Reports the findings of a group of experts commissioned to evaluate the acute short-term effects on health caused by exposure to air pollutants during episodes of smog. Winter- and summer-type smog exposures are considered separately. The report responds to both the public demand to be protected and the corresponding need of health authorities to know when concentrations of specific indicator pollutants reach dangerous levels, what will be the effects on health and which preventive measures should be taken. Of central concern was the question of whether protective measures currently used by European countries are effective. In its most important achievement, the report grades health effects observed at different concentrations of sulfur dioxide, particulate matter, and ozone according to degree of severity, and then outlines appropriate protective measures in line with the significance of health effects. For ozone concentrations, the report also defines the percent of the population likely to be affected at different concentrations. Concerning measures to protect the general public, the report concludes that short-term measures, such as traffic bans and temporary reductions in industrial emissions, are ineffective. Because traffic bans lead to extreme overloading of the public transport system, outdoor exposure to pollutants is likely to increase as people wait for buses or trains, walk to stations and bus stops, or walk or bicycle to work. The report further concludes that long-term measures to reduce baseline levels of pollution represent the most sensible and effective preventive measure.