What is Ozone?

Ozone is a form of oxygen with three atoms, instead of the usual two atoms. It is a photochemical oxidant and, at ground level, is the main component of smog. Ozone is not emitted directly into the air but is formed through chemical reactions between natural and man-made emissions of volatile organic compounds (VOCs) and oxides of Nitrogen in the presence of sunlight. These gaseous compounds mix like a thin soup in the ambient, or outdoor, air, and when they interact with sunlight, ozone is formed. Sources of these pollutants include automobiles, gas-powered motors, refineries, chemical manufacturing plants, solvents used in dry cleaners and paint shops, and wherever natural gas, gasoline, diesel fuel, kerosene, and oil are combusted.

Ozone pollution is the periodic increase in the concentration of ozone in the ambient air, the natural air that surrounds us. It is mainly a daytime problem during summer months because warm temperatures play a role in its formation. When temperatures are high, sunshine is strong, and winds are weak, ozone can accumulate to unhealthful levels.

Ground-level ozone is the most prevalent air pollutant in Texas and the nation. Ozone is often one of several pollutants that make up "smog," which you may recognize as the reddish-brown haze that forms when air quality is particularly poor. But because ozone itself is colorless, the air can look clear even when high ozone concentrations are present.

Ozone is measured in parts per billion. Parts per billion (ppb) is a unit of measurement used to represent the concentration of a substance in a certain medium (air, water, soil). For example, 70 ppb of ozone means that for every 1 billion parts of air studied, 70 of those parts are ozone.

Ozone Is a Health Hazard

The biggest concern with high ozone concentration is the damage it causes to human health, vegetation, and to many common materials we use. High concentrations of ozone can cause shortness of breath, coughing, wheezing, headaches, nausea, eye and throat irritation, and lung damage. People who suffer from lung diseases like bronchitis, pneumonia, emphysema, asthma, and colds have even more trouble breathing when the air is polluted. These effects can be worse in anyone who spends significant periods of time exercising or working outdoors.

Children often play outside for long periods during the summer. Their lungs are still developing, and they breathe more rapidly and inhale more air pollution per pound of body weight than adults. On days when ozone levels are high, these factors put children at increased risk for respiratory problems.

Adults breathe more than 10,000 times each day. During exercise or strenuous work, we breathe more often and draw air more deeply into the lungs. When we exercise heavily, we may increase our intake of air by as much as 10 times our level at rest. The interaction between air pollution and exercise is so strong that health scientists typically use exercising volunteers in their research.

Ozone levels are considered by the U.S. Environmental Protection Agency (EPA) to be "unhealthful" and exceed the National Ambient Air Quality Standard when they are measured at 125 parts per billion (ppb) or higher under the one-hour standard or at 75 ppb or higher under the eight-hour standard. When a single monitoring site has exceeded the one-hour standard on more than three days in three years, the EPA classifies the surrounding county or metropolitan area as not attaining the ozone standard, or "nonattainment" for ozone. Those areas in "attainment" of the one-hour standard are required to meet the eight-hour standard of a three-year average of the fourth-highest daily maximum eight-hour concentration measured at each site not to be at or exceed 75 ppb.

Ozone Readings in Texas

The TCEQ collects daily ozone measurements at several monitors across the state. Peak ozone concentrations in the state's major metropolitan areas are available as well as daily peaks since January 1, 1998. Ozone concentrations are also available. These concentrations are used in determining if the National Ambient Air Quality Standard has been exceeded.

The Air Quality Index (AQI), formerly known as the Pollution Standard Index (PSI), is derived from air pollutant measurements and is used to determine an AQI rating of "Good," "Moderate," or "Unhealthy." Because ozone measurements are usually higher in Texas than those of other air pollutants, the AQI is normally based on ozone levels.