Houston Air quality today

What is ozone, and how does it form over Houston?

Ground-level ozone is a harmful gas and the main ingredient in smog ^[5]. It is not released directly into the air ^[5]. It forms when two kinds of pollution, nitrogen oxides and volatile organic compounds, react in sunlight ^[5]. These come from cars, power plants, refineries, chemical plants and other sources ^[5].

In the Houston area, high ozone usually happens on hot, sunny, dry days with slow winds that circle back over the region ^[3]. Studies cited by the state air agency link the highest ozone to still winds and a land and sea breeze that turns through all directions over a day ^[3]. Slow winds let ozone and the pollution that forms it build up ^[3].

What role does the Houston Ship Channel play?

The Houston Ship Channel is an area with a large amount of industrial activity ^[3]. The Texas Commission on Environmental Quality, the state air agency, calls it a large source of industrial volatile organic compounds ^[3]. Its emissions combine with emissions from the city to create ozone at monitors downwind ^[3].

Some of these compounds are called highly reactive, because they are more likely to make large amounts of ozone ^[3]. Texas regulates six of them, including ethylene, propylene and 1,3-butadiene ^[3]. The state agency says cutting these compounds is likely to do more for ozone than equal cuts in less reactive ones ^[3]. It also says cutting nitrogen oxides is likely to do more to lower the ozone figure used to judge the area against the standard ^[3].

A University of Houston study looked at two ozone episodes in September 2021 ^[4]. It found that most ozone hotspots were over the urban core and industrial districts like the Ship Channel, where pollution from industry and vehicles was high ^[4].

Does all of Houston's ozone come from Houston?

No. The state agency says the area makes much of its own ozone, but some high ozone days come from air carried in from the north and northeast ^[3]. In the University of Houston study of September 2021, about 63 percent of the extra ozone in those episodes was carried in from the central and northern parts of the country ^[4]. About 37 percent came from chemistry over the Houston area itself ^[4]. The researchers said local emission control is still critical ^[4].

When is ozone worst in Houston?

Officially, Houston's ozone season runs all year ^[3]. But from 2012 through 2022, no high ozone days happened outside March through October ^[3]. High ozone days peak in May and June and again in August and September, with a dip in July ^[3]. The state agency says the July dip across much of Texas is likely caused by air flowing in from the Gulf of Mexico, which carries less ozone ^[3].

In general, ozone is usually worse in the afternoon and early evening, according to the EPA's AirNow guide ^[6].

How does Houston compare with other US cities?

In the American Lung Association's 2026 State of the Air report, based on 2022 to 2024 data, the Houston-Pasadena area ranked 6th worst in the US for ozone ^[1]. Harris County, its worst county, averaged 43.3 unhealthy ozone days a year and got an F grade ^[1]. In the 2025 report, the area ranked 7th worst, with 34.8 days a year ^[1].

The area also has a problem with fine particle pollution, tiny bits of soot from sources such as diesel engines and coal-fired power plants ^[1]. In the 2026 report it ranked 8th worst in the US for year-round particle levels ^[1]. Harris County failed that measure because its levels were above the federal standard ^[1]. For short spikes in particles, the area ranked 89th worst ^[1].

What is being done about Houston's ozone?

When an area does not meet a national air quality standard, the EPA calls it a nonattainment area, and the state must write a plan to clean the air ^[5]. In October 2015, the EPA lowered the ozone standard to 0.070 parts per million ^[2]. In 2018, six counties around Houston, including Harris, were designated as not meeting it ^[2].

In 2022, the EPA moved the area to a stricter class, moderate ^[2]. In June 2024, it moved the area up again to serious, with a deadline of August 3, 2027 ^[2]. Under an older, looser standard from 2008, set at 0.075 parts per million, an eight-county Houston area was rated severe in 2022 ^[2].

There has been progress. From 2012 through 2022, ozone levels in the area fell by 11 percent ^[3]. In 2012, only one monitor met the 2008 standard; by 2022, only one monitor was above it ^[3].

How to protect yourself in Houston

Check the Air Quality Index, or AQI, before planning time outside, and watch for Ozone Action Days ^[6]. When the AQI is unhealthy, choose a less intense activity, take more breaks, or move your activity indoors, where ozone levels are usually lower ^[6]. Because ozone is usually worse in the afternoon and early evening, plan outdoor activities for the morning ^[6].

People most at risk from ozone include people with lung disease such as asthma, children and teenagers, older adults, and people who are active outdoors, including outdoor workers ^[6]. People with asthma should follow their asthma action plan ^[6]. In extreme heat, staying inside with the windows closed and no air conditioning may be dangerous; go somewhere with air conditioning or find a local cooling center ^[6].

You can also help cut ozone by driving less, not letting your engine idle, not spilling fuel when you fill up, and using low-VOC paints and cleaning products ^[6].