Historical PM2.5 records
Riverside's worst air quality days
Based on the available regulatory-monitor record since 1999, November 5, 1999 ranks as Riverside's most severe daily PM2.5 day: 121.4 µg/m³ (AQI 197, unhealthy), about 5.5 cigarettes in a day.
- Daily rank
- #1
- Period of record
- Since 1999
- Peak daily PM2.5
- 121.4 µg/m³
- About
- 5.5 cigarettes
- Status
- Finalized
Current conditions on the smoke map → Data through Apr 30, 2026 · page built August 26, 2026
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 17 EPA monitors in the Riverside-San Bernardino-Ontario, CA metro area (CBSA). Days after April 30, 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 212 on July 5, 2004 (defining pollutant Ozone).
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 Riverside's worst days?
| Rank | Date | Air event | PM2.5 / cigarettes |
|---|---|---|---|
| #1 | November 5, 1999 | Temperature inversion Possible | 121.4 µg/m³ · 5.5 cigs |
| #2 | October 20, 20002-day stretch | Temperature inversion Possible | 119.6 µg/m³ · 5.4 cigs |
| #3 | October 22, 20052-day stretch | Stagnant pollution mixture Possible | 106.2 µg/m³ · 4.8 cigs |
| #4 | October 26, 20033-day stretch | Wildfire smoke EPA monitor flag | 104.3 µg/m³ · 4.7 cigs |
| #5 | October 19, 2000 | Temperature inversion Possible | 100.2 µg/m³ · 4.6 cigs |
| #6 | November 4, 2001 | Temperature inversion Possible | 98 µg/m³ · 4.5 cigs |
| #7 | January 18, 1999 | Temperature inversion Possible | 97.9 µg/m³ · 4.5 cigs |
| #8 | October 23, 2005 | Stagnant pollution mixture Possible | 95.9 µg/m³ · 4.4 cigs |
| #9 | March 19, 2004 | Temperature inversion Possible | 93.8 µg/m³ · 4.3 cigs |
| #10 | July 5, 2004 | Fireworks EPA monitor flag | 93.4 µg/m³ · 4.2 cigs |
| #11 | October 9, 2000 | Regional secondary PM Possible | 89.8 µg/m³ · 4.1 cigs |
| #12 | April 7, 2000 | Temperature inversion Possible | 89.2 µg/m³ · 4.1 cigs |
| #13 | October 29, 2003 | Wildfire smoke EPA monitor flag | 89.2 µg/m³ · 4.1 cigs |
| #14 | January 1, 2018 | Fireworks EPA monitor flag | 89.1 µg/m³ · 4.0 cigs |
| #15 | October 27, 2003 | Wildfire smoke EPA monitor flag | 88.9 µg/m³ · 4.0 cigs |
What caused Riverside's worst days?
The air-event catalog has named 20 episodes affecting Riverside, 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 September 9–11, 2024 Southern California wildfire-smoke episode
- Fireworks Fireworks episode, 2017-12-31 through 2018-01-02
- Stagnant pollution mixture 2005-10-21 to 2005-10-23 Riverside-San Bernardino-Ontario, CA stagnant mixed-pollution episode
- Fireworks 2005-07-05 fireworks episode affecting Riverside-San Bernardino-Ontario, CA
- Fireworks 2004-07-05 fireworks episode affecting Los Angeles-Long Beach-Anaheim, CA, Riverside-San Bernardino-Ontario, CA
- Temperature inversion 2004-03-19 Oxnard-Thousand Oaks-Ventura, CA and Riverside-San Bernardino-Ontario, CA temperature-inversion pollution episode
14 more, plus every named episode and its evidence is in the air events.
