants;
III. Data reported to the federal toxics release inventory pursuant to 42 U.S.C. § 11023 and data prepared by the federal environmental protection agency's (EPA) air toxics screening assessment (AirToxScreens) program;
IV. Any other relevant data submitted to the Commission during the rule-making process concerning the amount of emissions and concentrations of toxic air contaminants in the ambient air of the state, including data collected through community-led monitoring programs; and V. Input from the scientific community.
Purpose As directed in § 25-7-109.5(6)(a)(I), C.R.S., the Commission adopted a new Regulation Number 30, which identifies five PTACs.
Regulation Number 30 Regulatory Language and Outline In Part A, General Provisions for Toxic Air Contaminants, the Commission adopted necessary definitions. In Part B, Identification of Priority Toxic Air Contaminants, Section 1, the Commission identified five PTACs.
Prioritization Framework The Commission adopted the five PTACs based on a prioritization framework developed by Colorado Department of Public Health and Environment (CDPHE) staff. This prioritization framework involved input from a scientific community technical working group, feedback from stakeholders, and months of research and data analysis, consistent with the requirements set forth in § 25-7-109.5(6)(a)(I), C.R.S. CDPHE staff collected data on statewide toxic air contaminants (TACs), following requirements in § 25-7- 109.5(6)(a)(I), C.R.S. These data sources included existing data concerning toxic air pollutants gathered through Division monitoring programs, data reported to the Division concerning emission of toxic air pollutants, data reported to or prepared by the EPA for the Toxic Release Inventory (TRI) and EPA’s AirToxScreen program, data collected through community-led monitoring programs, and input from the scientific community. These data sources were supplemented with additional monitoring and modeled data suggested by the scientific community technical working group, and data from the first annual reporting cycle of TAC that was required under § 25-7-109.5(4)(a), C.R.S. Using these data to understand the prevalence of different TACs in Colorado, staff then estimated cancer and non-cancer health risks using pollutant-specific toxicity values. These toxicity values were adjusted slightly during the rulemaking process based on updates that EPA made to its regional screening level (RSL) table in November 2024. Toxicity values are numerical indicators of the potential of a chemical to cause harmful effects and are specific to either cancer or non-cancer health impacts. Cancer and non- cancer health impacts are analyzed separately due to differences in dose-response modeling. Chemicals that cause non-cancer impacts are generally considered to have a threshold below which there are no health effects. By contrast, even small exposures to chemicals that cause cancer are thought to increase a person’s cancer risk.
Non-cancer health impacts are expressed as a Hazard Quotient (HQ). A HQ below 1 indicates that negative health effects are not expected. If the HQ exceeds 1, adverse health effects may be more likely to occur. Cancer health impacts i.e., the increased potential to develop cancer–is expressed as the increased risk of cancer per 1 million people.
For pollutants with adequate monitored or modeled data from the sources described above, the Division calculated screening level risk estimates for both cancer and non-cancer health impacts using the pollutant-specific toxicity values. This analysis showed a measure of health risk based on modeled or monitored ambient concentrations of certain TACs.
CDPHE staff then assigned groupings to TACs using a tiered health risk approach. For the purposes of prioritization, CDPHE used a cancer screening risk level of greater than or equal to 1 in 1 million excess cancer cases. For non-cancer health impacts, CDPHE used a screening risk level of a HQ of greater than or equal to 0.1. The threshold of a HQ of 0.1 is commonly used during screening when multiple chemicals are present. Though negative health impacts are not expected for a single chemical at an HQ of 0.1, it is used in screening in order to account for exposures to multiple chemicals that could act together to cause negative health impacts.
Once the pollutants meeting or exceeding the screening levels for modeled or monitored data were identified, staff considered additional factors in analyzing those pollutants. The Division compared the existing monitored or modeled data from the list of approximately 40 pollutants to the various reporting program datasets it had available (e.g., APEN, National Emissions Inventory (NEI) and TRI). The emissions reported in each dataset were weighted by a relative toxicity factor from the EPA Risk Screening Environmental Indicator (RSEI) model. These toxicity-weighted emissions were ranked from highest to lowest. All of the top 11 toxicity-weighted pollutants in each dataset were also identified in the list of 40 pollutants screened in the aforementioned risk thresholds, further supporting the potential prioritization of pollutants on that list.
For modeled risk, staff evaluated the predominant source of each TAC that fell into this category. TACs predominantly from sources that the Commission has authority to regulate were prioritized over TACs predominantly resulting from sources outside the scope of Commission rules (e.g., biogenic sources and wildfires). Separately, staff limited its assessment for modeled risk to non-mobile sources. This was done to narrow the list of potential TACs to no more than five PTACs, consistent with the statutory direction. Staff determined that excluding mobile source emissions was reasonable because § 25-7-109.5(7)(b)(II), C.R.S. specifically directs the Commissions to consider stationary source emissions in developing emission control regulations for PTACs and because § 25-7-109.5(8)(a), C.R.S. specifically directs the Division to assess its needs to administer a PTAC stationary source permitting program. Additionally, Colorado is currently pursuing several incentive and regulatory programs to reduce emissions from mobile sources, such as state enterprises created under Senate Bill 21-260, including the Clean Fleet Enterprise, Nonattainment Area Pollution Mitigation Enterprise, Clean Transit Enterprise and Community Access Enterprise, the Colorado Clean Cars Program and the Advanced Clean Trucks and Heavy-Duty Low NOx rules. These mobile source programs are anticipated to begin achieving significant reductions of TACs as a co-benefit. These modeled risks were then ranked from highest to lowest for cancer risk and non-cancer HQ.
Over three meetings with the scientific community technical working group, staff posed questions and gathered feedback from group members on the prioritization framework. Specifically, the group provided insight into quantifying risk from monitored and modeled data, how to develop a risk-based framework, use of different toxicity values, and which health effects to focus on during prioritization of TACs. Group members also supplemented additional scientific research and sources of monitoring and modeling data to be used in the prioritization framework.
