Ground-level ozone is formed in complex atmospheric photochemical reactions between nitrogen oxides and hydrocarbons. The levels are highest in summertime and outside city centers. Elevated concentrations of ground-level ozone is mainly due to chemical reactions of emissions of volatile organic compounds and nitrogen oxides. The average concentrations of ground-level ozone occurring in nonurban areas in western Europe range from 40 to 70 µg m3. One-hour maximum values may reach 300 µg m3 in central Europe and 200 µg m3 in Sweden. Numerous studies, ranging from experimental studies with laboratory animals to controlled studies with humans to epidemiologic studies, have been performed on the health effects of ozone. In animal experiments, morphological effects on lung tissue, effects on host defense mechanisms, inflammation, fibrosis and tumors have all been induced by the inhalation of ozone. The lowest concentration tested that has given rise to these effects is 200 µg m3. It led to hyperplastic changes, proinflammatory changes, and sensitization to allergens.

In chamber exposure studies of 1- to 6-h duration, respiratory symptoms and pulmonary effects such as lung function decrements, increased airway responsiveness, and inflammatory reactions have all been demonstrated in humans. These effects have occurred even at the lowest concentration tested, 160 µg m3 for 6 h. Reduced lung function at 1-h maximum daily values of 160 µg m3 has also been found in epidemiologic studies, and some reports indicate effects at even lower levels. Other effects of ozone seen in epidemiologic studies include respiratory symptoms, hospital admissions, and emergency room visits for respiratory conditions, all of which correlated with daily 1-h maximum values of 240 µg m3 and lower. From these data on humans, and with the use of a safety factor of 2, a 1-h guideline value of 80 µg m3 is recommended. This level is also intended to protect against health effects in the most sensitive sections of the population. The proposed guideline level is currently exceeded and should be regarded as a goal for action to avoid adverse health effects in the future.

The experimental and epidemiologic basis for a risk assessment concerning long-term exposure is much weaker than for short-term exposure, and no explicit long-term guideline value is recommended. However, the proposed short-term guideline would probably offer some protection against potential effects of ozone following long-term exposure.