Outdoor Air

Issue Summary
U.S. EPA
States
Organizations/Non-Government Programs
Publications
Databases and Tools

Issue Summary

Outdoor air quality (also called ambient air quality) is affected by pollutants emitted from stationary sources such as power plants, waste incinerators, wastewater treatment facilities, landfills and industrial facilities, as well as from mobile sources like cars and trucks. Local governments have compliance obligations under the federal Clean Air Act (CAA) and the state implementation plans and air pollution laws approved by EPA under the CAA. These obligations affect local government operations, facilities and vehicle fleets. Some local governments also have regulatory and enforcement authority under state and local air pollution control laws. 

Clean Air Act Overview

The CAA, passed in 1970 and substantially amended in 1990, forms the basis for the nation's air pollution control effort and is intended to protect the public's health and welfare. The CAA directs EPA to establish national standards for ambient air quality and defines how EPA and the states (and authorized Tribes) will implement, maintain and enforce these standards. Under the CAA, some local government operations (e.g., solid waste incinerators) are required to obtain air permits. Relevant portions of the CAA are summarized below.

National Ambient Air Quality Standards (NAAQS) 

Pursuant to Title I of the CAA, EPA established National Ambient Air Quality Standards (NAAQS) to set threshold levels for control of six criteria pollutants:

• carbon monoxide (CO)
• oxides of nitrogen (NOx)
• ground-level ozone (O3)
• lead (Pb)
• sulfur dioxide (SO2)
• particulate matter (PM)

For each criteria pollutant, EPA establishes primary and secondary NAAQS. The primary standard defines a maximum level of the pollutant in the ambient air that will allow for the protection of human health with an adequate margin of safety. The secondary standard is established to protect the public welfare, which includes preventing environmental and property damage, as well as the effects of air pollution on economic values and personal well-being.

EPA designates geographical areas throughout the nation as attainment or non-attainment areas based on whether or not their air quality meets the NAAQS. If the ambient level of a pollutant is below the NAAQS, the area will be designated as in attainment for that pollutant. However, if the pollution limits are exceeded for several consecutive years, EPA will designate an area as non-attainment. The area will subsequently be subject to more stringent regulatory requirements. Areas can be in attainment for some pollutants, while designated as non-attainment for others. Some areas are designated as maintenance areas. These are regions that were initially designated as non-attainment or unclassifiable and have since attained compliance with the NAAQS. The designation of areas as nonattainment affects the actions that must be taken by states and permittees to improve air quality and move to attainment status. 

Use EPA's AirData System to check on an area's status.

National Emission Standards for Hazardous Air Pollutants (NESHAPs)

EPA establishes and enforces National Emission Standards for Hazardous Air Pollutants (NESHAPs), nationally uniform standards for particular hazardous air pollutants (HAPs). Section 112(c) of the CAA further directed EPA to develop a list of sources that emit any of 189 HAPs (later revised down to 187) and to develop regulations for these sources. To date, EPA has listed 174 categories of sources and developed a schedule for establishing  emission standards for each. The emission standards are developed for both new and existing sources based on "maximum achievable control technology" (MACT). The MACT is defined as the control technology achieving the maximum degree of reduction in the emission of the HAPs, taking into account cost and other factors. Unless a local government operates a hazardous waste treatment storage and disposal facility or stores significant quantities of organic chemicals, it is not likely to be subject to the NESHAPs requirements.

New Source Performance Standards (NSPSs)

Title I also directed EPA to establish New Source Performance Standards (NSPSs), which are nationally uniform emission standards for new stationary sources within particular industrial categories. NSPSs are based on the pollution control technology available to that category of industrial source. New and modified municipal waste combustors or sewage sludge incinerators may be subject to these standards.

Major Source Permit Program

Title V of the CAA of 1990 created a permit program for all "major sources" (and certain other sources) regulated under the CAA. One purpose of the operating permit is to include in a single document all air emissions requirements that apply to a given facility. States develop permit programs in accordance with guidance and regulations from EPA. Once a state program is approved by EPA, permits are issued and monitored by that State. Local government facilities that may require Title V permits include municipal waste combustors (large and small), hospital/medical/infectious waste incinerators and sewage sludge incinerators.