Using the prioritization framework, staff identified three PTACs that had an estimated HQ over 1 for non- cancer health impacts or over 100 in 1 million cancer risk. These were acrolein, ethylene oxide, and hydrogen sulfide. Staff then considered populations exposed to TACs that ranked among the top five modeled risks for cancer or non-cancer to assess risk based on how many people may be impacted by these pollutants, including how many of those people are located in a disproportionately impacted community as prioritized in the Air Quality Control Commission (AQCC) Regulation (Reg.) 3 Disproportionately Impacted Community Layer in Version 1.0 of Colorado EnviroScreen. The Division used this layer because it was designed to prioritize areas that are relevant for stationary source regulations, and staff focused on stationary source emissions in selecting the PTAC. Using census block and block group averages from AirToxScreen 2020 data, staff estimated the number of people exposed to TACs from non-mobile, anthropogenic sources of emissions. This analysis allowed staff to identify three additional pollutants, formaldehyde, benzene, and hexavalent chromium, for prioritization, over others with similar risk characterizations, given the exposure and risk posed to more people state-wide. The Commission prioritized formaldehyde over acrolein based on this population exposure analysis and feedback and data submitted during the rulemaking process. Identification of Five Priority Toxic Air Contaminants Applying the foregoing analysis, the Commission adopted benzene, chromium compounds (hexavalent), ethylene oxide (EtO), formaldehyde and hydrogen sulfide (H2S) as the initial five PTACs. Benzene Benzene emissions met the screening threshold of over 1 in 1 million cancer risk based on the estimated health risk from both monitoring and modeling data. Benzene is a federally designated HAP. In Colorado, based on the EPA AirToxScreen modeling results for human-caused, non-mobile sources, and considering adjustments made to correct for inventory data used in the AirToxScreen modeling, benzene was among the top five highest cancer risk based on AirToxScreen 2019 and among the top six highest cancer risk based on AirToxScreen 2020 modeling results. In addition to this pollutant ranking among the highest modeled cancer risk pollutants, staff further prioritized this pollutant based on population-level exposures, finding there are approximately 180 people, of which 157 are located in a disproportionately impacted community, exposed to benzene in Colorado at levels with an estimated risk of at least 10 excess cancer cases per million people. There are also 1.4 million people, of which 697,000 are located in a disproportionately impacted community, exposed to benzene in Colorado at levels with an estimated risk of between 1 and 10 excess cancer cases per million people. In addition to the modeled cancer risk, the available annual average monitoring data for over 100 different monitored benzene concentrations is in the 1-10 per million excess cancer cases range for the most part. Specifically, the available monitoring data ranges from 1 to 27 per million excess cancer cases, with a mean of approximately 6 per million excess cancer cases. Benzene is a volatile organic compound (VOC), a class of chemicals released into the air from numerous sources, including vehicle exhaust, cleaning products, paint, and gasoline. In Colorado, oil and gas operations, along with other industrial activities, are significant sources of benzene emissions. Benzene is naturally present in crude oil and natural gas, and significant emissions occur during various stages of the handling of crude oil and natural gas. During the extraction process, benzene is released when oil and gas is brought to the surface, as the volatile compounds are exposed to the atmosphere. In the refining process, crude oil is separated into different products, releasing benzene during the breakdown of hydrocarbons. Similarly, when natural gas is processed for sale, benzene may be released during various processing steps. Storage and transportation of oil and gas also contribute to benzene emissions, especially if containment systems are not properly sealed, allowing volatile compounds to escape into the air. Additionally, benzene emissions occur from combustion of wood including residential burning and forest fires. Chronic exposure to benzene may cause disorders in the blood by affecting bone marrow. Aplastic anemia (a risk factor for a specific type of leukemia), excessive bleeding, and damage to the immune system (by changes in blood levels of antibodies and loss of white blood cells) may develop. Benzene exposure may cause structural and numerical chromosomal abnormalities. Animal and human studies have provided evidence that exposure to benzene may impact reproductive and developmental organs. The EPA classifies benzene as a carcinogen. Long term exposure to benzene can cause leukemia. There have also been studies that show an association between benzene exposure and types of lymphoma, myeloma, and lung cancers. Chromium Chromium compounds, hexavalent, also called hexavalent chromium, had an estimated excess cancer risk that met the screening threshold of over 1 in 1 million cancer risk based on the estimated health risk from modeled data after corrections to inventory data were made. Chromium compounds are federally designated HAPs and hexavalent chromium is a state designated HAP under § 25-7-109.3(5)(a) C.R.S. In Colorado, based on the EPA AirToxScreen modeling results for human-caused, non-mobile sources, and considering adjustments made to correct for inventory data used in the AirToxScreen modeling, hexavalent chromium was among the top five highest cancer risk based on AirToxScreen 2019 and 2020. Staff further prioritized this pollutant based on population-level exposures, finding that, based on the updated RSL values and EPA AirToxScreen 2020 modeling results for human-caused, non-mobile sources, approximately 33 people, all of which are located in a disproportionately impacted community, are exposed to hexavalent chromium at levels with an estimated risk of at least 10 excess cancer cases per million people and 7,800 people, of which 5,400 are located in a disproportionately impacted community, are exposed to hexavalent chromium at levels with an estimated risk of between 1 and 10 excess cancer cases per million people. This population exposure is the next highest after benzene among pollutants that had the top five highest modeled cancer or non-cancer health impacts, with the exception of formaldehyde.
Hexavalent chromium is generated when chromium compounds combine with other elements during industrial processes. These compounds are widely used in various manufacturing applications, such as in the production of metal alloys (i.e., stainless steel, brass, aluminum, or plastic) and in decorative chromium electroplating for car parts, furniture, hydraulic cylinders, and aircraft landing gears. Hexavalent chromium is also utilized in anodizing operations to prevent corrosion in aerospace components and their machined parts. Hexavalent chromium is released into the air during the combustion of oil, natural gas, coal, and in certain manufacturing processes, including cement and hazardous waste facilities. Cigarette smoke and industrial emissions from metal processing are additional sources. Furthermore, it is released from plating tanks through control devices and vents, windows, and doors as fugitive emissions during chrome plating operations.
