Houston-The Woodlands-Sugar Land, TX Air Quality
EPA monitor data through 2025 · Updated August 13, 2026 · Methods
In 2025, Houston-The Woodlands-Sugar Land, TX recorded 46 days above an Air Quality Index of 100, the level at which EPA begins warning sensitive groups, out of 365 days with monitoring data. Most of those bad days were driven by ozone. Under the AQI rules that applied before 2013, the same air would have scored 33 days. Since 2000, unhealthy days are 55% lower on a consistent definition.
Unhealthy days by year, scored two ways
EPA changed what an AQI of 100 means in 2013 and again in 2015, so a count published before those changes is not comparable to one published after. Each panel below applies a single fixed rulebook to every year, so each is internally consistent. Why this matters.
Scored by today's rules
Scored by the pre-2013 rules
Ozone since 1990
The index counts above only start in 2000, because nationwide fine-particle monitoring did not exist before 1999 and an index without it would measure the monitoring network rather than the air. Ozone does have a 35-year record. This is Houston-The Woodlands-Sugar Land, TX's, from EPA's long-running trend monitors: 119 ppb in 1990 to 83 ppb in 2024, 30% lower.
Peak ozone, 1990 to 2024
What 2025 looked like
| Air quality category | Days | Share of monitored days |
|---|---|---|
| Good | 37 | 10% |
| Moderate | 282 | 77% |
| Unhealthy for sensitive groups | 35 | 10% |
| Unhealthy or worse | 11 | 3% |
Median AQI 63 · 90th-percentile AQI 112 · worst day 179. Scored under current rules.
What drove the unhealthy days
Of the 46 days above 100 in 2025, 44 were driven by ozone and 0 by fine particles, with 2 from another pollutant. The two behave differently. Ozone peaks on hot sunny afternoons; fine particles now spike mainly with wildfire smoke. In the eastern United States, ozone has stopped responding to emission cuts: nitrogen oxide emissions kept falling after 2014 and ambient ozone barely moved.
The unhealthy days in 2025
Every day the index passed 100, what it reached, and which pollutant drove it. 13 of these 46 would not have counted as unhealthy under the pre-2013 rules.
| Date | Peak AQI | Driven by | Pre-2013 score |
|---|---|---|---|
| Jan 17 | 168 | Coarse particles | 168 |
| Feb 1 | 140 | Ozone | 116 |
| Mar 1 | 179 | Ozone | 154 |
| Mar 15 | 169 | Coarse particles | 169 |
| Mar 25 | 136 | Ozone | 114 |
| Apr 9 | 126 | Ozone | 106 |
| Apr 10 | 133 | Ozone | 111 |
| Apr 12 | 133 | Ozone | 111 |
| Apr 16 | 105 | Ozone | 90 (not unhealthy) |
| May 7 | 105 | Ozone | 90 (not unhealthy) |
| May 12 | 140 | Ozone | 116 |
| May 29 | 126 | Ozone | 106 |
| Jun 2 | 126 | Ozone | 106 |
| Jun 9 | 105 | Ozone | 90 (not unhealthy) |
| Jul 24 | 161 | Ozone | 137 |
| Jul 29 | 172 | Ozone | 147 |
| Jul 30 | 156 | Ozone | 132 |
| Aug 3 | 159 | Ozone | 135 |
| Aug 4 | 112 | Ozone | 97 (not unhealthy) |
| Aug 8 | 101 | Ozone | 87 (not unhealthy) |
| Aug 18 | 122 | Ozone | 104 |
| Aug 19 | 108 | Ozone | 93 (not unhealthy) |
| Aug 20 | 161 | Ozone | 137 |
| Aug 21 | 129 | Ozone | 109 |
| Aug 25 | 147 | Ozone | 122 |
| Aug 26 | 119 | Ozone | 101 |
| Sep 3 | 108 | Ozone | 93 (not unhealthy) |
| Sep 4 | 122 | Ozone | 104 |
| Sep 9 | 140 | Ozone | 116 |
| Sep 10 | 150 | Ozone | 124 |
| Sep 12 | 147 | Ozone | 122 |
| Sep 13 | 151 | Ozone | 127 |
| Sep 14 | 115 | Ozone | 100 (not unhealthy) |
| Sep 15 | 154 | Ozone | 129 |
| Sep 16 | 112 | Ozone | 97 (not unhealthy) |
| Sep 17 | 140 | Ozone | 116 |
| Sep 18 | 136 | Ozone | 114 |
| Sep 19 | 101 | Ozone | 87 (not unhealthy) |
| Sep 29 | 112 | Ozone | 97 (not unhealthy) |
| Oct 1 | 151 | Ozone | 127 |
| Oct 2 | 126 | Ozone | 106 |
| Oct 11 | 112 | Ozone | 97 (not unhealthy) |
| Oct 12 | 101 | Ozone | 87 (not unhealthy) |
| Oct 13 | 133 | Ozone | 111 |
| Oct 14 | 143 | Ozone | 119 |
| Oct 15 | 136 | Ozone | 114 |
Statewide context
Houston-The Woodlands-Sugar Land, TX sits in Texas.
The full series
| Year | Unhealthy days, current rules | Pre-2013 rules | Days monitored |
|---|---|---|---|
| 2000 | 88 | 74 | 366 |
| 2001 | 84 | 68 | 365 |
| 2002 | 67 | 56 | 365 |
| 2003 | 78 | 65 | 365 |
| 2004 | 74 | 62 | 366 |
| 2005 | 100 | 81 | 365 |
| 2006 | 78 | 60 | 365 |
| 2007 | 54 | 44 | 365 |
| 2008 | 46 | 31 | 366 |
| 2009 | 32 | 29 | 365 |
| 2010 | 43 | 34 | 365 |
| 2011 | 57 | 38 | 365 |
| 2012 | 48 | 33 | 366 |
| 2013 | 24 | 18 | 365 |
| 2014 | 14 | 7 | 365 |
| 2015 | 45 | 30 | 365 |
| 2016 | 23 | 12 | 366 |
| 2017 | 25 | 20 | 365 |
| 2018 | 35 | 18 | 365 |
| 2019 | 31 | 24 | 365 |
| 2020 | 24 | 14 | 366 |
| 2021 | 30 | 19 | 365 |
| 2022 | 39 | 20 | 365 |
| 2023 | 57 | 46 | 365 |
| 2024 | 63 | 38 | 366 |
| 2025 | 46 | 33 | 365 |
Sources
- EPA AirData daily monitor files, recomputed under both breakpoint tables. Methods and validation.
- Download the full dataset for every state and metro.