The air is dirty nationwide. The report said the hot summer weather of 1999 contributed to the amount of ozone in many parts of the country. Principal sources of these pollutants are tailpipe emissions, power plants and certain kinds of factories. Ground-level ozone is the main component in smog. The terms often are used interchangeably.
Ozone is a particular risk for people with asthma and other respiratory ailments.
Children, the elderly and people who exercise outdoors also tend to be at a greater risk. For healthy adults, excessive exposure to ozone can cause shortness of breath, wheezing, coughing and chest pain. The report also lists 12 cities that had no days with unhealthy ozone levels.