Mobile Sources 

Title II of the CAA pertains to mobile sources, such as cars, trucks, buses and planes. Reformulated gasoline, automobile pollution control devices and vapor recovery nozzles on gas pumps are a few of the mechanisms EPA uses to regulate mobile air emission sources. Local governments may be subject to these standards if they operate vehicles or large fleets of vehicles, or conduct fueling operations. Check LGEAN's Transportation page for more information helpful to local governments.

State Implementation Plans (SIPs)

States are required to submit a "plan" for EPA approval, describing how the CAA will be implemented within that state. The SIP is a collection of the regulations, programs and policies to achieve compliance with federal air quality and permitting requirements. The SIP is a vehicle for imposing enforceable emissions controls on individual sources within the "entire geographic area" of the state that are stringent enough for the state to meet and maintain NAAQS by the required deadline. The SIP must be revised whenever the NAAQS or other federal standards are updated. Some of these sources requiring permits and subject to emissions limitations will be local government facilities.  

Acid Rain: Sulfur Dioxide/Nitrogen Oxide Emissions

Title IV of the CAA establishes an emissions program designed to reduce the formation of acid rain. Local governments that operate municipal waste combustors, sewage sludge incinerators or large boilers generators may be subject to these requirements.

Stratospheric Ozone Protection

Title VI of the CAA is intended to protect stratospheric ozone by phasing out the manufacture of ozone-depleting chemicals and restrict their use and distribution. Local governments that conduct vehicle or building air conditioner maintenance and repair are subject to these requirements.

EPA sets and updates NAAQS and NESHAPs. The agency reviews and approves or disapproves SIPs. if EPA finds a SIP incomplete or defective, EPA must issue a Federal Implementation Plan (FIP), which operates in its place. If EPA does not approve a SIP revision, it can take over enforcing that particular provision in that state. EPA oversees compliance with permitting requirements and carries out permitting where the state has not received authority to do so. EPA can also take direct enforcement action for violations of the CAA. EPA conducts research, air quality monitoring, rulemaking and enforcement to support achieving CAA goals.

EPA Office of Air and Radiation (OAR). OAR develops national programs, technical policies and regulations to control air pollution and radiation exposure. OAR implements the nation's CAA programs for both stationary and mobile sources of air pollution, including the greenhouse gases contributing to climate change.

Integrated Urban Air Toxics Strategy. Framework for addressing air toxics emissions in urban areas.

Selected EPA air pollution regulations potentially relevant to local government facilities and operations:

• Stationary Refrigeration and Air-Conditioning. CAA regulations covering service, repair or disposal of refrigeration and air-conditioning equipment.
• Solid Waste:
• Municipal Solid Waste Landfills
• Solid Waste Combustion 
• Hospital/Medical/Infectious Waste Incineration (HMIWI) 
• Municipal Waste Combustors (MWC) - Large Units 
• Municipal Waste Combustors - Small Units 

States

The CAA program is primarily administered by states and local air pollution control agencies, with federal oversight by EPA. For CAA purposes, federally recognized Tribes may be treated the same as states. 

In general, states and Tribes may implement more stringent air pollution laws, but their laws cannot have weaker limits than those set by EPA. To learn more about stringency, visit the National Association of Clean Air Agencies.

SIPs are submitted by states (or recognized Tribes) and specify the contents of the regulatory program. These plans require reasonable notice and public hearing. They include enforceable emissions limitations, efforts to monitor and model air quality data, measures to ensure sources do not significantly contribute to out-of-state pollution and a process for consulting with local governments and relevant federal land managers. States develop permit programs in accordance with guidance and regulations from EPA. Once a state program is approved by EPA, permits will be issued and monitored by that state. 

Many local government activities are potential sources of air pollution and accordingly may need a permit to operate or may have other compliance obligations. Examples of the types of local government operations that may be subject to CAA requirements include:

• Landfill operation
• Waste recycling
• Municipal solid waste combustors
• Hazardous waste combustors 
• Boiler and furnace operations 
• Wastewater/contaminated groundwater treatment
• Construction activities, including buildings and infrastructure
• Air conditioner maintenance and repair
• Vehicle fleets and fleet maintenance
• Vehicle refueling
• Paint stripping and vehicle painting

Attainment Areas/Prevention of Significant Deterioration (PSD). SIPs must include measures to prevent significant deterioration and ensure visibility protection in areas that comply with NAAQS requirements (so-called PSD areas). Within a PSD area, permits are required for major emitting facilities that will undergo a modification leading to an increase in emissions.