Coal-fired power plants are the largest emitter in Colorado followed by fabricated metal product manufacturing, particularly processes used in electroplating operations and abrasive blasting of metal parts. The sources driving the highest risk from hexavalent chromium are these smaller sources due to their proximity to more densely populated areas. Chronic exposure to hexavalent chromium may cause respiratory effects, including perforations and ulcerations of the septum, bronchitis, decreased pulmonary function, pneumonia, asthma, and nasal itching or soreness. There is limited evidence that exposure to hexavalent chromium may result in complications during pregnancy and childbirth. Epidemiological studies of workers have found clear associations between chromium exposure and lung cancer. EPA, DHHS, and IARC have all classified hexavalent chromium as a carcinogen. Ethylene Oxide Ethylene oxide (EtO) emissions had an estimated cancer risk exceeding 100 in 1 million exposed people, based on the estimated health risk from both modeling data, including only human-caused, non-mobile sources, and monitoring data in the state. EtO is a federally designated HAP. EtO is a colorless gas, used in making a range of products such as antifreeze, textiles, plastics, detergents, and adhesives. EtO is also used to sterilize equipment and plastic devices, often medical equipment, unable to be cleaned by steam. In Colorado, sterilizers located in commercial sterilization facilities, or supporting other industrial processes or research and development applications, are the largest contributors to EtO emissions. The storage of sterilized materials is also one source of emissions. While the EPA finalized a regulation on April 5, 2024, see 89 Fed. Reg. 24090, to reduce emissions from four existing commercial sterilizers as well as any new sterilizer facility that should be built in the state, there are other sources of EtO in the state that are not covered by EPA’s rule. Additionally, this PTAC had the highest estimated cancer risk of any pollutant in the state. Chronic exposure to EtO may cause eye, skin, and respiratory irritation, as well as impacts to the nervous system, including headache, nausea, memory loss, and numbness. EtO exposure may cause an increased rate of miscarriages as well as other reproductive effects. The EPA classifies EtO as a carcinogen. EtO has been shown to cause lymphoid cancer and breast cancer in humans.
Formaldehyde Formaldehyde met the screening threshold of over 1 in 1 million cancer risk based on the estimated health risk from both monitoring and modeling data. Additionally, formaldehyde met the screening threshold of an HQ over 0.1 from both monitoring and modeling data. Formaldehyde is a federally- designated HAP. Formaldehyde is a colorless, flammable gas at room temperature with a strong, pungent odor and is highly reactive and is classified as a VOC. In Colorado, based on the EPA AirToxScreen modeling results for human-caused, non-mobile sources, and considering adjustments made to correct for inventory data used in the AirToxScreen modeling, formaldehyde was among the top five highest cancer risk based on AirToxScreen 2019 and AirToxScreen 2020 modeling results. Formaldehyde also had the sixth highest HQ for AirToxScreen 2020 modeling results. In addition to this pollutant ranking among the highest modeled cancer risk pollutants, staff further prioritized this pollutant based on population-level exposures, finding there are approximately 400,000 people, of which 2,000 are located in a disproportionately impacted community, exposed to formaldehyde from human-caused, non-mobile sources in Colorado at levels with an estimated risk of at least 10 excess cancer cases per million people. There are also 4.15 million people, of which 93,000 are located in a disproportionately impacted community, exposed to formaldehyde in Colorado at levels with an estimated risk of between 1 and 10 excess cancer cases per million people. Some of this same population, approximately 229,000 people, of which 2,000 are located in a disproportionately impacted community, is also exposed to an HQ between 0.1 and 0.6. In addition to the modeled cancer risk, the available annual average monitoring data for 13 different monitored formaldehyde observations ranges from 15 to 57 per million excess cancer cases, with a mean of approximately 38 per million excess cancer cases. For monitored HQ, the available data is in the range of 0.17 - 0.63 with a mean of 0.41 HQ. In Colorado, formaldehyde is a notable byproduct of incomplete combustion including the burning of organic matter, like wood (e.g., wildfires, residential wood combustion, or open burning), other combustion associated with industrial operations and electricity generation, motor vehicle exhaust, and decaying organic matter. Much of the combustion occurring in the industrial sector is associated with oil and gas extraction and production including combustion engines used for drilling or compression activities or flaring, as well as combustion to operate glycol dehydrators and regeneration units at natural gas processing plants. Formaldehyde is also emitted from surface coating or solvent usage and during the production or of composite wood products. Secondary formation is also a significant source of formaldehyde emissions, resulting from atmospheric reaction between VOCs in the atmosphere. Chronic exposure to formaldehyde may cause respiratory symptoms, sensitization leading to asthma and contact dermatitis, neurological effects, and eye, nose, and throat irritation. Occupational and animal studies have shown epithelial lesions from chronic inhalation exposure. There is limited evidence that pulmonary functions may also be adversely affected. EPA has classified formaldehyde as a probable human carcinogen. Occupational studies have found limited evidence of associations between formaldehyde exposure and increased incidence of lung and nasopharyngeal cancer. The International Agency for Research on Cancer (IARC) and Department of Health and Human Services (DHHS) have classified formaldehyde as a human carcinogen.
