Ozone is a highly reactive gas that might be described as “heavy oxygen.” The life-sustaining oxygen we breathe is O₂, or two molecules combined. Three oxygen molecules combine to form ozone (O₃).

There’s good news and bad news with ozone. The U.S. Environmental Protection Agency coined a slogan –– “good up high, bad nearby” – to illustrate the difference.

Ozone located in the stratosphere absorbs ultraviolet light, preventing most UV-B rays from reaching the earth’s surface. This shields humans, plants and animals alike from their damaging effects including skin cancer and cataracts.

Nearby or at ground level, however, the gas is a clear-cut hazard. While ground levels of ozone are a small fraction of the concentrations found in the stratosphere, even low levels of O₃ are toxic to organisms. How low? A number of federal agencies have established maximum public-health thresholds ranging from 50 to 100 parts per billion, depending on the circumstances.

Ozone causes breathing difficulties, exacerbates chronic lung conditions, and decreases resistance to respiratory disease. Children, senior citizens and patients with heart or lung ailments are most susceptible to elevated O₃. Even in healthy adults, higher-than-normal ozone concentrations can cause coughing, shortness of breath and lung inflammation.

Ozone’s readiness to combine with other gases in the atmosphere is a major cause of photochemical smog in summer. Electrical arcs force formation of ozone from atmospheric oxygen, which is why there’s a sharp odor in the air immediately after a strong thunderstorm. For similar reasons, major electrical appliances also produce small amounts of ozone.

Ozone generators have been marketed as a method of combating odors caused by bacteria, mold, volatile organic compounds or smoke following a fire. However, the ozone concentration needed to achieve these benefits is far above maximum levels specified in public health standards. In some cases, these concentrations are as much as 10 times greater than the highest regulatory threshold. Not only that, ozone cannot remove allergy-causing particles from the air, and its ability to eliminate odor and bacteria from porous surfaces is limited at best.

For these reasons, home ozone treatments should be performed by experienced contractors only. Due to the potential health hazards involved, no one should remain inside a house while ozone treatment is under way, and the home should be allowed to air out completely afterwards.

For further information about atmospheric ozone, a partnership of federal and state agencies in the U.S. operates AirNOW, a Web site offering daily updates on air quality and ozone levels throughout the nation. The site also has a provision inviting users to sign up for notification about air-quality alerts via email, pager or cell phone.