Nonattainment Areas. SIPs must provide for "reasonable further progress" in areas failing to meet NAAQS. This means there will be sufficient annual incremental reductions in the nonattainment pollutant to ensure timely attainment. SIPs will also require "reasonable available control measures" that employ "reasonably available control technology." The technology will vary depending on the emissions source, and EPA maintains a Clearinghouse to promote information sharing on these various technologies.

Permitting Programs. Most permitting programs are administered by state or local agencies ("CAA part 70" permits), while a small number are issued by EPA ("CAA part 71" permits). New or modified stationary sources require a permit (New Source Review). 

Permits for new or modified major sources in nonattainment areas include more stringent requirements and can be granted only if all of the following criteria are met: 

• Emissions in the nonattainment area will decrease (after accounting for offsets)
• The source incorporates technology designed to achieve the "lowest achievable emission rate" 
• The owner or operator is in compliance with all applicable emissions limitations
• The economic benefits outweigh the environmental and social costs associated with the location, construction or modification of the source

Permits for new or modified major sources in attainment areas can be granted only if all of the following criteria are met:

• The owner or operator has conducted pre-construction monitoring
• All pollutants regulated under CAA, including criteria and hazardous pollutants, are subject to the best available control technology (BACT) 
• Emissions from the source will not cause or contribute to air pollutant that will exceed the maximum allowable increases or maximum allowable concentrations for any pollutant (also known as PSD increments), NAAQS or any other emissions standards 

Air Pollution State Resource Locator. Provides links to state and local air pollution agencies and regulations along with points of contact.

Air and Waste Management Association (A&WMA). Nonprofit, nonpartisan professional organization that enhances knowledge and expertise by providing a neutral forum for information exchange, professional development, networking opportunities, public education and outreach to more than 8,000 environmental professionals in 65 countries.

National Association of Clean Air Agencies (NACAA). Nonprofit, nonpartisan association of the air pollution control agencies of 35 states, the District of Columbia, four territories and 116 metropolitan or local clean air regulators. Based in Washington, D.C., NACAA provides policy information, advocacy and state and local perspectives on national CAA policy proposals.

Association of Air Pollution Control Agencies. Nonprofit association, founded in 2012 and based in Kentucky, includes 22 state and 47 local air pollution control agencies, chiefly in the Southeast, Lower Midwest and Mountain West.

American Lung Association. Nonprofit organization with a long history of research and advocacy on air pollution issues affecting human health and welfare. It annually publishes a State of the Air Report, which compiles location-specific air quality information on ground level ozone and fine particulate matter compiled from official air quality monitors across the nation's states, metro regions and localities.

National Tribal Air Association. Nonprofit organization of federally recognized Tribes and Native Alaskan villages (principal members) and tribal organizations, consortia, state-recognized Tribes and individuals (associate members) supporting air quality management policies and programs, information and advancement consistent with the needs, interests, and status of American Indian Tribes and Alaska Natives.

Clean Air Act: A Summary of the Act and its Major Requirements. Convenient summary of CAA requirements, including standards for non-attainment areas, permitting and other provisions, prepared by the Congressional Research Service.

Citizen Science Programs at Environmental Agencies Series. Compilation of 15 case studies of agency programs that actively involve the public to complement official action and to establish a collaborative role in protecting the environment. Highlights successful methods of data collection and how they have been used by various agencies in air and water programs. (Environmental Law Institute)

Office of Air and Radiation Publications. EPA search tool that may be used to locate OAR publications.

EPA Annual Report, "Our Nation's Air: Trends Through 2022". Report summarizing the nation's air quality status and trends through December 2022. The report features interactive graphics that allow decision makers to access detailed information by pollutant, location and year.

Air Data: Air Quality Data Collected at Outdoor Monitors Across the US. EPA data, including daily air quality tracker and historic data on air quality, with information from state, tribal and local monitoring stations.

Technical Air Pollution Resources. EPA technical resources on air pollution prevention, technology, regulation, measurement and science.

Plume Labs City Air Quality Maps. Provides real time air quality index data at the city level across the United States.

American Lung Association State of the Air. Provides air quality data including ozone and particle pollution as well as information on at-risk demographics for zip codes, counties and states.

EPA P2 (Pollution Prevention) Calculators. Calculators for P2 program grantees and technical assistance providers to assess organizations' environmental impact, develop pollution prevention strategies, and track progress towards sustainability goals.