On May 5, 2024, a day with 10 micrograms of fine particles in every cubic meter of air was officially rated Good. On May 6, the same air was rated Moderate.
Nothing happened to the air. The ruler changed.
That shift came out of a rulemaking most people never heard about, and federal courts are still fighting over it. The number at its center, 9.0 micrograms per cubic meter, decides which counties get labeled, which permits get harder, and what your phone tells you about today.
This article is about the layer underneath the app. Our article on wildfire smoke covers reading a forecast day to day.
The six pollutants the federal government puts a number on
The Clean Air Act directs the Environmental Protection Agency to set national limits for six widespread pollutants, known as criteria pollutants: carbon monoxide, lead, nitrogen dioxide, ozone, particle pollution and sulfur dioxide. Those limits are the National Ambient Air Quality Standards, usually shortened to NAAQS.
Each pollutant gets two kinds of limit. A primary standard is health-based. A secondary standard protects everything else pollution damages: crops, livestock, buildings, visibility.
Here is EPA’s table, as the agency published it with a last-updated date of November 4, 2025.
| Pollutant | Type | Averaging time | Level | Form |
|---|---|---|---|---|
| Carbon monoxide | Primary | 8 hours | 9 ppm | Not to be exceeded more than once per year |
| Carbon monoxide | Primary | 1 hour | 35 ppm | Not to be exceeded more than once per year |
| Lead | Primary and secondary | Rolling 3-month average | 0.15 µg/m³ | Maximum arithmetic mean of 3 consecutive monthly means in a 3-year period |
| Nitrogen dioxide | Primary | 1 hour | 100 ppb | Annual 98th percentile of 1-hour daily maximums, averaged over 3 years |
| Nitrogen dioxide | Primary and secondary | 1 year | 53 ppb | Annual mean |
| Ozone | Primary and secondary | 8 hours | 0.070 ppm | Annual fourth-highest daily maximum 8-hour concentration, averaged over 3 years |
| PM2.5 | Primary | 1 year | 9.0 µg/m³ | Annual mean, averaged over 3 years |
| PM2.5 | Secondary | 1 year | 15.0 µg/m³ | Annual mean, averaged over 3 years |
| PM2.5 | Primary and secondary | 24 hours | 35 µg/m³ | 98th percentile, averaged over 3 years |
| PM10 | Primary and secondary | 24 hours | 150 µg/m³ | Not to be exceeded more than once per year on average over 3 years |
| Sulfur dioxide | Primary | 1 hour | 75 ppb | Annual 99th percentile of 1-hour daily maximums, averaged over 3 years |
| Sulfur dioxide | Secondary | 1 year | 10 ppb | Annual mean, averaged over 3 years |
Three features of that table get overlooked.
The Form column matters as much as the Level column. The 24-hour PM2.5 standard is not a promise that no day exceeds 35. It is the 98th percentile of daily values, averaged across three years, so a monitor can read above 35 on roughly seven days a year and the area still meets the standard. Ozone works the same way.
PM2.5 carries three separate standards that people mix up constantly: a health-based annual limit of 9.0, a welfare-based annual limit of 15.0 that mostly protects visibility, and a daily limit of 35 serving both purposes.
The sulfur dioxide rows changed recently. EPA’s footnote says a 2024 rulemaking added a new annual secondary standard without altering the 1971 secondary 3-hour standard, and the table now carries that annual figure of 10 ppb. It assigns no level to the older 3-hour standard the footnote mentions.
What a standard is, and what it is not
The statute tells the Administrator to set primary standards at a level that, “allowing an adequate margin of safety, are requisite to protect the public health.” That phrase does enormous work.
Two consequences follow, and most readers have them backwards.
A standard is not the concentration at which harm begins. It is a legal line drawn across an uncertain dose-response curve, with a cushion built in because Congress expected the science to be incomplete. Our articles on hazard versus risk and on the hazard-dose-exposure framework work through why a regulatory ceiling and a biological threshold are different objects that happen to share units.
A standard also moves on a calendar, not on the arrival of new evidence. Section 109(d)(1) requires a thorough review of the science and the standards at five-year intervals, and reviews routinely run longer.
One more boundary. NAAQS govern outdoor air in places the public can reach. They do not govern the air inside your house, and they do not govern workplace air, which falls to OSHA under a separate statute with separate numbers.
The number that moved
On February 7, 2024, EPA announced a final rule lowering the primary annual PM2.5 standard from 12.0 micrograms per cubic meter to 9.0. The agency kept the primary 24-hour standard at 35 and left the secondary standards alone.
