What is an Ozone Action Day?

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Take Care of Texas
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Each day during ozone season (roughly March through November in Texas), the TCEQ informs the public when forecasted conditions can lead to Ozone Action Days. These are days when ozone levels are above normal and can impact our health through the quality of the air we breathe.

Sign-up for e-mails and text alerts about TCEQ’s Air Quality Forecast through the TCEQ newsletter page to stay in the know about ozone levels near you. It’s especially important to stay informed about Ozone Action Days during the summer months, so we can take steps to reduce pollutants contributing to ozone formation.

What is ozone?

Ozone, sometimes referred to as smog, is a gas that is formed in the atmosphere when three atoms of oxygen combine. The chemical structure of ozone is the same wherever it is found; however, there are two categories of ozone.

  • Stratospheric ozone is found naturally in the Earth’s upper atmosphere — 6 to 30 miles above the Earth’s surface — where it forms a protective layer that shields us from the sun’s harmful ultraviolet rays.
  • Ground-Level ozone is found at ground level (it is also called tropospheric ozone). It is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOCs) in the presence of sunlight. Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources of NOx and VOCs. In addition, biogenic sources (living organisms or biological processes) release VOCs that can contribute to ground-level ozone.

Health effects of elevated ground-level ozone

Ground-level ozone is important because it is a respiratory toxic agent that can cause acute respiratory health effects when people breathe high concentrations of it over several hours. These effects can include decreased lung function and pain with deep breaths, as well as aggravated asthma symptoms.

What is an Ozone Action Day?

TCEQ meteorologists make daily Ozone Action Day (OAD) forecasts during the OAD forecast season for each of the nine metropolitan areas in Texas.

Ozone Action Day metropolitan area chart

The Ozone Action Day forecast seasons are based on when each region is likely to experience elevated ozone concentrations. Each forecast predicts whether ozone levels for the following day in an area are expected to reach or exceed the EPA’s AQI Level Orange — an eight-hour average of 71 parts per billion or a one-hour average of 125 ppb.

TCEQ meteorologists use a set of criteria from historic meteorological data, ozone measurements, and ozone prediction models to make these predictions. When they forecast an Ozone Action Day, TCEQ meteorologists contact the National Weather Service, which then broadcasts the information across its “weather wire.” TCEQ also contacts officials in affected areas so that local community clean air coalitions can notify their media, government, business, and industry. Ozone Action Days are issued, in most cases, by 2 p.m. local time and are valid for the next day.

What can I do to limit ozone formation?

  • Tips to Decrease Ozone infographicLimit driving and idling. Instead, carpool, combine errands, use public transportation, bike, or walk.
  • Refuel your vehicle in the late afternoon or evening and don’t top off the tank.
  • Keep your vehicle maintained, including proper tire pressure.
  • Maintain your yard equipment, including changing the oil and replacing air filters regularly. Also consider using tools without motors. Hand tools such as shears, edgers, and push reel mowers are lightweight, quiet, and easy to use, and do not generate emissions.
  • Don’t burn yard waste.
  • Use paint and cleaning products with less or zero volatile organic compounds.

Receive Ozone Action Day Notifications

You can sign up for e-mail and text alerts about TCEQ’s Air Quality Forecast and other topics on the TCEQ newsletter page.

You can also sign up to receive e-mail alerts through the EPA EnviroFlash website.

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