Findings

Cold, sparsely populated states hold the cleanest air

The clearest pattern over the five years is geographic. Alaska averaged good air on 89.33% of days across 2020-2024, and it gets there the way the cleanest places usually do: few people, little heavy industry, and a median air-quality reading near the floor of the index. North Slope Borough logged just 10 days above the unhealthy threshold across the entire 2020 to 2024 span, a level most urban counties pass in a single bad fire week. Where population thins out and the wind keeps moving, the good-day share stays high year after year.

Warmer, denser, more industrial regions sit lower. Alabama, for instance, averaged 75.44% good days across 2020-2024, a respectable figure but one weighed down by summer ozone and fine-particle pollution that the colder northern states rarely accumulate. The gap between a place like Alaska and a Gulf Coast state is not noise. It reflects real differences in climate, traffic, and the mix of nearby emission sources.

The national trend drifted upward, but unevenly

Pooling every monitored county and metro, the nationwide good-day share rose modestly over the window. That gentle climb hides a lot of churn underneath it. Coastal and high-latitude areas held steady near their ceilings, while several Western counties swung hard in wildfire years and then recovered. Averaging these opposite movements produces a calm national line that no single county actually experienced, which is the usual hazard of a country-wide number.

Fine particles do most of the damage to the good-day count. When PM2.5 days pile up, whether from smoke, traffic, or stagnant summer air, the good-day percentage falls in step. That is why the cleanest states share a profile of low particle exposure, and why the next chart ranks states by the share of days that stayed in the good range. Read it alongside the caveat that two thirds of U.S. counties have no monitor at all, so the ranking describes the places we can actually measure.

Comparative jurisdictional notes

State Implementation Plans (SIP) and federal preemption

Under the Clean Air Act (CAA), each state submits a State Implementation Plan (SIP) to the EPA documenting how the state will achieve and maintain National Ambient Air Quality Standards (NAAQS) for the six criteria pollutants. SIPs incorporate emissions inventories, control measures, and demonstration of attainment timelines. When EPA determines a SIP is inadequate, the agency may impose a Federal Implementation Plan (FIP) under CAA section 110(c). California operates the country's only state-level mobile-source program, granted authority under CAA section 209 waivers issued by EPA, over 100 waivers have been issued since 1968 covering tailpipe standards, zero-emission vehicle mandates, and Low-NOx fleet rules. Other states adopt California standards under section 177 piggyback authority.

Nonattainment area classifications (marginal, moderate, serious, severe, extreme for ozone; moderate or serious for PM2.5 and PM10) trigger progressively stricter permitting, offset ratios, and Reasonable Available Control Technology (RACT) requirements. The South Coast Air Basin (Los Angeles), San Joaquin Valley, and Houston-Galveston-Brazoria are designated extreme or severe ozone nonattainment under the 2015 8-hour standard. New York, New Jersey, and Connecticut sit in the Ozone Transport Region (OTR), a multistate nonattainment zone established by the 1990 CAA Amendments under section 184.

Cross-state pollution and the Good Neighbor framework

The Cross-State Air Pollution Rule (CSAPR), promulgated 2011 and revised through CSAPR Update Rule (2016), Revised CSAPR Update (2021), and Good Neighbor Plan (2023), addresses upwind state contributions to downwind nonattainment under CAA section 110(a)(2)(D)(i)(I). EPA quantifies each upwind state's significant contribution to ozone or PM2.5 violations in downwind states using photochemical air-quality modeling (CAMx, CMAQ) and sets emissions budgets for power plants and industrial sources. The 2023 Good Neighbor Plan also covers non-EGU sources (cement kilns, iron and steel mills, glass furnaces, boilers, engines) for the first time.

The Mercury and Air Toxics Standards (MATS), finalized 2012 under CAA section 112, set technology-based limits for mercury, acid gases, and non-mercury heavy metals from coal- and oil-fired electric utilities. The Regional Haze Rule, established 1999 and revised 2017, requires states to demonstrate progress toward natural visibility conditions in 156 Class I federal areas (national parks and wilderness areas larger than 5,000 acres or 6,000 acres, respectively) by 2064. Best Available Retrofit Technology (BART) determinations apply to facilities built between 1962 and 1977.

Air-quality reference notes

Monitoring infrastructure and measurement methods

The Air Quality System (AQS) is the federal repository administered by EPA Office of Air Quality Planning and Standards (OAQPS) at Research Triangle Park, North Carolina. Submissions arrive from approximately 4,000 monitors operated by state, local, and tribal (SLT) air agencies under 40 CFR Part 58. Each criteria pollutant has prescribed Federal Reference Methods (FRM) and Federal Equivalent Methods (FEM) per 40 CFR Part 53. PM2.5 sampling uses gravimetric FRM samplers (Rupprecht & Patashnick Partisol-Plus, BGI PQ200, Thermo Andersen RAAS) for 24-hour integrated mass measurement. Continuous PM2.5 monitoring uses Tapered Element Oscillating Microbalance (TEOM), Beta Attenuation Monitors (BAM), or Synchronized Hybrid Ambient Real-time Particulate (SHARP) instruments with FEM designation.

