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Several of Texas’s biggest cities, including Houston, Dallas, and San Antonio, have been blanketed by multiple ozone action days this week.
Issued by the state’s environmental agency and relayed by the National Weather Service, these warnings inform residents that air pollution in a given area may reach potentially harmful levels.
Newsweek spoke with experts about why many of the Lone Star State’s largest cities have been under so many of the alerts this week.
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What Is Ozone?
The Texas Commission on Environmental Quality (TCEQ) defines ozone, also known as smog, as “a gas that is formed in the atmosphere when three atoms of oxygen combine.” Ozone can be categorized in two main ways.
The first is stratospheric ozone, which occurs naturally in the Earth's upper atmosphere and provides a shield against the sun’s ultraviolet rays.
Individuals most at risk from breathing ozone-contaminated air include those with asthma, children, older adults, and people who are physically active outdoors, according to the Environmental Protection Agency (EPA). Health problems caused by ozone may include coughing, difficulty breathing, and inflamed and damaged airways, according to the agency.
“September and early October is the peak season for ozone pollution in Texas due to a combination of factors,” Texas State Climatologist John Nielsen-Gammon told Newsweek. “First, there's still plenty of sunlight, which makes the photochemistry that produces ozone possible.
“Second, wind speeds tend to be lightest in the fall, allowing local emissions to accumulate into greater concentrations over a smaller area (which is why the pollution is concentrated near major cities)," John Nielsen-Gammon, who is also a professor at Texas A&M University, said.
"Lastly, the pollution levels coming into Texas from the east and northeast are already high because of all the pollution sources in the rest of the country. Other times of year, Texas air typically comes from the south (cleaner tropical air) or from the north (cleaner polar air). A broad area of high pressure near the Great Lakes is ideal for high pollution levels in Texas. Usually, the pattern persists for a few days.
“Houston and Dallas in particular have had to implement stringent air pollution controls over the past couple of decades to stay close to federal ozone standards. Other cities in Texas have not had such systematic pollution levels, though they do occasionally have bad air days, Nielsen-Gammonsaid.”
What Is an Ozone Action Day?
When TCEQ expects ozone concentrations to reach potentially harmful levels, they may issue an ozone action day. These are issued “so citizens, businesses, and industry can take steps to reduce the pollutants that contribute to ozone formation."
Advice from the agency to help residents reduce pollution in their areas on such days includes sharing rides, walking, biking, using public transportation, and keeping vehicles properly tuned.
TCEQ told Newsweek that it isn’t unusual for Texas’s more densely populated cities to have multiple stretches of ozone action day issuances.
According to data from the agency, the Houston metropolitan area saw nearly 50 ozone action days in 2024, while Austin and the Dallas-Fort Worth areas saw eight and 43, respectively.
It is worth noting that different states have varying environmental bodies, and reporting methods may differ from state to state.
As of early Friday, ozone action days were in place for both the San Antonio and Dallas-Fort Worth areas.
Read a Q&A between Newsweek and Zong-Liang Yang, a professor in the Department of Earth and Planetary Sciences at The University of Texas at Austin, below:
Q: Is it usual to see so many ozone action days on consecutive days, or is this quite normal?
Consecutive ozone action days are not entirely unusual in Texas, especially during the peak ozone season (roughly March through November), but extended streaks like those seen this week in Houston and San Antonio can indicate above-average conditions driven by persistent weather patterns. They occur when high temperatures, abundant sunlight, and low winds allow ground-level ozone (formed from pollutants like vehicle emissions and industrial volatile organic compounds reacting with nitrogen oxides) to build up without dispersing.
In short, consecutive days happen regularly during peak heat, but prolonged or more intense periods like this week are becoming more noticeable amid rising temperatures.
Q: What makes locations like Houston and San Antonio prone to ozone action days? Is it simply due to dense population, or are there ongoing long-term air quality issues?
It is a combination of factors: dense population plays a role, but it is amplified by geography, meteorology, industrial activity, infrastructure, and long-term environmental challenges. These cities are not just populous; they have unique conditions that trap and exacerbate ozone precursors. Both have histories of ozone non-attainment under EPA standards, meaning they've struggled to meet federal air quality limits for years, leading to ongoing regulatory efforts.
In summary, while population density is a factor, the proneness stems from a mix of natural (hot, stagnant weather) and human-driven (emissions from traffic, industry, and energy use) issues, compounded by long-term climate trends. This creates persistent air quality challenges beyond just urban size.
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