Hydrogen Sulfide Hydrogen Sulfide emissions had an estimated health risk from monitoring data that exceeded a HQ of 1 for non-cancer health impacts. Estimated health risk from modeling data, including only human-caused, non-mobile sources, met the screening criteria of a HQ over 0.1. Hydrogen Sulfide is not a HAP under federal or state law, but it is a “covered air toxic,” as defined at § 25-7-141(2)(b), C.R.S. Since Hydrogen Sulfide is not a HAP, it is not reported to the EPA National Emission Inventory (NEI) or used in EPA AirToxScreen modeling analysis. The scientific community technical working group provided feedback to expand the data sources considered to include CDPHE modeling studies to address this gap. Staff developed dispersion modeling conducted in a selected 20-kilometer area near Commerce City North Denver to assess the health risk from this pollutant. This area was selected for modeling because it has the highest concentration of reported hydrogen sulfide emissions in the state. Additionally, 3 other stationary monitors had monitoring data available for hydrogen sulfide that were above the screening criteria of a HQ of 0.1.
Hydrogen sulfide is a colorless gas released from both natural sources, such as the decay of organic matter, and human activities, including waste disposal (wastewater treatment and landfilling), industrial processes like petroleum refining, natural gas processing, wastewater treatment, roofing and asphalt material manufacturing, meat processing, gas processing plants and compressions stations, renewable natural gas processing from biogas and molybdenum mining. Chronic exposure to hydrogen sulfide may cause low blood pressure, headache, nausea, loss of appetite, weight loss, eye-membrane inflammation, loss of muscle control, chronic cough, and psychological disorders. Low concentrations can result in irritation of the eyes, nose, and throat, while higher concentrations may lead to more severe respiratory issues, headaches, dizziness, and chronic conditions.
As resources allow, the Commission encourages the Division to move forward with identifying additional PTACs sooner than 2029. In addition to other toxins the Division deems deserving of further study after consultation with scientific experts, stakeholders, and the affected public, we would encourage the Division to also study polycyclic organic matter (including benzo(a)pyrene), ultrafine particles, and radionuclides for potential identification as a PTAC.
The Commission recognizes both the importance and complexity of this statutorily-directed rulemaking and notes that it will be followed by subsequent rulemakings to adopt health-based standards and control measures, as well as the opportunity for designating additional PTACs in the future. In selecting these five initial PTACs, the Commission relied upon the information in the record, while being responsive to community input about compounds of concern to their lived experience. The Commission was statutorily constrained to a maximum of five toxic air contaminants in designating the initial set of PTACs. The Commission believes that the Division appropriately filtered its potential list of initial PTACs to focus first on those from stationary sources. However, there is no numeric limit in the statutes on how many TACs can be designated as PTACs throughout the life of this program. The Commission encourages the Division to not limit its study and analysis just to TACs associated with stationary sources but to also consider those associated with mobile sources to the extent supported by data.
Additional Considerations There are no direct federal requirements that correspond with the new rule. However, to the extent the new rule could be construed to exceed or differ from any related requirements of federal law, the Commission provides the following additional statement, consistent with § 25-7-110.5(5)(a), C.R.S.:
(I) Any federal requirements that are applicable to this situation with a commentary on those requirements;
(II) Whether the applicable federal requirements are performance-based or technology-based and whether there is any flexibility in those requirements, and if not, why not; The federal requirement to list hazardous air pollutants is not performance-based or technology- based. EPA adds pollutants which present, or may present, through inhalation or other routes of exposure, a threat of adverse human health or environmental effects.
(III) Whether the applicable federal requirements specifically address the issues that are of concern to Colorado and whether data or information that would reasonably reflect Colorado's concern and situation was considered in the federal process that established the federal requirements; The federal requirement to list hazardous air pollutants do not specifically address Colorado’s statutory directive to identify up to five priority toxic air contaminants.
(IV) Whether the proposed requirement will improve the ability of the regulated community to comply in a more cost-effective way by clarifying confusing or potentially conflicting requirements (within or cross-media), increasing certainty, or preventing or reducing the need for costly retrofit to meet more stringent requirements later;
(V) Whether there is a timing issue which might justify changing the time frame for implementation of federal requirements;
(VI) Whether the proposed requirement will assist in establishing and maintaining a reasonable margin for accommodation of uncertainty and future growth; The identification of five priority toxic air contaminants does not directly establish additional requirements on the regulated community and, therefore, does not impact future growth.
(VII) Whether the proposed requirement establishes or maintains reasonable equity in the requirements for various sources;
(VIII) Whether others would face increased costs if a more stringent rule is not enacted; The identification of five priority toxic air contaminants does not directly establish additional requirements on the regulated community.
(IX) Whether the proposed requirement includes procedural, reporting, or monitoring requirements that are different from applicable federal requirements and, if so, why and what the “compelling reason” is for different procedural, reporting, or monitoring requirements; The identification of five priority toxic air contaminants does not directly establish additional procedural, reporting, or monitoring requirements.
(X) Whether demonstrated technology is available to comply with the proposed requirement; The identification of five priority toxic air contaminants does not directly establish additional requirements on the regulated community.
(XI) Whether the proposed requirement will contribute to the prevention of pollution or address a potential problem and represent a more cost-effective environmental gain; The identification of five priority toxic air contaminants does not directly establish additional requirements on the regulated community.
(XII) Whether an alternative rule, including a no-action alternative, would address the required standard.
Findings of Fact § 25-7-110.8, C.R.S., requirements do not apply to this rulemaking because the adopted rule is not directly intended to reduce air pollution. To the extent § 25-7-110.8, C.R.S. does apply to this rulemaking, after considering all the information in the record, the Commission hereby makes the determination that:
(I) The rule is based upon reasonably available, validated, reviewed, and sound scientific methodologies, and the Commission has considered all information submitted by interested parties.
(II) The rule is not intended, directly, to address a reduction in air pollution, or, in the alternative, is administrative in nature.
(III) Evidence in the record supports the finding that the rule provides benefits that justify the costs to government to implement the rule, and the rule does not result in any cost to the regulated community or public.
(IV) The rule has no regulatory impact on any person, facility, or activity and, therefore, the analysis under § 25-7-110.5(4), C.R.S. is not required. Additionally, the rule is not intended, directly, to achieve a reduction in air pollution.
