Historical PM2.5 records
Houston's worst air quality days
Based on the available regulatory-monitor record since 1999, January 1, 2020 ranks as Houston's most severe daily PM2.5 day: 110.6 µg/m³ (AQI 190, unhealthy), about 5.0 cigarettes in a day.
- Daily rank
- #1
- Period of record
- Since 1999
- Peak daily PM2.5
- 110.6 µg/m³
- About
- 5.0 cigarettes
- Latest event
- 1 day, January 2026
- Status
- Finalized
Current conditions on the smoke map → Data through Mar 31, 2026 · page built July 26, 2026
Photo: The original uploader was XxTrillvillexX9 at English Wikipedia. (CC BY-SA 2.0)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 25 EPA monitors in the Houston-The Woodlands-Sugar Land, TX metro area (CBSA). Days after March 31, 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 241 on September 13, 2002 (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 Houston's worst days?
| Rank | Date | Daily PM2.5 |
|---|---|---|
| #1 | January 1, 20202-day stretch | 110.6 µg/m³ |
| #2 | January 1, 2024 | 103.1 µg/m³ |
| #3 | January 1, 2019 | 90.8 µg/m³ |
| #4 | January 1, 2026 | 85 µg/m³ |
| #5 | September 13, 20023-day stretch | 57.5 µg/m³ |
| #6 | September 12, 2002 | 56.7 µg/m³ |
| #7 | September 14, 2002 | 53.5 µg/m³ |
| #8 | June 26, 2020 | 50.9 µg/m³ |
| #9 | January 17, 2021 | 47.4 µg/m³ |
| #10 | September 6, 2000 | 46.8 µg/m³ |
| #11 | June 19, 2004 | 46.2 µg/m³ |
| #12 | December 31, 2019 | 45.7 µg/m³ |
| #13 | November 11, 2001 | 45.2 µg/m³ |
| #14 | May 9, 2024 | 44.7 µg/m³ |
| #15 | August 2, 2006 | 44.2 µg/m³ |
What caused Houston's worst days?
The event catalog has named 13 episodes affecting Houston, 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.
- Fireworks 2026-01-01 fireworks episode affecting Houston-The Woodlands-Sugar Land, TX
- Wildfire smoke May 19-27, 2024 Mexico and Central America fire-smoke episode in Texas
- Wildfire smoke May 8-10, 2024 Mexico and Central America fire-smoke episode in Texas
- Fireworks 2024-01-01 Houston-The Woodlands-Sugar Land, TX fireworks episode
- Prescribed fire 2023 March 15 Houston-The Woodlands-Sugar Land prescribed-fire smoke episode
- Dust 2021-01-17 Houston-The Woodlands-Sugar Land dust episode
7 more, plus every named episode and its evidence is in the event catalog.
How does it compare?
The record is 7% higher than the next-worst day: 110.6 µg/m³ (January 1, 2020) against 103.1 µg/m³ (January 1, 2024).
How unusual is air like this?
Every measured day near Houston, by daily PM2.5; the record stands far out on the right. 52 days at "unhealthy for sensitive groups" or worse, 6 at "unhealthy", 0 at "very unhealthy" (2024 EPA thresholds, across 9,616 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 Houston?
- January 1, 2020, when daily PM2.5 reached 110.6 µg/m³ (AQI 190), about 5.0 cigarette-equivalents. It ranks first of 9,616 monitored days since 1999.
- What is the highest AQI recorded in Houston?
- On the worst day, the daily PM2.5 AQI reached 190 (Unhealthy). EPA's official metro AQI, which uses the single worst monitor of any pollutant, peaked at 241 on September 13, 2002.
- 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 Houston get air this bad?
- Rarely. The record sits at the far end of 9,616 measured days; Houston averages about one "unhealthy" day every 5 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: houston.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: Houston, TX. iaqng.com/records/houston. Data: U.S. EPA AQS and AirNow. Accessed July 26, 2026.