Ambient Air Quality Standards: NAAQS Pollutants and Nonattainment

The National Ambient Air Quality Standards, known as NAAQS, are the concentration limits the Environmental Protection Agency sets for six pollutants in outdoor air across the United States. The Clean Air Act gives EPA the authority to set these limits, and federal regulations define the “ambient air” they cover as the portion of the atmosphere outside buildings to which the general public has access.¹ Every region in the country is measured against these limits, and areas that fall short face escalating federal consequences that reach industrial permitting, state highway funding, and the fees major polluters pay per ton of emissions.

The Six Regulated Pollutants and Their Limits

Under 42 U.S.C. § 7408, EPA maintains a list of pollutants whose emissions endanger public health or welfare and come from numerous or diverse sources.² These are called “criteria pollutants” because the agency develops health-based and environmental criteria as the scientific foundation for each standard. Six pollutants currently have NAAQS under 40 CFR Part 50: carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide.³ The list is not permanently capped; EPA can add pollutants if the science supports it, but the six have remained unchanged for decades.

Each pollutant has a specific ceiling measured over a defined averaging period, and some carry both short-term and long-term limits. The current standards as of 2026 are:⁴

Carbon monoxide: 9 ppm over 8 hours and 35 ppm over 1 hour, neither to be exceeded more than once per year. Main sources are vehicles and industrial fossil-fuel combustion.
Lead: 0.15 µg/m³ as a rolling 3-month average. Comes primarily from ore and metals processing and from aircraft burning leaded fuel.
Nitrogen dioxide: 100 ppb over 1 hour (98th percentile of daily maximums, averaged over 3 years) and 53 ppb as an annual mean. Generated by high-temperature combustion and a key ingredient in smog.
Ozone: 0.070 ppm over 8 hours, based on the annual fourth-highest daily maximum averaged over 3 years. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight; it is distinct from the protective upper-atmosphere ozone layer.
PM₂.₅ (fine particles up to 2.5 micrometers): 9.0 µg/m³ annual mean primary standard, strengthened in 2024 from the previous 12.0 µg/m³, plus 35 µg/m³ over 24 hours. The secondary annual standard remains 15.0 µg/m³.
PM₁₀ (particles up to 10 micrometers): 150 µg/m³ over 24 hours, not to be exceeded more than once per year on average over 3 years.
Sulfur dioxide: 75 ppb over 1 hour (primary) and 10 ppb annual mean (secondary). Emitted largely by fossil-fuel power plants and industrial facilities.

When EPA tightens a standard, as it did with PM₂.₅ in 2024, areas that previously passed can be reclassified as nonattainment overnight, triggering new obligations for states and industries in those regions.

Primary and Secondary Standards

Each criteria pollutant carries two categories of standard. Primary standards must be set at levels “requisite to protect the public health” with “an adequate margin of safety.”⁵ That margin is why limits are set stricter than the exact threshold where harm begins, particularly to protect children, the elderly, and people with asthma or heart disease.

Secondary standards protect “public welfare,” a broader category that covers damage to crops and vegetation, harm to animals, reduced visibility in national parks, and deterioration of buildings and monuments.⁵ Often the two are identical, but not always. The PM₂.₅ annual standard is the clearest example: 9.0 µg/m³ primary against 15.0 µg/m³ secondary.⁴

How the Standards Get Updated

EPA is required to review both the scientific criteria and the standards themselves at five-year intervals.⁵ After each review, the agency must revise any standard the evidence shows is no longer adequate or leave it in place if the science supports the current level. In practice, EPA has rarely completed reviews on the statutory schedule; most have taken closer to a decade.

The Clean Air Scientific Advisory Committee, a panel of independent scientists established by the 1977 amendments to the Clean Air Act, advises the Administrator during every NAAQS review. CASAC evaluates the underlying science, identifies research gaps, and recommends whether revisions are warranted, weighing public health, welfare, and economic effects.⁶ Its recommendations carry weight, but the Administrator makes the final call.