(V) The rule does not directly result in air quality benefits. APPENDICES, BY PART Part B:
Appendix A: Priority Toxic Air Contaminants CAS Number Toxic Air Contaminant Date Identified 71432 Benzene 1/17/2025 18540299 Chromium Compounds, Hexavalent 1/17/2025 75218 Ethylene Oxide 1/17/2025 50000 Formaldehyde 1/17/2025 7783064 Hydrogen Sulfide 1/17/2025 II. Adopted: September 19, 2025 This Statement of Basis, Specific Statutory Authority, and Purpose complied with the requirements of the Colorado Administrative Procedure Act section 24-4-103(4), C.R.S., the Colorado Air Pollution Prevention and Control Act sections 25-7-110, -110.5, and -110.8 C.R.S. (the State Air Act), and the Air Quality Control Commission’s (Commission) Procedural Rules, 5 Code Colo. Reg. section 1001-1. Basis On June 2, 2022, House Bill (HB) 22-1244: Public Protections for Toxic Air Contaminants was signed into law. The law, now primarily codified at section 25-7-109.5, C.R.S., directed the Commission to propose health-based standards for each identified Priority Toxic Air Contaminant (PTAC) by April 30, 2026, for General Assembly approval. On January 17, 2025, the Commission adopted Regulation Number 30 (Regulation 30) and identified five initial PTACs. In this action, the Commission adopted revisions to Regulation 30 to propose chronic health-protective benchmarks, which meet the statutory definition of health-based standards, for each identified PTAC for General Assembly approval. Further, the Commission adopted revisions to address typographical, grammatical, and formatting errors found throughout the regulation.
The Commission did not define use-cases for the chronic health-protective benchmarks at the time of rule adoption. Without limiting the Division’s existing authority under the State Air Act or Commission regulations, the Commission acknowledges that before the Division may use the chronic health-protective benchmarks as a level that could trigger a regulatory or economic impact, or as the determining factor that could trigger a regulatory or economic impact, the Commission must have promulgated an enabling regulation.
Specific Statutory Authority The State Air Act, specifically section 25-7-105(1), C.R.S., directs the Commission to promulgate such rules and regulations as are consistent with the legislative declaration set forth in section 25-7-102, C.R.S., and that are necessary for the proper implementation and administration of the State Air Act. Section 25-7-109.5(6)(a), C.R.S. required the Commission to adopt rules that, no later than April 30, 2026, propose health-based standards for each identified PTAC for approval by the General Assembly. Section 25-7-109.5(1)(e), C.R.S. defines health-based standards as “the chronic exposure limits for each priority toxic air contaminant required to protect the public from adverse health effects of that priority toxic air contaminant, allowing for an ample margin of safety, represented as benchmark numerical concentrations in the ambient air.” “Adverse health effects” is defined at section 25-7-109.5(1)(a), C.R.S. as “the detrimental health effects from exposure to emissions of a toxic air contaminant, including the cumulative effects to health from exposure to the combined air emissions of the toxic air contaminant from multiple sources, whether the emissions are emitted routinely, intermittently, or accidentally.” In determining the health-based standards, the statute required:
I. Consideration of the best available peer-reviewed toxicity values regarding the levels of exposure to PTACs that may cause or contribute to adverse health effects;
II. Consideration of standards adopted in other states to reduce or limit concentrations of toxic air contaminants (TACs) in the ambient air;
III. Consideration of the effects of exposure to PTACs on vulnerable groups of the state, including disproportionately impacted communities, infants, children, fetuses, the elderly, and people with disabilities;
IV. Consideration of both cancer-related health risks and non-cancer-related health risks and identification of the excess cancer and non-cancer risk levels used in determining the health- based standards;
V. Provision of a sufficient margin of safety that accounts for the various effects that different populations may experience from exposure to PTACs; and VI. Consultation with the scientific community through holding at least one public hearing. Purpose Chronic Health-Protective Benchmarks In this Regulation Number 30, the Commission adopted proposed chronic health-protective benchmarks for each PTAC to satisfy the statutory directive in section 25-7-109.5(6)(b), C.R.S. to adopt proposed health-based standards, as informed by the factors identified in section 25-7-109.5(6)(b), C.R.S. As levels below which there is a high degree of scientific confidence that adverse health effects are unlikely to occur from exposure to each PTAC on an individual basis, the chronic health-protective benchmarks adopted by the Commission satisfy the statutory direction to adopt chronic exposure limits for each PTAC, represented as benchmark numerical concentrations in the ambient air, that protect the public from adverse health effects of that PTAC, allowing for an ample margin of safety. The Commission included the defined term “chronic health-protective benchmark” in Regulation Number 30 for two reasons.
First, the term “chronic health-protective benchmarks” accurately describes the proposed values the Commission adopted. As explained above, the adopted values are set at levels the Commission determined to be health-protective for each individual PTAC, with an ample margin of safety. As such, the adopted values meet the statutory definition of health-based standards when that definition is read as a whole and together with the considerations listed under section 25-7-109.5(6)(b), C.R.S. on an individualized PTAC basis. Party comments during the rulemaking raised the potential for confusion around parallels between the National Ambient Air Quality Standards (NAAQS) and chronic health- protective benchmarks due to similar, though not identical, language in the federal Clean Air Act and HB 22-1244. Based on Division research for how ambient air concentrations for air toxics were developed for other state programs, the Commission determined that the chronic health-protective benchmarks are not intended to function as ambient air quality standards similar to the NAAQS. To avoid future conflation of the chronic health-protective benchmarks and ambient air quality standards, the Commission described the adopted values as chronic health-protective benchmarks, clarifying that the values represent levels below which adverse health effects are unlikely to occur, not levels that must be achieved to ensure public health is protected.
Second, adopting a definition of chronic health-protective benchmarks will allow for distinguishing the adopted values in this action from future iterations of health-based standards. Section 25-7-109.5(6)(c), C.R.S. provides that the Commission must consider in the future whether to include acute exposure limits for PTACs in the definition of health-based standards. By identifying the proposed values adopted in this action as chronic health-protective benchmarks, the Commission can easily distinguish such values from any future acute values.