PM2.5 is the fraction measuring 2.5 micrometers across or less, the size range the standard is written around. EPA’s page on particle health effects says particles under 10 micrometers pose the greatest problems because they reach deep into the lungs, and that exposure can affect both the lungs and the heart. It names who is most likely to be affected: people with heart or lung disease, children, older adults, minority populations and people of low socioeconomic status.
EPA’s overview fact sheet projected the new standard could produce as many as 4,500 avoided premature deaths and net public health benefits as high as $46 billion in 2032.
The part that rewrote your air quality app
Here is the distinction this article exists for.
Air Quality Index categories are fixed. Good, Moderate, Unhealthy for Sensitive Groups, Unhealthy, Very Unhealthy and Hazardous are definitions, and the 2024 rule did not touch them. What the rule touched was the concentration that earns each label.
That mapping sits in federal regulation at 40 CFR Part 58, Appendix G, carrying an amendment citation of 89 FR 16403, dated March 6, 2024. EPA’s fact sheet says the updates took effect May 6, 2024. Here is what moved, in micrograms per cubic meter of 24-hour average PM2.5.
| Category | Old concentration range | Current concentration range |
|---|---|---|
| Good | 0.0 to 12.0 | 0.0 to 9.0 |
| Moderate | 12.1 to 35.4 | 9.1 to 35.4 |
| Unhealthy for Sensitive Groups | 35.5 to 55.4 | 35.5 to 55.4, unchanged |
| Unhealthy | 55.5 to 150.4 | 55.5 to 125.4 |
| Very Unhealthy | 150.5 to 250.4 | 125.5 to 225.4 |
| Hazardous | 250.5 and above | 225.5 and above |
Look where the two most-seen boundaries now sit. The Good-to-Moderate line is 9.0, the level of the annual standard, applied to one day’s average. The Moderate ceiling, 35.4, is essentially the 24-hour standard of 35. EPA anchored the everyday index to the two legal limits, which is tidy, but it means the daily label is calibrated partly against a standard written for a yearly average. The top of the scale moved further than the bottom: air once called Unhealthy at 130 micrograms is now Very Unhealthy.
That produces a consequence almost nobody states. A count of good-air days is not comparable across May 6, 2024. Any chart showing a city’s good days rising or falling through that date is comparing two rulers and reporting the difference as weather.
The court fight, including the part where EPA tried to kill its own rule
States and industry petitioners challenged the 2024 rule in the US Court of Appeals for the D.C. Circuit. The case was argued on December 16, 2024, then sat while a new administration reviewed the rule.
In November 2025, according to the court’s own opinion, EPA filed a motion asking the court to vacate EPA’s rule, and reversed course in two ways: the agency agreed with industry petitioners that it had exceeded its statutory authority and had unreasonably ignored costs.
On June 26, 2026, the D.C. Circuit ruled against everyone. Judges Millett and Childs sat with Senior Judge Ginsburg, who wrote for the court. The opinion concludes: “the petitions for review and the EPA’s motion for vacatur are Denied.” The court held that the Administrator acted within his statutory authority, and that the five-year review provision does not require a thorough review before an off-cycle revision.
The case then went up. On August 28, 2026, the states petitioned the Supreme Court, docketed September 2, 2026 as Kentucky, et al. v. Environmental Protection Agency, et al., No. 26-277, paired with No. 26-298. On October 2, 2026, the federal respondents, meaning EPA, filed a brief supporting the petition against the agency’s own rule. The response deadline has been extended twice, most recently to November 17, 2026, and as of October 20, 2026 the Court had neither granted nor denied review.
So 9.0 is the operative standard today, upheld once, and genuinely unsettled. Any sentence calling it settled should carry a date.
Look up your county today and you get an old map
A new standard does not instantly produce a new map. EPA must designate areas as attainment or nonattainment, and that runs on its own schedule after a rule is final.
EPA’s lookup tool is the Green Book. When this article was written, its landing page said data was current as of April 30, 2025, and its fine-particle area pages were organized around the 2012 PM2.5 standard, with original areas designated April 15, 2015. The PM2.5 2012 page carried a last-updated date of February 26, 2026.
That is the reader trap. Check whether your county is in nonattainment for fine particles right now and you are almost certainly reading a map drawn against 12.0 micrograms, not 9.0. The label is real and its legal consequences are real, but it is not answering the question most people think they are asking.
What the evidence says about harm below the line
The largest study on this question is a federal-scale cohort, and its headline finding runs against intuition.
Di and colleagues, in the New England Journal of Medicine in 2017, built an open cohort of 60,925,443 Medicare beneficiaries across the continental United States from 2000 through 2012, giving 460,310,521 person-years of follow-up. Each 10 microgram per cubic meter increase in annual PM2.5 was associated with a 7.3 percent increase in all-cause mortality, 95 percent confidence interval 7.1 to 7.5.
