Historical PM2.5 records
Denver's worst air quality days
Based on the available regulatory-monitor record since 1999, May 19, 2023 ranks as Denver's most severe daily PM2.5 day: 95.6 µg/m³ (AQI 179, unhealthy), about 4.3 cigarettes in a day.
- Daily rank
- #1
- Period of record
- Since 1999
- Peak daily PM2.5
- 95.6 µg/m³
- About
- 4.3 cigarettes
- Status
- Finalized
Current conditions on the smoke map → Data through May 2, 2026 · page built August 26, 2026
Photo: Quintin Soloviev (CC BY 4.0)How this is measured sources, the daily measure, and what this ranking is not
Source. Daily PM2.5 from the U.S. EPA Air Quality System (AQS) , back to 1999, across 19 EPA monitors in the Denver-Aurora-Lakewood, CO metro area (CBSA). Days after May 2, 2026 are preliminary AirNow readings, marked provisional, and re-rank once EPA certifies them (usually the following spring).
The daily measure. "Worst area" is the metro's highest credible monitor each day, with lone bad-monitor spikes dropped (the raw single-monitor max before that filter is in the data but not charted). "Metro average" averages every monitor. The unit choice re-reads the same value as µg/m³, cigarette-equivalents (PM2.5 ÷ 22, Berkeley Earth, an illustration rather than a medical reading), or the EPA 2024 PM2.5 AQI.
Official AQI. EPA's reported metro AQI uses the single worst monitor of any pollutant, so it can run higher than the values here. The highest official daily AQI among the ranked days is 179 on May 19, 2023 (defining pollutant PM2.5).
What this ranking is not. It does not attribute days to wildfire smoke: a high-PM2.5 day can be dust, fireworks, an inversion, or industry. The wildfire-smoke event ranking below arrives with a published attribution method.
What were Denver's worst days?
| Rank | Date | Air event | PM2.5 / cigarettes |
|---|---|---|---|
| #1 | May 19, 20232-day stretch | Wildfire smoke Agency determination | 95.6 µg/m³ · 4.3 cigs |
| #2 | February 15, 20012-day stretch | Temperature inversion Possible | 68.4 µg/m³ · 3.1 cigs |
| #3 | February 17, 2001 | Temperature inversion Possible | 68 µg/m³ · 3.1 cigs |
| #4 | August 8, 20212-day stretch | Wildfire smoke Agency determination | 64.8 µg/m³ · 2.9 cigs |
| #5 | September 4, 2017 | Wildfire smoke EPA monitor flag | 62.3 µg/m³ · 2.8 cigs |
| #6 | May 20, 2023 | Wildfire smoke Agency determination | 60.7 µg/m³ · 2.8 cigs |
| #7 | February 8, 20072-day stretch | Temperature inversion Possible | 60.5 µg/m³ · 2.8 cigs |
| #8 | March 6, 2019 | Winter inversion Corroborated | 58.6 µg/m³ · 2.7 cigs |
| #9 | August 7, 2021 | Wildfire smoke Agency determination | 57.1 µg/m³ · 2.6 cigs |
| #10 | February 10, 2014 | Stagnant pollution mixture Likely | 57 µg/m³ · 2.6 cigs |
| #11 | March 20, 2021 | Other Corroborated | 55.7 µg/m³ · 2.5 cigs |
| #12 | August 20, 2018 | Wildfire smoke Corroborated | 55.4 µg/m³ · 2.5 cigs |
| #13 | February 10, 2007 | Cause not established after standard review Review complete | 53.6 µg/m³ · 2.4 cigs |
| #14 | November 29, 2001 | Cause not established after standard review Review complete | 53.4 µg/m³ · 2.4 cigs |
| #15 | July 7, 2015 | Wildfire smoke High confidence | 53 µg/m³ · 2.4 cigs |
What caused Denver's worst days?
The air-event catalog has named 15 episodes affecting Denver, each resolved to a cause only where retained evidence supports it: EPA monitor qualifiers, NWS alerts, agency statements, and reporting. The ranking above stays cause-neutral; this is where the causes live.