The Commission also adopted revisions at Part A, Section I.A. to establish definitions essential to understanding how the chronic health-protective benchmarks were calculated. In Part B, Section II.A. the Commission adopted proposed chronic health-protective benchmarks, as set out in Appendix B. Part B, Section II.A. and Appendix B may be revised with future iterations of PTAC identification and health- based standard adoption, as required under section 25-7-109.5(6)(d), C.R.S. They may also be revised in the future to modify existing health-based standards, consistent with section 25-7-109.5(6)(c), C.R.S. The chronic health-protective benchmark table in Part B, Appendix B, differentiates between cancer risk and/or non-cancer health risk. The table also denotes where the toxicology values were derived and the most recent year the values have been updated.
The proposed chronic health-protective benchmarks were calculated using (1) pollutant-specific toxicity values and (2) selected cancer and non-cancer health risk levels. These two inputs are discussed below. After considering all the information in the record, the Commission selected the risk levels used in the chronic health-protective benchmarks as the appropriate levels for the state of Colorado as a matter of policy. The selected levels were informed by the statutory definitions of “health-based standards” and “adverse health effects,” and by the considerations at section 25-7-109.5(6)(b). The Commission determined the toxicity values used in the chronic health-protective benchmarks were the best available peer-reviewed toxicity values, see section 25-7-109.5(6)(b)(I), considering evidence in the record and largely in accordance with longstanding CDPHE policy.
Toxicity Values Toxicity values are numerical values that indicate the relative toxicity of individual chemicals and their capacity to cause adverse health effects. Pollutants can have toxicity values for either or both cancer and non-cancer health risk. Toxicity values for carcinogenic pollutants are expressed as an inhalation unit risk (IUR), as defined in Section I.A.3, while toxicity values for pollutants with non-cancer health impacts are expressed as a reference concentration (RfC), as defined in Section I.A.5. Consideration of the best available toxicity values (section 25-7-109.5(6)(b)(I), C.R.S.) The best available toxicity values used to determine the proposed chronic health-protective benchmarks were selected largely based on a Colorado Department of Public Health & Environment (CDPHE) policy entitled On Use Of Human Health Toxicity Values In Environmental Risk Assessment And Remediation Management, published in 2004. The policy establishes a hierarchy of federal and some state sources to consult when deciding upon a toxicity value to use for various risk-management or policy-related decision making. This policy aligns with EPA’s approach in other programs including the Office of Air Quality Planning and Standards, AirToxScreen, and the Risk-Screening Environmental Indicators Model. The Commission recognizes the value in following a systematic approach to the selection of toxicity values. For the non-cancer toxicity value for benzene, however, the Commission determined to use the value from the California Office of Environmental Health Hazard Assessment rather than the value from the EPA Integrated Risk Information System (IRIS), as would have been dictated by existing CDPHE policy. The Commission made this decision based on evidence presented by parties to the rulemaking and the Commission’s informed judgment. The Commission may revisit toxicity values, including the selection process, in future rulemakings to adopt new chronic health-protective benchmarks or to revise existing chronic health-protective benchmarks, as appropriate. Consideration of margin of safety and vulnerable groups in toxicity values (sections 25-7-109.5(1)(e) and 25-7-109.5(6)(b)(III), (V), C.R.S.) The proposed chronic health-protective benchmarks, by definition, had to allow for “an ample margin of safety.” Further, in determining the chronic health-protective benchmarks, the Commission had to consider the effects of exposure to PTACs on vulnerable groups in the state and provide a sufficient margin of safety that accounts for the various effects that different populations may experience from PTAC exposure. The selected toxicity values satisfied these requirements because all of the sources of toxicity values chosen to calculate the proposed chronic health-protective benchmarks incorporate uncertainty factors to account for variations in susceptibility within the human population. These uncertainty factors are used by different sources, like the EPA Integrated Risk Information System (IRIS), when translating scientific studies, which often only include a single group (for example a group of workers), into values that are protective for the larger public. The application of these uncertainty factors means that the toxicity values selected include considerations for differences in sensitivity between humans that may arise from differences like age and health status. The toxicity values used are all chronic toxicity values, which are developed to be protective for exposures which may occur over a person’s lifetime.
Consultation with the scientific community on toxicity values (section 25-7-109.5(6)(b)(VI), C.R.S.) In meetings with a scientific community technical working group, staff presented options for toxicity values for the identified PTACs. Several members of the working group were supportive of using the CDPHE policy for toxicity values. Further, as required by section 25-7-109.5(6)(b)(VI), C.R.S., the Commission convened a public hearing on February 21, 2025, during which the Commission heard from the scientific community technical working group and the Division regarding the proposed framework to calculate chronic health-protective benchmarks. This briefing provided the Commission with an opportunity to receive technical perspectives and better understand the context surrounding the toxicity values being considered for the proposed chronic health-protective benchmarks. Working group members noted that the CDPHE policy was consistent with other EPA policies on toxicity value hierarchies. Some members expressed concern that newer values should be prioritized regardless of the tier of the source in the CDPHE policy.
Consideration of toxicity values used in other states (section 25-7-109.5(6)(b)(II), C.R.S.) CDPHE staff conducted research on numerous other state regulatory agency programs for air toxics. This research included how ambient air concentrations were derived and applied across the different programs. A majority of the programs used similar sources of toxicity values as the CDPHE policy, with the most common source being EPA’s IRIS (tier 1 in CDPHE’s policy). A few states derive their own toxicity values either informed by existing sources or developed with help from scientific boards or committees. There are several states that have opted to use occupational values from agencies such as The Occupational Safety and Health Administration, The National Institute for Occupational Safety and Health, or American Conference of Governmental Industrial Hygienists. Since these values are developed for healthy workers exposed for a typical work-span, these states may make adjustments to these values when they are used for a population level risk assessment or screening. Risk levels In addition to toxicity values, the other key input in determining the proposed chronic health-protective benchmarks was risk level. The statute required consideration of both cancer and non-cancer health impacts and associated risk levels when determining the proposed chronic health-protective benchmarks. Cancer risk, for the purposes of risk assessment, is often expressed as the excess cancer cases per 1 million people exposed to a given pollutant. Non-cancer health impacts are expressed using a HQ, which shows the ratio between potential exposure to a pollutant and the level at which no adverse health effects are expected from a single pollutant.