Then the authors restricted the analysis to person-years spent below 12 micrograms, the standard in force at the time. The association did not vanish. It grew, to 13.6 percent per 10 micrograms, confidence interval 13.1 to 14.1. Risk ran higher among men, among Black beneficiaries, and among people eligible for Medicaid.
A second study supports that shape at finer grain. Wang and colleagues, in Epidemiology in 2017, followed 13.1 million Medicare beneficiaries across seven southeastern states from 2000 to 2013 and found a hazard ratio for death of 1.021 per single microgram increase in annual PM2.5, confidence interval 1.019 to 1.022.
EPA’s own analysts attempted a national burden estimate. Fann and colleagues, in Risk Analysis in 2011, modeled 2005 air quality and estimated 130,000 PM2.5-related deaths and 4,700 ozone-related deaths, with the share of all deaths attributable to the two pollutants ranging from 3.5 percent in San Jose to 10 percent in Los Angeles among the ten most populous counties. That is a modeled burden, produced by applying cohort risk coefficients to population and concentration maps. It is not a count of death certificates.
One limitation runs across all of it. These are observational cohorts, and nobody randomizes people into dirty air. The researchers adjust for income, Medicaid status and area characteristics, and the findings survive, but no adjustment removes everything that travels with living in a polluted place. Well established: the association, its persistence below the old standard, its larger size in poorer and non-white populations. Less settled: the shape of the curve at the lowest concentrations.
What the World Health Organization recommends, and why it is not law
WHO published updated Global Air Quality Guidelines on September 22, 2021. The recommended levels sit well below American standards.
| Pollutant and averaging time | WHO 2005 | WHO 2021 |
|---|---|---|
| PM2.5, annual | 10 µg/m³ | 5 µg/m³ |
| PM2.5, 24-hour | 25 µg/m³ | 15 µg/m³ |
| PM10, annual | 20 µg/m³ | 15 µg/m³ |
| PM10, 24-hour | 50 µg/m³ | 45 µg/m³ |
| Ozone, peak season | none | 60 µg/m³ |
| Ozone, 8-hour | 100 µg/m³ | 100 µg/m³ |
| Nitrogen dioxide, annual | 40 µg/m³ | 10 µg/m³ |
| Nitrogen dioxide, 24-hour | none | 25 µg/m³ |
| Sulfur dioxide, 24-hour | 20 µg/m³ | 40 µg/m³ |
| Carbon monoxide, 24-hour | none | 4 mg/m³ |
The American annual PM2.5 standard of 9.0 is 1.8 times WHO’s guideline of 5, and the daily standard of 35 is a little over twice WHO’s 15. One value moved the other way in 2021: the 24-hour sulfur dioxide guideline doubled.
Do not push the comparison too hard. WHO’s 24-hour guidelines are set at the 99th percentile, allowing three or four exceedance days a year, while the American daily PM2.5 standard is the 98th percentile averaged over three years. Comparable units, different forms.
The legal point matters more. WHO says its air quality guidelines are not standards and not legally binding criteria. They are evidence-based recommendations to national and city governments, and WHO advises weighing local conditions before turning any of them into law. No country is obliged to adopt them, in Europe or anywhere else. WHO’s ambient air fact sheet, dated October 24, 2024, attributes 4.2 million premature deaths worldwide to outdoor air pollution in 2019, when 99 percent of the world’s population lived where guideline levels were not met.
Has American air gotten cleaner?
Yes, substantially, and the headline hides the exceptions.
EPA’s national air quality summary, last updated May 6, 2025, reports that between 1970 and 2023 gross domestic product rose 321 percent, vehicle miles traveled rose 194 percent, energy consumption rose 42 percent and the population grew 63 percent, while total emissions of the six principal pollutants fell 78 percent.
Measured concentrations are more uneven. Against 2023, sulfur dioxide is down 95 percent since 1980, carbon monoxide 88 percent, annual nitrogen dioxide 68 percent. Annual PM2.5 is down 37 percent since 2000 and 15 percent since 2010.
Now the exceptions. Eight-hour ozone is down only 26 percent since 1980 and just 1 percent since 2010. Coarse particles, 24-hour PM10, are down 36 percent since 2000 but have not moved at all since 2010, a flat 0 percent.
And one line in EPA’s table runs the wrong way. The 24-hour PM2.5 measure, built from daily peaks rather than a yearly average, is up 1 percent since 2010. Annual PM2.5 fell 15 percent over the same stretch. The typical day got cleaner while the worst days did not, and the 78 percent headline swallows that whole.
If you live near farmland, unpaved roads, heavy construction or desert dust, PM10 may be the pollutant that matters most where you are, and it is the least covered of the six.