- Wildfire smoke 2023 mid-May Canadian wildfire-smoke episode
- Wildfire smoke 2021-08-07 2-area wildfire-smoke episode
- Other 2021-03-19 Denver-Aurora-Lakewood other-cause episode
- Wildfire smoke 2020-09-17 2-area wildfire-smoke episode
- Wildfire smoke 2020-09-06 2-area wildfire-smoke episode
- Wildfire smoke 2020-08-22 2-area wildfire-smoke episode
9 more, plus every named episode and its evidence is in the air events.
Denver air season · summer so far, through July 27
Denver's summer is the 19th-worst of 28 on record.
The summer season in every year since 1999, measured over the same window, June 1 to July 27, so each is compared at the same point. All fine particulate the metro's monitors recorded, from every source.
Where 2026 stands
The race, summer by summer
Every summer on record
| Rank | Season | Cigs | Average µg/m³ | Worst day | Unhealthy days | Coverage |
|---|---|---|---|---|---|---|
| 1 | 2002 | 31.1 cig · 12.4 µg/m³ avg | 12.4 | 42.5 | 0 | 97% |
| 2 | 20211 documented smoke event | 30.8 cig · 11.9 µg/m³ avg | 11.9 | 25.7 | 0 | 100% |
| 3 | 2005 | 26.6 cig · 10.3 µg/m³ avg | 10.3 | 19.7 | 0 | 100% |
| 4 | 20181 documented smoke event | 26.6 cig · 10.3 µg/m³ avg | 10.3 | 17.1 | 0 | 100% |
| 5 | 2006 | 26.6 cig · 10.3 µg/m³ avg | 10.3 | 36.7 | 0 | 100% |
| 6 | 2008 | 26.4 cig · 10.2 µg/m³ avg | 10.2 | 25.9 | 0 | 100% |
| 7 | 20171 documented smoke event | 25.9 cig · 10 µg/m³ avg | 10 | 18 | 0 | 100% |
| 8 | 2013 | 25.7 cig · 9.9 µg/m³ avg | 9.9 | 21 | 0 | 100% |
| 9 | 2014 | 25.6 cig · 9.9 µg/m³ avg | 9.9 | 22.6 | 0 | 100% |
| 10 | 2011 | 25.4 cig · 9.8 µg/m³ avg | 9.8 | 20.9 | 0 | 100% |
| 11 | 2003 | 25.4 cig · 10.8 µg/m³ avg | 10.8 | 18.4 | 0 | 91% |
| 12 | 2007 | 25.4 cig · 9.8 µg/m³ avg | 9.8 | 16.7 | 0 | 100% |
| 13 | 2015 | 25.2 cig · 9.7 µg/m³ avg | 9.7 | 53 | 0 | 100% |
| 14 | 2024 | 24.7 cig · 9.5 µg/m³ avg | 9.5 | 35.8 | 0 | 100% |
| 15 | 2012 | 24.4 cig · 9.4 µg/m³ avg | 9.4 | 31.5 | 0 | 100% |
| 16 | 2016 | 24.2 cig · 9.4 µg/m³ avg | 9.4 | 15.8 | 0 | 100% |
| 17 | 20203 documented smoke events | 23.6 cig · 9.1 µg/m³ avg | 9.1 | 24.4 | 0 | 100% |
| 18 | 2001 | 23.3 cig · 9.2 µg/m³ avg | 9.2 | 17.5 | 0 | 98% |
| 19 | 2026 | 22.2 cig · 8.7 µg/m³ avg | 8.7 | 26 | 0 | 98% |
| 20 | 2022 | 22.2 cig · 8.6 µg/m³ avg | 8.6 | 22.7 | 0 | 100% |
| 21 | 2000 | 21.9 cig · 9.1 µg/m³ avg | 9.1 | 14.5 | 0 | 93% |
| 22 | 20231 documented smoke event | 21.5 cig · 8.3 µg/m³ avg | 8.3 | 18.5 | 0 | 100% |
| 23 | 2010 | 21.2 cig · 8.2 µg/m³ avg | 8.2 | 13 | 0 | 100% |
| 24 | 2025 | 21.2 cig · 8.2 µg/m³ avg | 8.2 | 15.4 | 0 | 100% |
| 25 | 2009 | 20.9 cig · 8 µg/m³ avg | 8 | 14.4 | 0 | 100% |
| 26 | 2004 | 20.4 cig · 9 µg/m³ avg | 9 | 16 | 0 | 88% |
| 27 | 2019 | 20.2 cig · 7.8 µg/m³ avg | 7.8 | 22.8 | 0 | 100% |
| 28 | 1999 | 13.5 cig · 8.2 µg/m³ avg | 8.2 | 17.9 | 0 | 63% |
What this measures, and what it does not
Each bar is the sum of Denver's robust metro daily PM2.5 readings across the summer season through July 27, in µg/m³-days, divided by 22 µg/m³ to give cigarette-equivalents. Earlier seasons are cut at the same point in the season, so this is like-for-like rather than full-season totals. The cigarette figure is an illustration of exposure, not a medical equivalence.