By definition, the proposed chronic health-protective benchmarks are required to be set at a level to protect the public from adverse health effects, allowing for an ample margin of safety. Further, in adopting the proposed chronic health-protective benchmarks, the Commission had to consider the effects of exposures on vulnerable groups of the state (section 25-7-109.5(6)(b)(III)) and provide a sufficient margin of safety that accounts for the various effects that different populations may experience from PTAC exposures (section 25-7-109.5(6)(b)(V)). In alignment with these directives, the Commission used a cancer risk level of 1 excess cancer case in 1 million exposed people and a non-cancer HQ of 1.0 to calculate the proposed chronic health-protective benchmarks. A cancer risk level of 1 in 1 million aligns with long-standing precedent in federal environmental policy. In particular, EPA identifies 1 in 1 million as its target to protect the greatest number of people in its residual risk program and uses it as a screening threshold in programs like EPA Superfund when assessing risk from multiple pollutants. The cancer risk level of 1 in 1 million provides an ample margin of safety when considering vulnerable populations exposed to multiple TAC as it allows for the summing of multiple cancer risks while staying below the 100 in 1 million level, which EPA considers unacceptable. An HQ of 1 represents the threshold level at which no adverse health effects are expected from exposure to that level of a single pollutant. A level below an HQ of 1 is common practice in federal and state risk assessments where multiple pollutants and shared target organs are involved. Consideration of adverse health effects, margin of safety, and vulnerable groups in risk levels (sections 25-7-109.5(1)(a), 25-7-109.5(1)(e), 25-7-109.5(6)(b)(III), (V), C.R.S.) The risk levels selected by the Commission to calculate the proposed chronic health-protective benchmarks considered that the standards must provide an ample margin of safety and consider vulnerable groups. The Commission recognizes that Coloradans are not exposed to emissions from a single pollutant at a time, and that the additive effects from multiple exposures may exacerbate risk. Because the toxicological data that are the basis of the CHPB are typically collected for only a single pollutant at a time, multi-pollutant impacts are not included in these CHPB values. This important consideration will be addressed in future rulemakings on this topic. Consultation with the scientific community on risk levels (section 25-7-109.5(6)(b)(VI), C.R.S.) In meetings with the scientific community technical working group, staff presented options for risk levels for the identified PTACs. The scientific community working group did not provide a consensus recommendation on the cancer or non-cancer risk levels to be used in the chronic health-protective benchmarks .Some conveyed interest in using the most conservative values, considering that people do not breathe a single PTAC in isolation, while others were concerned that they could not advise on the risk levels without knowing how the resulting chronic health-protective benchmarks might be used. This feedback was shared with the Commission during a public hearing on February 21, 2025, with the Commission.
Consideration of risk levels adopted in other states (section 25-7-109.5(6)(b)(II), C.R.S.) Staff also considered risk levels used in other states. Based on CDPHE staff research, the most commonly chosen cancer risk level in other state programs was 1 in 1 million excess cancer cases. Maine, Minnesota, Texas, and Maryland were in the minority by choosing 10 in 1 million excess cancer cases. Importantly, Minnesota uses the same cancer risk level, of 10 in 1 million excess cancer cases, for an individual carcinogen and for the sum of multiple carcinogens from a source. For non-cancer risk levels, the majority of state programs calculate their benchmark values using an HQ of 1. Massachusetts and Texas both use values that are lower than an HQ of 1. The values that the Texas uses in their permitting program are set at a level that is effectively an HQ of 0.3 in order to “account for exposure to chemicals from multiple sources.” Massachusetts uses an HQ of 0.2. Calculations for Proposed Chronic Health-Protective Benchmarks Considering the above factors, staff considered the best available peer-reviewed information concerning PTAC toxicity values, collaborated internally across departments, and sought input from a variety of stakeholders, including the scientific community, to select the appropriate level of risk to calculate the proposed chronic health-protective benchmarks. The risk levels adopted by the Commission reflect statutory directives to propose chronic health-protective benchmarks that consider adverse health effects and incorporate an ample margin of safety, including for vulnerable groups. To calculate the proposed cancer chronic health-protective benchmarks, which are presented as a concentration in the ambient air, micrograms per meter (µg/m3), standard methodologies were used to convert the toxicity values and risk levels into concentrations. The cancer risk level (0.000001, representing 1 excess cancer case per 1 million people) is divided by the IUR, for each PTAC. For non- cancer risk, the RfC is multiplied by the risk level (HQ of 1.0) for each PTAC and then multiplied by 1,000 to align the units of the RfC with the desired units of each proposed chronic health-protective benchmark (µg/m3).
