Dr. Ditto says ozone causes inflammation and heightens sensitivity to outdoor allergens such as pollen or mold.

Some of the cigarette companies' 50 planned witnesses are expected to argue that ozone, cosmic radiation or nontobacco cabin contaminants could have caused the diseases that the plaintiffs attribute to tobacco.

Because of these claims, and because ozone can cause health problems at high concentrations, several federal government agencies have worked in consultation with the U.S. Environmental Protection Agency to produce this public information document.

Tropospheric ozone causes approximately 21,000 premature deaths per year in 25 countries in the European Union.

Ground-level ozone causes health problems such as difficulty breathing, lung damage, and reduced cardiovascular functioning.

Potassium permanganate and sulfuric acid react to produce some ozone, which has a high oxidising power and rapidly oxidises the alcohol, causing it to combust.

Even computer laser printers have ozone filters to protect people from the ozone they produce, Dr. Friedman said, and ozone causes the funny smell after a short circuit makes sparks.

Other recent studies show that ground-level ozone (or smog) triggers asthma attacks, leaves long-term lung tissue damage and causes emergency-room visits to skyrocket during heat waves.

Medical researchers have found that ozone in the lower atmosphere can cause serious upper respiratory problems.

Ground-level ozone can cause wheezing, coughing, a burning sensation in the respiratory tract and, over time, permanent lung damage.