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Occupational & Environmental Epidemiology


Ozone is a gas made up of three atoms of oxygen. You can't see it and, in low concentrations, you can't smell it. It is found in the upper atmosphere and at ground level, as well as in some indoor environments.

You might be confused when you hear that ozone can be bad, because you've probably heard a great deal about the importance of the "ozone layer" and how it protects us from the sun's harmful ultraviolet rays. This stratospheric ozone — found 10 to 30 miles above the Earth's surface — is good.

However, when ozone is created at ground level, it can become a health problem, irritating the respiratory system, impairing breathing, and aggravating existing conditions such as asthma, allergies, emphysema or bronchitis.

The federal outdoor air quality standard for ozone is 0.075 parts per million (ppm) averaged over an eight-hour period. Ozone concentrations exceeding this standard pose a potential health risk, either to sensitive groups or to everyone. The N.C. Division of Air Quality (DAQ) maintains ozone monitors across the state and issues daily air quality forecasts. DAQ issues air quality advisories when forecasted ozone levels pose a health risk. These advisories are available on the Division of Air Quality website.

Three things are necessary to create ground-level ozone — volatile organic compounds (VOCs), oxides of nitrogen (NOx), and sunlight. Cars, power plants, industrial boilers, refineries, chemical plants and other sources emit VOCs and NOx. Sunlight "cooks" VOCs and NOx, creating ground-level ozone. For North Carolina, this means that ozone is primarily a hot-weather problem, with most high readings occurring in June, July and August. It is also highest in urban areas with lots of cars like Charlotte, the Triangle, the Triad (Greensboro area), Fayetteville and Asheville. High levels can even occur at high elevations in the Blue Ridge and Smoky Mountains when man-made ozone travels in upper atmospheric layers across mountain peaks. In urbanized areas, problems are more likely from 2:00 to 6:00 p.m. In the mountains, the problem usually occurs after 7:00 p.m.

Indoor ozone concentrations vary widely, from less than 10 percent of the outdoor level to 80 percent of the outdoor level. This large variation is caused by many factors such as air infiltration or exchange rate of the structure, interior air circulation, interior surface composition (e.g., rugs, draperies, furniture) and reaction with other indoor air compounds. In situations where there is an indoor ozone source, such as ozone-generating air cleaners, indoor ozone concentrations have been reported to range between 0.12 to 0.80 parts per million (ppm).

While the indoor ozone sources (e.g., ozone generators, electrostatic air cleaners, photocopiers, laser printers) can be responsible for higher concentrations indoors, outdoor ozone appears to be the major source of ozone exposure. In general, it is safer to be indoors on high-ozone days unless there is an ozone-generating device like an air cleaner present.

Health Effects & Symptoms

There is lots of good, scientific research concerning the effect of ozone on people. Ozone can affect your lungs and respiratory system in several ways:

  • Respiratory system irritation. This might come in the form of coughing or an uncomfortable feeling in your chest. Symptoms may last a few hours after exposure.
  • Impaired breathing. You may not be able to breathe in as much air as you normally do. Your breathing might be more rapid and shallow.
  • Aggravated asthma. Doctors report that high ozone levels result in a greater number of asthma attacks. That's because asthmatics are more greatly affected by the irritant. Ozone also makes you more sensitive to allergens that cause asthma attacks. Ozone can also aggravate chronic lung diseases like emphysema and bronchitis.
  • Lung inflammation and damage. Ozone can damage the cells that line your lungs. Eventually, these damaged cells are replaced. But, repeated damage may result in permanent problems.

Some health effects are short-term. Scientists are researching ozone's long-term effects. There is a concern that the developing lungs of children may be damaged by repeated exposure to high levels of ozone. Some studies in animals suggest that ozone may also harm the body's ability to fight off respiratory infections.

Ozone damage to the body can occur without any noticeable signs. People who live in areas where ozone levels are frequently high may find that their initial symptoms go away over time, particularly when exposure to high ozone levels continues for several days. However, ozone continues to cause lung damage even when the symptoms have disappeared. The best way to protect your health is to find out when ozone levels are forecasted to be elevated in your area and take simple precautions to minimize exposure, even when you do not feel obvious symptoms.

The EPA has developed an Air Quality Index for some air pollutants, including ozone, to alert the public to levels that may pose a threat to people's health. The index reports pollution levels in color-coded categories that represent health risk: green (good), yellow (moderate), orange (unhealthy for sensitive groups), red (unhealthy) and purple (very unhealthy). Days on which air pollution levels are forecasted to be code orange or worse are Air Quality Action Days. Most Air Quality Action Days in North Carolina are for ozone, although air quality advisories are also sometimes issued for particle pollution, particularly in areas downwind of forest fires.

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