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By Joe Edwards
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Millions of Texans are being advised to limit vehicle use on Monday after the National Weather Service issued air quality alerts.
The alerts cover the Dallas-Fort Worth region as well as the Houston, Galveston, and Brazoria areas, warning that atmospheric conditions are likely to support elevated levels of ozone—a harmful air pollutant.
Residents are encouraged to help reduce ozone pollution by carpooling, walking or biking, bringing lunch from home, avoiding drive-through lanes, conserving energy, and ensuring their vehicles are properly maintained.
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Ozone Action Days have been issued for these areas, the alerts said. Ozone Action Day forecast seasons are based on the periods when each region is most likely to see elevated ozone levels. Forecasters assess whether concentrations are expected to reach or exceed the EPA’s AQI Level Orange threshold—defined as an eight-hour average of 71 parts per billion or a one-hour average of 125 ppb in a given area, according to the Take Care of Texas website.
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To make these determinations, Texas Commission on Environmental Quality (TCEQ) meteorologists draw on historical weather patterns, ozone monitoring data, and predictive models. When an Ozone Action Day is issued, they coordinate with the National Weather Service, which then disseminates the alert.
What Is Ozone?
"Ozone is a secondary pollutant, meaning it's not emitted directly from sources, but is formed through chemical reactions," AccuWeather meteorologist Brandon Buckingham previously told Newsweek. "These reactions require sunlight and higher temperatures, making warmer months more prone to ozone formation. When air is stagnant, pollutants don't get dispersed, allowing ozone to build up to unhealthy levels."
Ozone, sometimes referred to as smog, is a gas formed in the atmosphere. There are two categories of ozone, stratospheric and ground-level, although the chemical structure of ozone is the same wherever it is found.
Stratospheric ozone is found naturally in the Earth’s upper atmosphere, where it forms a protective layer that shields the planet from the sun’s harmful ultraviolet rays. Ground-level ozone is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen and volatile organic compounds in the presence of sunlight.
The Environmental Protection Agency (EPA) states that those most vulnerable to ground-level ozone exposure include people with asthma, children, older adults and individuals who spend time outdoors, particularly outdoor workers.
Depending on the level of exposure, ozone can cause coughing and a sore or scratchy throat. It can make it harder to breathe deeply and lead to pain when taking a deep breath, as well as inflame and damage the airways, increase susceptibility to infection, worsen lung conditions such as asthma, emphysema, and chronic bronchitis, and raise the frequency of asthma attacks.
How the EPA Measures Air Quality
The EPA employs the Air Quality Index (AQI) to measure air quality across America. It is a scale from 0 to 301+ that rates air quality into six categories ranging from good to hazardous, based on the public health risk.
- Green—Good (0 to 50): Air quality is satisfactory, and air pollution poses little or no risk.
- Yellow—Moderate (51 to 100): Air quality is acceptable. However, there may be a risk for some people, particularly those who are unusually sensitive to air pollution.
- Orange—Unhealthy for Sensitive Groups (101 to 150): Members of sensitive groups may experience health effects. The general public is less likely to be affected.
- Red—Unhealthy (151 to 200) Some members of the general public may experience health effects. Members of sensitive groups may experience more serious health effects.
- Purple—Very Unhealthy (201 to 300) Health alert: The risk of health effects is increased for everyone.
- Maroon—Hazardous (301 and higher) Health warning of emergency conditions; everyone is more likely to be affected.
Update, 5/25/2026, 5:30 a.m. ET: This article was updated with additional information.
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