It counts all fine particulate the monitors recorded, whatever produced it: wildfire smoke, traffic, industry, fireworks. It is not a smoke-only measure. The documented smoke events shown alongside explain particular spikes; they do not define the ranking, and that catalog only begins in 2017.
Context for reading the ranking: regional fine-particulate levels have fallen substantially since the early 2000s, which is why seasons from that era rank high on season totals while recent seasons rank high on single-day peaks.
A season is ranked only where monitors reported on at least 60% of the window's days, since below that a low total would reflect missing instruments rather than clean air. No season currently falls below that threshold. Readings for 2026 after the certified EPA archive are provisional AirNow values and may be revised.
How does it compare?
The record is 40% higher than the next-worst day: 95.6 µg/m³ (May 19, 2023) against 68.4 µg/m³ (February 15, 2001).
How unusual is air like this?
Every measured day near Denver, by daily PM2.5; the record stands far out on the right. 79 days at "unhealthy for sensitive groups" or worse, 11 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 9,762 measured days).
Year by year (1999–2026)
Worst single day each year, by the selected measure.
Why did reported numbers differ?
During a major event, AirNow, weather apps, news reports, and neighborhood monitors can all show different figures at the same moment. All of them can be right:
- Concentration vs AQI. µg/m³ is the measurement; AQI is a 0 to 500 index derived from it. "330 µg/m³" and "AQI 500" can describe the same air.
- Hourly vs NowCast vs daily. An hourly peak, the NowCast rolling estimate apps display, and the full-day average answer different questions; this page ranks full days.
- One monitor vs a composite. A single sensor can read far above or below a citywide figure. The measure toggle above shows both framings.
- Live vs certified. Real-time feeds get corrected when EPA certifies data months later, which is why recent days here carry a provisional label.
What about indoors?
Citywide monitors can tell you what happened outside. They cannot tell you what happened inside a particular home, school, or facility: filtration, ventilation, and building operation decide that. During and after days like the ones above, outdoor particles become an indoor load question.
Check current conditions and the next clean-air window → Estimate recent filter load where you are →
Common questions
- What was the worst air-quality day in Denver?
- May 19, 2023, when daily PM2.5 reached 95.6 µg/m³ (AQI 179), about 4.3 cigarette-equivalents. It ranks first of 9,762 monitored days since 1999.
- What is the highest AQI recorded in Denver?
- On the worst day, the daily PM2.5 AQI reached 179 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 179 on May 19, 2023.
- Were these days caused by wildfire smoke?
- This page ranks measured PM2.5 without assigning a cause: a high day can be smoke, fireworks, dust, an inversion, or industry. A separate wildfire-smoke event ranking, using a published attribution method with its version disclosed, is the next layer of this catalog.
- How often does Denver get air this bad?
- Rarely. The record sits at the far end of 9,762 measured days; Denver averages about one "unhealthy" day every 3 years.
- Why did different apps show different numbers that day?
- Concentration (µg/m³) and AQI are different scales; hourly, NowCast, and daily values answer different questions; a single monitor differs from a citywide composite; and live feeds get revised when EPA certifies data. This page uses the certified daily record, with the provisional tail labeled.
Data and citation
Ranked days for both measures are downloadable, with units and status flags: denver.csv. Source: U.S. EPA Air Quality System (AQS) daily PM2.5, with preliminary AirNow readings for the uncertified tail. Cite as:
IAQng Air Records: Denver, CO. iaqng.com/records/denver. Data: U.S. EPA AQS and AirNow. Accessed August 26, 2026.