How Your Area Is Judged

After EPA sets or revises a standard, it evaluates every region in the country and assigns one of three designations for each pollutant:⁷

Attainment: air quality meets the standard.
Nonattainment: air quality fails the standard, or the area contributes to failure in a neighboring area.
Unclassifiable: EPA lacks enough monitoring data to decide.

Designations are pollutant-specific. The same county can be in attainment for lead and nonattainment for ozone, and the consequences apply only to the pollutant that exceeded the limit.

Ozone’s Severity Tiers

Ozone gets special treatment. Instead of a simple pass/fail, ozone nonattainment areas are sorted into five severity tiers based on how far concentrations exceed the standard: Marginal, Moderate, Serious, Severe, and Extreme.⁸ Higher severity means tighter deadlines and stricter requirements. Extreme areas originally received 20 years to reach attainment; Marginal areas got only 3. The classification also drives the offset ratios new industrial sources must meet and the size threshold that makes a facility a “major source.”

What Happens in a Nonattainment Area

A nonattainment designation is not just a label. It triggers a cascade of regulatory requirements on states, local governments, and businesses.

Emission Offsets for New Industry

Any new or modified major industrial source in a nonattainment area must secure emission offsets: the new emissions it produces must be more than offset by reductions from existing sources in the same area. For ozone, the required ratio escalates with severity:⁹

Marginal: 1.1-to-1 (every ton of new emissions requires 1.1 tons of reductions)
Moderate: 1.15-to-1
Serious: 1.2-to-1
Severe: 1.3-to-1, or 1.2-to-1 if existing sources already use best available control technology
Extreme: 1.5-to-1, or 1.2-to-1 under the same BACT condition

Building the same factory in an Extreme area costs significantly more in offsets than in a Marginal one, a strong economic pull toward cleaner regions or toward reductions at existing plants.

Federal Sanctions on States

When a state fails to submit an adequate compliance plan or fails to implement one that’s been approved, EPA follows a mandatory sanctions sequence. The first sanction lands 18 months after EPA makes a formal finding of deficiency: the emission offset ratio for new sources in the affected area jumps to 2-to-1 regardless of classification.¹⁰ Six months later, highway funding for the nonattainment area can be withheld. EPA can also reverse this order in specific circumstances, applying the highway sanction first.¹¹

If the state still hasn’t corrected the deficiency within 24 months, EPA has a mandatory duty to step in and write a Federal Implementation Plan covering what the state’s plan was supposed to address.¹² Both sanctions and the FIP clock only stop when EPA formally determines the state has fixed the problem.

Section 185 Fees for Persistent Polluters

Major stationary sources in Severe or Extreme ozone nonattainment areas that miss their attainment deadlines owe a per-ton fee on emissions exceeding 80 percent of their baseline. For calendar year 2025, that fee was $12,850.67 per ton of volatile organic compounds and nitrogen oxides, a figure that adjusts annually for inflation.¹³ Even modest excess emissions become six- or seven-figure annual bills at those rates.

What NAAQS Mean for Building or Expanding a Facility

The permitting requirements a facility faces depend on whether it sits in an attainment or nonattainment area and how much it emits.

Attainment Areas: PSD and BACT

In areas that meet the NAAQS, the Prevention of Significant Deterioration program applies to major new or modified stationary sources. For 28 listed source categories such as power plants, petroleum refineries, and cement plants, the major source threshold is 100 tons per year of any regulated pollutant. All other sources trigger PSD review at 250 tons per year.¹⁴ Sources that cross the threshold must install Best Available Control Technology, determined case by case with weight given to energy use, environmental effects, and cost.¹⁵

Nonattainment Areas: LAER and Lower Thresholds

In nonattainment areas, the bar is higher. New or modified major sources must achieve the Lowest Achievable Emission Rate, the most stringent emission limit any comparable source achieves in practice anywhere in the country, without regard to cost.¹⁶ Existing sources must install Reasonably Available Control Technology. BACT balances effectiveness against cost; LAER demands the maximum reduction whatever the expense.

The threshold for what counts as a “major source” also drops. For ozone-related pollutants in Serious nonattainment areas, a source emitting just 50 tons per year triggers major source permitting. In Extreme areas the threshold falls to 10 tons per year.¹⁷ Combined with offset requirements, businesses in severely polluted areas face far more scrutiny than identical operations in clean regions.