Scope of the Chronic Health-Protective Benchmarks Although the statute defines a health-based standard and sets forth statutory requirements for the development of health-based standards, it does not prescribe specific regulatory applications of the proposed health-based standards. In assessing how the proposed chronic health-protective benchmarks could be applied in the future, staff research concluded that most states use benchmark concentrations to compare to modeled emissions estimates from potential or existing sources during the permitting process in order to determine if additional information or regulatory action might be required, or if the permit application might require refinements to reduce the risk of exposure to one or more air toxic pollutants. Some states also use benchmark concentrations to evaluate monitoring data to identify potential areas of health concern for further study. There are also several examples where the benchmark concentrations are used as a requirement that permitted facilities must meet, as measured at the facilities’ fencelines. While the Commission is not adopting use-cases for the proposed chronic health-protective benchmarks at this time, these benchmark numerical concentrations will inform the recommended permitting framework that will be published in the permitting needs assessment submitted to the General Assembly in December 2025. The Commission also understands that staff will present the proposed chronic health- protective benchmarks for approval at this SMART Act hearing in January 2026 in order to maximize the time available during the 2026 session for the General Assembly to take action on the proposed values and provide any direction for a potential future permitting program. The Commission recognizes that section 25-7-109.5(6)(c), C.R.S. allows for an iterative review of the chronic health-protective benchmarks, including incorporation of updated toxicity values and/or risk levels over time. As part of this iterative review, the Commission may determine whether to include acute exposure limits for PTACs in this Regulation Number 30. Additional Considerations There are no direct federal requirements that correspond with the new rule. However, to the extent the new rule could be construed to exceed or differ from any related requirements of federal law, the Commission provides the following additional statement, consistent with section 25-7-110.5(5)(a), C.R.S.:
(I) Any federal requirements that are applicable to this situation with a commentary on those requirements;
(II) Whether the applicable federal requirements are performance-based or technology-based and whether there is any flexibility in those requirements, and if not, why not; Most NESHAPs are technology-based. However, the promulgation of the proposed chronic health-protective benchmarks is separate from and unrelated to these federal standards.
(III) Whether the applicable federal requirements specifically address the issues that are of concern to Colorado and whether data or information that would reasonably reflect Colorado's concern and situation was considered in the federal process that established the federal requirements; Although the federal NESHAP framework includes residual health risk assessment for certain types of source categories, and health risk was also considered in proposing the chronic health- protective benchmarks, the NESHAP framework does not specifically address Colorado’s statutory directive to propose chronic health-protective benchmarks for the identified PTACs. The statutory directive to propose such chronic health-protective benchmarks is separate from and does not mirror the source category-based NESHAP framework. Additionally, NESHAPs only apply to federally-designated HAPs, whereas the list of PTACs–for which chronic health- protective benchmarks must be proposed–includes pollutants that are not federally-designated HAPs.
(IV) Whether the proposed requirement will improve the ability of the regulated community to comply in a more cost-effective way by clarifying confusing or potentially conflicting requirements (within or cross-media), increasing certainty, or preventing or reducing the need for costly retrofit to meet more stringent requirements later;
(V) Whether there is a timing issue which might justify changing the time frame for implementation of federal requirements;
(VI) Whether the proposed requirement will assist in establishing and maintaining a reasonable margin for accommodation of uncertainty and future growth; The proposed chronic health-protective benchmarks for the five PTACs do not establish additional requirements on the regulated community and, therefore, do not impact future growth.
(VII) Whether the proposed requirement establishes or maintains reasonable equity in the requirements for various sources;
(VIII) Whether others would face increased costs if a more stringent rule is not enacted; There are no federal requirements applicable to Colorado’s proposal for chronic health-protective benchmarks and the proposed chronic health-protective benchmarks have no specified regulation application. As a result, whether others would face increased costs if a more stringent rule is not enacted is not relevant to this rulemaking.
(IX) Whether the proposed requirement includes procedural, reporting, or monitoring requirements that are different from applicable federal requirements and, if so, why and what the “compelling reason” is for different procedural, reporting, or monitoring requirements; The proposed chronic health-protective benchmarks do not establish additional procedural, reporting, or monitoring requirements.
(X) Whether demonstrated technology is available to comply with the proposed requirement; The proposed chronic health-protective benchmarks do not establish additional requirements on the regulated community.
(XI) Whether the proposed requirement will contribute to the prevention of pollution or address a potential problem and represent a more cost-effective environmental gain; The proposed chronic health-protective benchmarks do not establish additional requirements on the regulated community.
(XII) Whether an alternative rule, including a no-action alternative, would address the required standard.
Findings of Fact Section 25-7-110.8, C.R.S., requirements do not apply to this rulemaking because the adopted rule is not directly intended to reduce air pollution. To the extent section 25-7-110.8, C.R.S. does apply to this rulemaking, after considering all the information in the record, the Commission hereby makes the determination that:
(I) The rule is based upon reasonably available, validated, reviewed, and sound scientific methodologies, and the Commission has considered all information submitted by interested parties.
(II) The rule is not intended, directly, to address a reduction in air pollution, or, in the alternative, is administrative in nature.
(III) Evidence in the record supports the finding that the rule provides benefits that justify the costs to government to implement the rule, and the rule does not result in any cost to the regulated community or public.
(IV) The rule has no regulatory impact on any person, facility, or activity and, therefore, the analysis under section 25-7-110.5(4), C.R.S. is not required. Additionally, the rule is not intended, directly, to achieve a reduction in air pollution.
(V) The rule does not directly result in air quality benefits. Appendix B: Chronic Health-Protective Benchmarks for Priority Toxic Air Contaminants Table Notes:
Proposed Chronic Health-Protective Benchmarks (μg/m³) as of September 19, 2025.
Chronic Cancer2 Chronic Non-Cancer3 Cancer Chronic Toxicity Value Non-Cancer Toxicity Value Health- Reference Chronic Health- Reference Protective Protective Benchmark Benchmark CAS Priority Toxic (μg/m³) S ource Year ( μg/m³) Source Year Number Air Contaminant 71432 Benzene 0.13 IRIS 2000 3 CalEPA 2014 18540299 Chromium 0.000056 IRIS 2024 0.03 IRIS 2024 Compounds, Hexavalent 75218 Ethylene Oxide 0.0002 IRIS 2016 30 CalEPA 2000 50000 Formaldehyde 0.091 IRIS 2024 7 IRIS 2024 7783064 Hydrogen Sulfide 2 IRIS 2003 _________________________________________________________________________ Editor’s Notes History New rule eff. 03/17/2025.
Entire rule eff. 11/14/2025.