Riverside air season · summer so far, through July 27
Riverside's summer is the 27th-worst of 27 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 | 20031 documented smoke event | 74.4 cig · 29.8 µg/m³ avg | 29.8 | 60.5 | 1 | 97% |
| 2 | 2002 | 72.1 cig · 27.8 µg/m³ avg | 27.8 | 60 | 1 | 100% |
| 3 | 2004 | 67.2 cig · 25.9 µg/m³ avg | 25.9 | 93.4 | 1 | 100% |
| 4 | 2000 | 59.8 cig · 25.3 µg/m³ avg | 25.3 | 47.8 | 0 | 91% |
| 5 | 2017 | 57.6 cig · 22.2 µg/m³ avg | 22.2 | 67.8 | 1 | 100% |
| 6 | 2001 | 57.1 cig · 28.6 µg/m³ avg | 28.6 | 64.2 | 2 | 77% |
| 7 | 2008 | 55.1 cig · 21.3 µg/m³ avg | 21.3 | 54.2 | 0 | 100% |
| 8 | 2009 | 53.3 cig · 20.6 µg/m³ avg | 20.6 | 45.5 | 0 | 100% |
| 9 | 2018 | 52.9 cig · 20.4 µg/m³ avg | 20.4 | 48.5 | 0 | 100% |
| 10 | 2005 | 52.8 cig · 22.8 µg/m³ avg | 22.8 | 79.8 | 1 | 90% |
| 11 | 2010 | 49.9 cig · 19.3 µg/m³ avg | 19.3 | 44 | 0 | 100% |
| 12 | 2013 | 49.6 cig · 19.2 µg/m³ avg | 19.2 | 32.9 | 0 | 100% |
| 13 | 2016 | 49.0 cig · 18.9 µg/m³ avg | 18.9 | 58.8 | 1 | 100% |
| 14 | 2019 | 48.8 cig · 18.9 µg/m³ avg | 18.9 | 60.5 | 1 | 100% |
| 15 | 2014 | 48.8 cig · 18.8 µg/m³ avg | 18.8 | 27.6 | 0 | 100% |
| 16 | 2007 | 48.1 cig · 20.3 µg/m³ avg | 20.3 | 77.5 | 1 | 91% |
| 17 | 2006 | 46.8 cig · 20.2 µg/m³ avg | 20.2 | 42.6 | 0 | 90% |
| 18 | 2015 | 46.5 cig · 17.9 µg/m³ avg | 17.9 | 50.5 | 0 | 100% |
| 19 | 20241 documented smoke event | 46.0 cig · 17.8 µg/m³ avg | 17.8 | 44.5 | 0 | 100% |
| 20 | 2023 | 45.6 cig · 17.6 µg/m³ avg | 17.6 | 45.4 | 0 | 100% |
| 21 | 2021 | 42.2 cig · 16.3 µg/m³ avg | 16.3 | 30.2 | 0 | 100% |
| 22 | 2012 | 42.1 cig · 16.2 µg/m³ avg | 16.2 | 39.9 | 0 | 100% |
| 23 | 2020 | 41.7 cig · 16.1 µg/m³ avg | 16.1 | 36.2 | 0 | 100% |
| 24 | 2011 | 39.9 cig · 15.4 µg/m³ avg | 15.4 | 31 | 0 | 100% |
| 25 | 2022 | 39.5 cig · 15.3 µg/m³ avg | 15.3 | 41.8 | 0 | 100% |
| 26 | 2025 | 39.4 cig · 15.2 µg/m³ avg | 15.2 | 30.1 | 0 | 100% |
| 27 | 2026 | 39.1 cig · 15.4 µg/m³ avg | 15.4 | 27.3 | 0 | 98% |
What this measures, and what it does not
Each bar is the sum of Riverside'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 2003.
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. Excluded on those grounds: 1999. Readings for 2026 after the certified EPA archive are provisional AirNow values and may be revised.
How does it compare?
The record is 2% higher than the next-worst day: 121.4 µg/m³ (November 5, 1999) against 119.6 µg/m³ (October 20, 2000).
How unusual is air like this?
Every measured day near Riverside, by daily PM2.5; the record stands far out on the right. 799 days at "unhealthy for sensitive groups" or worse, 187 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 9,569 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 Riverside?
- November 5, 1999, when daily PM2.5 reached 121.4 µg/m³ (AQI 197), about 5.5 cigarette-equivalents. It ranks first of 9,569 monitored days since 1999.
- What is the highest AQI recorded in Riverside?
- On the worst day, the daily PM2.5 AQI reached 197 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 212 on July 5, 2004.
- 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 Riverside get air this bad?
- Rarely. The record sits at the far end of 9,569 measured days; Riverside averages 7 "unhealthy" days a year.
- 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: riverside.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: Riverside, CA. iaqng.com/records/riverside. Data: U.S. EPA AQS and AirNow. Accessed August 26, 2026.