Title V Operating Permits

Title V of the Clean Air Act requires most major sources and certain other facilities to hold a comprehensive operating permit. The default threshold is 100 tons per year, with lower cutoffs of 10 tons per year for any single hazardous air pollutant or 25 tons per year for any combination.¹⁷ A Title V permit consolidates all of a facility’s air quality obligations into a single, enforceable document.

How States Are Supposed to Comply

The NAAQS system is a federal-state partnership. EPA sets the concentration limits; each state has to figure out how to meet them. Within three years of any new or revised standard, every state must adopt and submit a State Implementation Plan (SIP) laying out enforceable emission limits, compliance schedules, monitoring programs, and permitting procedures for each air quality control region within its borders.¹⁸

A SIP is not a single document but a collection of regulations, technical analyses, and enforcement programs that grows and shifts as standards change.¹² EPA reviews every submission and must formally approve it. Inadequate or missing plans start the sanctions clock.

Every SIP must also include a “good neighbor” provision preventing sources within the state from emitting pollutants that significantly contribute to nonattainment in another state or interfere with another state’s ability to maintain clean air.¹⁸ Air pollution does not respect state lines, and upwind states can undermine downwind states’ attainment efforts. The provision has generated extensive litigation, particularly around power plant emissions that drift across borders.

When Bad Readings Don’t Count

Wildfires, dust storms, volcanic eruptions, and other natural or extraordinary events can push pollution readings well above NAAQS limits. The Exceptional Events Rule gives states a way to exclude that data from attainment determinations, so an area isn’t penalized for pollution it had no realistic way to prevent.¹⁹ Qualifying events include wildfires, high-wind dust events, prescribed fires on wildlands, and stratospheric ozone intrusions.

The exclusion is not automatic. The state must show the event was not reasonably controllable and that the reading spike was directly caused by it. EPA reviews each request, and the data, the state’s justification, and EPA’s decision are all subject to public disclosure. As wildfire smoke events grow more frequent, the rule has moved from procedural footnote to one of the most practically significant pieces of the NAAQS framework for western and, increasingly, eastern states.

1
eCFR. 40 CFR 50.1 – Definitions
2
Office of the Law Revision Counsel. 42 USC 7408 – Air Quality Criteria and Control Techniques
3
eCFR. 40 CFR Part 50 – National Primary and Secondary Ambient Air Quality Standards
4
U.S. Environmental Protection Agency. NAAQS Table
5
Office of the Law Revision Counsel. 42 USC 7409 – National Primary and Secondary Ambient Air Quality Standards
6
Federal Register. Request for Nominations to the EPA Clean Air Scientific Advisory Committee (CASAC)
7
Environmental Protection Agency. Process to Determine Whether Areas Meet the NAAQS (Designations Process)
8
Office of the Law Revision Counsel. 42 USC 7511 – Classifications and Attainment Dates
9
Office of the Law Revision Counsel. 42 USC Chapter 85, Subchapter I, Part D – Plan Requirements for Nonattainment Areas
10
eCFR. 40 CFR 52.31 – Selection of Sequence of Mandatory Sanctions
11
eCFR. 40 CFR 52.31 – Selection of Sequence of Mandatory Sanctions
12
Environmental Protection Agency. Basic Information about Air Quality State Implementation Plans (SIPs)
13
Environmental Protection Agency. Clean Air Act Section 185 Fee Rates Effective for Calendar Year 2025
14
eCFR. 40 CFR 52.21 – Prevention of Significant Deterioration of Air Quality
15
Legal Information Institute. 42 USC 7479 – Definitions
16
U.S. Environmental Protection Agency. RACT/BACT/LAER Clearinghouse (RBLC) Basic Information
17
U.S. Environmental Protection Agency. Who Has to Obtain a Title V Permit
18
Office of the Law Revision Counsel. 42 USC 7410 – State Implementation Plans for National Primary and Secondary Ambient Air Quality Standards
19
U.S. Environmental Protection Agency. Treatment of Air Quality Monitoring Data Influenced by Exceptional Events