Did the 2015 Ozone Standard Work?
EPA design values through 2023–2025 · Updated August 13, 2026 · Methods
Partly, and the two halves point opposite ways. Emissions of the pollutants that form ozone kept falling on schedule. Ambient ozone, the concentration actually measured in the air, did not fall nearly enough to match. Of the areas EPA designated as violating the 2015 standard, 83% of those given until 2021 missed that deadline, and 96% of those given until 2024 missed as well. Of 52 designated areas, 42 are still above the standard, 8 have reached it, and 2 lack enough data to say.
Two different things, and this page keeps them apart.
Emissions are the mass of pollution released: tons of nitrogen oxides out of a power plant stack or a tailpipe. That is what regulation directly controls, and what control programmes are measured on.
Ambient levels are the concentration in the air people actually breathe, in parts per billion. That is what the standard is written in and what legal compliance is judged on.
Ozone is not emitted by anything. It forms in the atmosphere when nitrogen oxides and volatile organic compounds react in sunlight. So the path from cutting emissions to lowering ambient ozone is chemical, indirect, and not proportional. Cutting emissions by half does not halve ozone. For twenty years that distinction did not much matter, because the two moved together. It matters now.
The test: a promise with a date on it
When EPA designated areas as violating the 2015 standard, effective August 2018, the Clean Air Act attached a legal deadline to each one. The category itself encodes EPA's expectation: a “marginal” area is the closest to compliance, expected to get there soonest with the least additional effort.
| Category | Deadline | Judged on | Result |
|---|---|---|---|
| Marginal | August 2021 | 2018–2020 air | 7 of 41 attained |
| Moderate | August 2024 | 2021–2023 air | 1 of 25 attained |
| Serious | August 2027 | 2024–2026 air | not yet due |
Missing a deadline is not a breakdown of the law. The Clean Air Act anticipates it and moves the area to a stricter category with more time. That is the designed remedy. But it is the administrative record that the timetable was not met, and 24 areas were moved up after 2021, including Houston, Denver, Dallas–Fort Worth, Phoenix, Chicago, Detroit, Baltimore, Philadelphia and Washington.
Two things about the timing cut in opposite directions and both should be said. The first window contains 2020, when the pandemic collapsed traffic. If anything that should have helped, and 83% still missed. The second contains 2023, the Canadian wildfire smoke year, which raised ozone in the East; concurred wildfire days are excluded from these design values, but states have to apply for that exclusion and not every affected day is removed.
How much air actually changed
Still furthest above the standard
| Area | Category | 2021–2023 design value (ppm) |
|---|---|---|
| Los Angeles-South Coast Air Basin, CA | Extreme | 0.106 |
| Morongo Band of Mission Indians, CA | Serious | 0.094 |
| Los Angeles-San Bernardino Counties (West Mojave Desert), CA | Severe 15 | 0.090 |
| San Joaquin Valley, CA | Extreme | 0.090 |
| Riverside County (Coachella Valley), CA | Severe 15 | 0.085 |
| Houston-Galveston-Brazoria, TX | Moderate | 0.083 |
| New York-Northern New Jersey-Long Island, NY-NJ-CT | Moderate | 0.082 |
| Dallas-Fort Worth, TX | Moderate | 0.081 |
| Denver Metro/North Front Range, CO | Moderate | 0.081 |
| Phoenix-Mesa, AZ | Moderate | 0.080 |
| El Paso-Las Cruces, TX-NM | Marginal | 0.079 |
| San Diego County, CA | Severe 15 | 0.079 |
A design value is the three-year average of the fourth-highest daily maximum 8-hour ozone concentration, the number compliance is judged on. The standard is met at 0.070 ppm or below.
What EPA promised, and what that promise depended on
EPA's regulatory impact analysis priced the health benefits of the standard at 96 to 160 avoided ozone-related deaths and 230,000 avoided asthma attacks a year by 2025, nationwide outside California. Those figures were explicitly conditional: EPA analysed the benefits of attaining 70 ppb, and stated that its 2025 analysis year “does not predict or prejudge attainment dates.”
Comparing what EPA's own modelling projected against what monitors actually recorded, roughly three-quarters of the population-weighted ozone reduction the benefits were priced on did occur, implying something like 73 to 122 avoided deaths rather than 96 to 160.
That sits oddly beside a 96% deadline-miss rate until you see the size of what was modelled: EPA's scenario represented an ozone change of roughly one part per billion. Delivering three-quarters of one ppb cannot move an area from 78 ppb to 70. The modelled benefits were mostly delivered; the standard was mostly not met. Those are different questions with different answers, and the gap between them is the gap between emissions and ambient levels.
What this page does not claim. It does not say the 70 ppb standard is set at the wrong level; that is a question about health evidence, not about delivery. It does not attribute the ozone improvement that did occur to the standard. The comparison world, in which the standard was never tightened, cannot be observed. And the avoided-death figures rest on a causal determination EPA itself downgraded in 2020, from “likely causal” to “suggestive,” for the link between short-term ozone and mortality.
Sources
- EPA ozone design value reports for 2018–2020 and 2021–2023, Table 1a, including EPA's own “Met NAAQS?” determination. These are used rather than our own calculation because legal attainment excludes concurred wildfire days that public monitor files do not carry.
- EPA Green Book nonattainment area designations and classifications.
- EPA, Regulatory Impact Analysis of the Final Revisions to the Ozone NAAQS, September 2015, Table ES-6 and Appendix 2A.
- Methods and validation.