Photochemical Assessment Monitoring Stations (PAMS) operate in serious-or-worse ozone nonattainment areas, providing speciated volatile organic compound (VOC) measurements (60 target compounds per the PAMS Type 2 site requirement) plus carbonyls and nitrogen oxides. The National Core Network (NCore) integrates 80+ multipollutant sites measuring trace-level NOx, ammonia, sulfur dioxide, ozone, PM2.5, PM2.5 speciation, PM10, and meteorology. AirNow is EPA's real-time AQI dissemination system pulling hourly data from over 4,000 monitors plus partner-agency feeds (NPS, USFS, BLM, tribal governments). The Federal Equivalent Method designation process at EPA Air & Energy Engineering Research Laboratory in RTP requires field-test demonstration of agreement within plus-or-minus 10 percent of the FRM.

Hazardous Air Pollutants (HAPs) and emission standards

The 1990 CAA Amendments Title III lists 187 Hazardous Air Pollutants (HAPs, also called Air Toxics) including benzene, formaldehyde, 1,3-butadiene, chromium VI, mercury, and polycyclic organic matter. EPA promulgates National Emission Standards for Hazardous Air Pollutants (NESHAP) under CAA sections 112(d) and 112(f) covering 174 source categories, codified at 40 CFR Parts 61 and 63, establishing Maximum Achievable Control Technology (MACT) floors based on the average emissions limitation achieved by the best-performing 12 percent of existing sources in the source category. Generally Available Control Technology (GACT) applies to area sources (smaller facilities). Residual risk reviews under section 112(f)(2) follow MACT compliance by 8 years.

New Source Performance Standards (NSPS) at 40 CFR Part 60 set technology-based emission limits for new, modified, or reconstructed sources in 80+ categories using Best System of Emission Reduction (BSER) determinations. Major-source Prevention of Significant Deterioration (PSD) permitting in attainment areas requires Best Available Control Technology (BACT) per CAA section 165, while major-source New Source Review (NSR) permitting in nonattainment areas requires Lowest Achievable Emission Rate (LAER) plus emission offsets per CAA sections 173 and 182. Title V operating permits under CAA Title V apply to major sources and document all applicable requirements in a single federally enforceable document; over 16,000 Title V permits are issued nationwide.

Control technology, vehicle standards, and combustion engineering

Stationary-source pollution control includes wet scrubbers (Venturi, packed bed, spray tower) for particulate and acid-gas removal, electrostatic precipitators (ESP) for PM control achieving 99.9-percent collection efficiency, baghouse fabric filters for fine particulate, catalytic oxidizers and regenerative thermal oxidizers (RTO) for VOC destruction, selective catalytic reduction (SCR) using ammonia or urea injection over a vanadium-titania catalyst for NOx control achieving 80-90 percent reduction, selective non-catalytic reduction (SNCR) injecting ammonia or urea at 870-1,150 degrees C, low-NOx burners (LNB), flue-gas recirculation, and overfire air staging.

Mobile-source standards include EPA Tier 3 light-duty vehicle and gasoline sulfur standards (10-ppm sulfur cap effective 2017), heavy-duty engine standards (0.20-g/bhp-hr NOx, 0.01-g/bhp-hr PM under the 2027 Phase 3 GHG/Criteria Pollutant Rule), Reformulated Gasoline (RFG) program in ozone nonattainment areas, oxygenated fuels program (carbon monoxide nonattainment), Ultra-Low-Sulfur Diesel (ULSD) at 15-ppm cap, and Marine Diesel Emission Control Areas (ECA) under MARPOL Annex VI Regulation 14 for ships within 200 nautical miles of US/Canadian coasts. The Greenhouse Gas Reporting Program (GHGRP) under 40 CFR Part 98 collects emissions from facilities exceeding 25,000 metric tons CO2-equivalent annually.

Health-effects evidence base

EPA Integrated Science Assessments (ISA) for criteria pollutants synthesize epidemiological and toxicological evidence supporting NAAQS revision. The 2019 Particulate Matter ISA documented causal relationships between long-term PM2.5 exposure and cardiovascular mortality, all-cause mortality, lung cancer, and respiratory effects. Long-term cohort studies (American Cancer Society Cancer Prevention Study II, Harvard Six Cities Study, Medicare Cohort, NIH-AARP Diet and Health Study) provide concentration-response functions used in EPA Benefits Mapping and Analysis Program (BenMAP) cost-benefit analyses. The Krewski et al. reanalysis (2009) of ACS CPS-II established the canonical 6-percent all-cause mortality risk per 10-microgram-per-cubic-meter PM2.5 increment.

The Office of Research and Development (ORD) Health and Environmental Effects Assessment Laboratory conducts controlled human exposure studies at the EPA Human Studies Facility on the University of North Carolina-Chapel Hill campus, examining cardiovascular and respiratory responses to ozone, PM2.5, diesel exhaust, and wood smoke. Human Exposure Modeling Group develops the Air Pollutants Exposure Model (APEX) used in NAAQS Risk and Exposure Assessment (REA). Multi-Pollutant Exposure Model (MPEM) integrates outdoor concentrations from EPA Community Multiscale Air Quality (CMAQ) modeling with indoor infiltration and time-activity patterns from the Consolidated Human Activity Database (CHAD).

For full methodological documentation including data lineage, monitor-network assumptions, and AQI computation steps, see the methodology page.