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Ozone: The Facts

Answers to general questions about ozone, such as what is ozone, how it is formed, and why we should be concerned about this air pollutant.

The TCEQ uses sound scientific research to reduce ozone and, as a result, has achieved decreases in ozone concentrations statewide. In fact, from 2000 to 2010, ozone levels across Texas decreased by 27 percent. By comparison, the rest of the nation averaged only a 14 percent decrease in ozone levels over the same period. Texas has also devoted more funds to air quality research during the past decade than any other state in the country, which has led to the dramatic decrease in ozone concentrations. The state has sponsored two major field studies-- the Texas Air Quality Study (TexAQS) 2000 and Texas Air Quality Study II (TexAQS II).

Sound science and targeted regulations have greatly reduced ozone in Texas cities and across the nation. The TCEQ’s goal is sensible regulation, based on sound science, which addresses real environmental risks while complying with state and federal statutes.

What is ozone?

Ozone is a gas that is formed in the atmosphere when three atoms of oxygen combine. Ozone is found high in the Earth’s upper atmosphere and at ground level. Ozone has the same chemical structure wherever it occurs.

Stratospheric Ozone: Ozone occurs 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: Ground-level ozone 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) also release VOCs that can contribute to ground-level ozone.

What are the health effects of ground-level ozone?

Breathing relatively high levels of ground-level ozone can cause acute respiratory problems like cough and respiratory irritation and can aggravate the symptoms of asthma.

What are the conditions that lead to elevated ground-level ozone?

Ozone, typically referred to as smog, mainly forms the highest concentrations on sunny days with slow wind speeds, which allow pollutants to accumulate. 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.

What can I do to limit ozone formation?

  • Limit driving and idling; instead, car pool, combine errands, use public transportation, bike, or walk.
  • Don’t top off the tank when refueling your vehicle.
  • Keep your vehicle maintained, including proper tire pressure.
  • Maintain your yard equipment, including changing the oil and replacing air filters. Use electric lawn equipment when possible.
  • Don’t burn yard waste.
  • Use paint and cleaning products with less or zero VOCs.
  • Conserve energy.
  • Refuel in late afternoon or evening.

What about indoor air quality?

The U.S. Environmental Protection Agency 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 most Americans spend up to 90 percent of their time. Indoor levels of air pollution may be up to two to five times higher, and occasionally 100 times higher, than outdoor levels. Common indoor air contaminants include radon, tobacco smoke, mold, irritant and allergenic asthma triggers, combustion by-products and VOCs. Indoor contaminants may be of natural origin (e.g., radon, allergens and molds), may derive from products used indoors (e.g., finishes, furnishings and cleaning products) and may result from indoor processes and behaviors (e.g., smoking, use of unvented combustion sources; or cleaning, operation and maintenance procedures). Building systems (e.g., heating, ventilating, and air conditioning) also have a direct influence on the type and amount of exposure building occupants may experience from environmental contaminants indoors. For more information on indoor air quality:
U.S. Environmental Protection Agency: Indoor Air Quality Exit TCEQ
Texas Department of State Health Services Exit TCEQ

What should I consider in weighing the EPA’s recommendations to limit exercise and stay indoors because of outside ozone concentrations?

The World Health Organization ranks physical inactivity as a major risk factor for heart disease, breast cancer, colon cancer, and diabetes. Recently, it has been estimated that over 30 percent of the U.S. population is fully inactive. For children, the risks of obesity are well-documented Exit TCEQ. Given that physical exercise is important for adults and children for reasons including prevention of disease and obesity Exit TCEQ, individuals must consider those benefits when making choices about following the EPA’s recommendations to limit exercise outdoors and stay indoors because of concentrations of ozone in ambient air. A personal decision to limit outdoor activities should consider weather conditions more than ozone levels. Exercise performance is compromised in the heat, and temperature should be considered when making personal choices about exercising.

What is background ozone and why it is important?

The study of background ozone is important in order to understand how much ozone is being added to an airshed by local contributions. Policy Relevant Background (PRB) ozone is currently defined as the ozone concentrations that would occur in an area in the absence of anthropogenic emissions of precursors in the U.S., Canada and Mexico. It is important in health risk and policy assessments. PRB ozone cannot be directly determined from monitored data because of the influence of local and regional precursor emissions on ozone formation in any given area. It is important for air quality planners to use modeling and other analytical tools to estimate day-specific ozone PRB levels for high ozone days as part of the regulatory development program. The PRB ozone is of concern because it can sometimes represent a substantial fraction of the observed ozone in Texas urban areas. Recent scientific studies using global-scale modeling have estimated that PRB ozone in Texas can range from 25 to 58 parts per billion.

“Background ozone” can also be described as the amount of ozone due to distant sources entering an airshed; this definition can help to differentiate between the natural and transported ozone, and the ozone produced locally. Using this definition, regional background ozone can be directly measured by monitors and can at times approach or even exceed levels that are allowed by the regulatory standard. This concept is important when understanding the contribution of a local area in consideration of control strategies.

What is an ozone forecast?

Ozone forecasts are made daily during the season for each of nine metropolitan areas in Texas. Each forecast is a simple yes or no prediction for the question: Will ozone levels reach or exceed a target level for a particular area?

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, the meteorologists contact the National Weather Service which then broadcasts the information across its weather wire. The agency also contacts officials in all affected areas so that local community clean-air coalitions can notify media, government, business, and industry.

The forecasts are made, in most cases, a day in advance by 2 p.m. local time and are valid for the next day. The only exception is for the Houston area, where the forecast can be updated as late as 9 a.m. local time on the same day that the forecast is in effect. The forecast target level for all areas is the U.S. EPA Air Quality Index (AQI) Level Orange, which corresponds to an 8-hour average of 76 ppb or a 1-hour average of 125 ppb. In the Houston and Dallas-Fort Worth areas, forecasts for AQI Level Red may also be issued and the target for these forecasts is an 8-hour average of 96 ppb or a 1-hour average of 165 ppb.

The ozone forecast seasons do not necessarily correspond to the "ozone seasons" defined for regulatory purposes by the EPA.

Participating Metropolitan Areas and Their Ozone forecast season Dates

Each day during ozone forecast season (roughly March through November in Texas), the TCEQ forecasts ozone levels for nine metropolitan areas:

Metropolitan AreaOzone Forecast Season BeginsOzone Forecast Season Ends
April 1
October 31
Beaumont-Port Arthur
May 1
October 31
Corpus Christi
April 1
October 31
Dallas-Fort Worth
March 1
October 31
El Paso
May 1
October 31
March 1
November 30
San Antonio
April 1
October 31
May 1
September 30
May 1
September 30


What is the Air Quality Index?

The EPA has a scale called the Air Quality Index for rating air quality. The AQI scale is based on the National Ambient Air Quality Standards and is described in Title 40, Code of Federal Regulations, Part 58, Appendix G. Each NAAQS pollutant has a separate AQI scale; an AQI rating of 100 corresponds to the concentration of the federal standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA.

How can I sign up to receive e-mail alerts and alerts on mobile devices?

You can sign up for free ozone forecast e-mail alerts for any of the nine metropolitan areas listed above through TCEQ’s website.

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