Five things to check before deciding your local air is fine
- Which standard the map is built on. A nonattainment designation you look up today is almost certainly drawn against 12.0, not 9.0.
- Whether the number is annual or daily. Different promises, and a monitor can satisfy one while repeatedly failing the other.
- Whether your trend line crosses May 6, 2024. Good-day counts on either side came from different breakpoints.
- What the Form column says. Most standards are percentiles averaged over three years, so meeting one never means never exceeding it.
- Whether PM2.5 is even your problem. Ozone peaks on hot, stagnant afternoons. PM10 spikes with dust and agriculture.
When the air becomes a medical question
Air quality numbers describe an average across a monitoring area. They are not a readout of what is happening in your chest.
If you have asthma or chronic obstructive pulmonary disease and find yourself using a rescue inhaler more often during hazy stretches, or waking short of breath on nights you would normally sleep through, that pattern is worth a call to your clinician rather than a week of waiting it out. The same goes for a child whose cough returns each summer during ozone season, or an older adult with heart failure whose breathlessness tracks bad air days.
New chest pain, shortness of breath at rest, confusion or blue-tinged lips are emergencies. Call 911.
Nothing here is medical advice, and no air quality reading should decide whether you take or skip a prescribed medicine.
What to take away
Nine point zero is a legal threshold with a date on it. An agency set it, then defended it in court against itself, lost that argument, and now watches a certiorari petition. It is not the concentration at which fine particles stop harming people. The largest American cohort study on the question found the mortality association got stronger below the old standard, not weaker.
So use the number for what it is good for, then look up the two things that are about you: which standard your county is designated against, and what your nearest monitor has been reporting. The standard cannot answer what follows, which is how your own lungs and heart behave on the days the number runs high.
Sources
- U.S. Environmental Protection Agency. “NAAQS Table (last updated 4 November 2025).” epa.gov
- Electronic Code of Federal Regulations. “40 CFR Part 58, Appendix G, Table 2 to Section 3.4: Breakpoints for the AQI (amended 89 FR 16403, 6 March 2024).” ecfr.gov
- U.S. Environmental Protection Agency. “Fact Sheet: Revisions to the Air Quality Index (AQI) for Particulate Matter.” epa.gov
- U.S. Environmental Protection Agency. “Final Rule Overview: Reconsideration of the National Ambient Air Quality Standards for Particulate Matter.” epa.gov
- U.S. Environmental Protection Agency. “Health and Environmental Effects of Particulate Matter (PM) (last updated 23 May 2025).” epa.gov
- U.S. Environmental Protection Agency. “Air Quality – National Summary (last updated 6 May 2025).” epa.gov
- U.S. Environmental Protection Agency. “Green Book: Nonattainment Areas for Criteria Pollutants (data current as of 30 April 2025).” epa.gov
- U.S. Environmental Protection Agency. “Green Book: PM-2.5 (2012) Area Information (last updated 26 February 2026).” epa.gov
- Cornell Law School Legal Information Institute. “42 U.S.C. Sec. 7409, National primary and secondary ambient air quality standards.” law.cornell.edu
- Supreme Court of the United States. “Docket for No. 26-277, Kentucky, et al. v. Environmental Protection Agency, et al..” supremecourt.gov
- Justia (U.S. Court of Appeals for the D.C. Circuit opinion). “Commonwealth of Kentucky v. EPA, No. 24-1050, decided 26 June 2026.” law.justia.com
- World Health Organization (via NCBI Bookshelf). “WHO global air quality guidelines, Table 3.26: Recommended 2021 AQG levels compared with 2005 air quality guidelines.” ncbi.nlm.nih.gov
- World Health Organization. “What are the WHO Air Quality Guidelines? (22 September 2021).” who.int
- World Health Organization. “Ambient (outdoor) air quality and health (fact sheet, 24 October 2024).” who.int
- PubMed. “Di Q, Wang Y, Zanobetti A, et al. "Air Pollution and Mortality in the Medicare Population." N Engl J Med, 2017;376(26):2513-2522. PubMed. DOI: 10.1056/NEJMoa1702747.” doi.org
- PubMed. “Wang Y, Shi L, Lee M, et al. "Long-term Exposure to PM2.5 and Mortality Among Older Adults in the Southeastern US." Epidemiology, 2017;28(2):207-214. PubMed. DOI: 10.1097/EDE.0000000000000614.” doi.org
- PubMed. “Fann N, Lamson AD, Anenberg SC, et al. "Estimating the national public health burden associated with exposure to ambient PM2.5 and ozone." Risk Analysis, 2011;32(1):81-95. PubMed. DOI: 10.1111/j.1539-6924.2011.01630.x.” doi.org