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Ozone: The Facts

Sound science and targeted regulations have greatly reduced ozone concentrations in Texas cities and across the nation.

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 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.

What are the health effects of ground-level ozone?

Ground-level ozone is of particular importance 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 include decreased lung function and pain with deep breaths, and aggravated asthma symptoms.

What conditions lead to elevated ground-level ozone concentrations?

Summer days in Texas can be conducive for ozone formation as high-pressure systems dominate our local weather patterns, giving us clear skies and stagnant winds. Ozone mainly forms in the highest concentrations on warm, sunny days with light wind speeds and low relative humidity which allows more of the pollutant to form and accumulate.


What about indoor air quality?

The United States Environmental Protection Agency (EPA) has identified and characterized significant risks to public health from indoor environmental contaminants that are commonly found in homes, schools, offices, and other buildings where, on average, Texans are spending about 90 percent or more of their time. It is possible for indoor levels of air pollutants to reach up to two to five times higher, and occasionally even 1,000 times higher, than outdoor levels—according to the Texas Department of State Health Services (DSHS).

Common indoor air contaminants include radon, tobacco smoke, cleaning products, chemicals in upholstery foam, mold, combustion by-products, and VOCs. Ozone is not typically an indoor air contaminant. However, ozone generators, which are sometimes used in homes as air purifiers, may cause harmful levels of ozone.

Building systems, such as heating, ventilating, and air conditioning, also have a direct influence on the type and amount of exposure occupants may experience from indoor environmental contaminants.

For more information on indoor air quality visit:

Should I limit exercise and stay indoors when ozone concentrations are elevated?

The World Health Organization ranks physical inactivity as a major risk factor for heart disease, breast cancer, colon cancer, and diabetes. The Centers for Disease Control and Prevention found that 27.2 percent of adults and 16.6 percent of youth in Texas were inactive in 2014. For children, the risks of obesity are well-documented.

Many people engage in physical exercise to prevent disease and obesity. Individuals must consider those benefits when making choices about whether to follow the EPA’s recommendation to limit exercise outdoors and stay indoors when concentrations of ozone in ambient air are elevated.

A personal decision to limit outdoor activities should consider more than ozone levels because there are other conditions that can increase health risks, such as high heat and humidity.

For more information on obesity risks, visit:

What is background ozone and why is it important?

Background ozone is the amount of ozone due to distant sources. It can come from natural processes such as movement from the stratosphere or wildfires. Also, it can be transported from international pollution sources. It is important to study the locations in which background ozone is formed and where that ozone is transported. This information is used to develop successful reduction strategies.

What is the Air Quality Index?

The AQI is a numerical scale accompanied by corresponding colors created by the EPA for rating air quality. The AQI scale is based on the National Ambient Air Quality Standards (NAAQS) and is used to report current conditions and forecast future conditions.

Each NAAQS pollutant, such as ozone, has a separate AQI scale. For a given locality, a pollutant other than ozone could be in the good range while the ozone level is unhealthy. Colors indicate the amount of threat to health posed by the concentration of a pollutant. For example, an AQI for a locality anywhere from 101 to 150 due to ozone is coded Level Orange, which means “unhealthy for sensitive groups.”

For more information on the AQI visit:

What is an Ozone Action Day forecast?

Ozone Action Day (OAD) forecasts are made daily by TCEQ meteorologists during the ozone-forecast season (roughly March through November in Texas) for each of nine metropolitan areas in Texas (see below).

Metropolitan Area

Ozone Forecast Season Begins

Ozone Forecast Season Ends


Beaumont-Port Arthur

Corpus Christi

Dallas-Fort Worth

El Paso


San Antonio



April 1

May 1

April 1

March 1

May 1

March 1

April 1

May 1

April 1

October 31

October 31

October 31

October 31

October 31

November 30

October 31

September 30

October 31

The OAD forecast seasons are based on when each area is likely to experience warmer temperatures and weather conditions that are favorable for increased ozone formation. Each forecast predicts whether ozone levels in the area may reach or exceed the EPA's Air Quality Index (AQI) Level Orange category (Unhealthy for Sensitive Groups). Level Orange on the AQI corresponds to a level of ozone that would exceed the level of the National Ambient Air Quality Standard (NAAQS).

TCEQ meteorologists use historic meteorological data, historic ozone measurements, and ozone-prediction models to make these predictions, which are forecasts and not actual measured ozone values. TCEQ issues OAD notifications a day in advance of when conditions are forecast to be favorable for the creation of elevated ozone levels so citizens, businesses, and industry can take steps to reduce the pollutants that contribute to ozone formation. When they forecast an OAD, 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 media, government, business, and industry. The forecasts are made, in most cases, by 2 p.m. local time and are valid for the next day.

TCEQ maintains records of when Ozone Action Days (OAD) are issued for each metropolitan area and has this information available going back to 2008. View Historical Ozone Action Days.

What can I do to limit ozone formation?

  • Limit 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 VOCs.

How can I sign up to receive email and text alerts?

You can subscribe for email and text alerts about tTCEQ’s Air Quality Forecast and other topics. Don’t worry, you can unsubscribe at any time.

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

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