Long-term exposure to ozone can greatly increase the risk of death from respiratory diseases, the Los Angeles Times reports. An 18-year study of half a million people in 96 cities found that an increase of 10 parts per billion in ozone concentrations correlated to a 4% increase in deaths from respiratory causes. Riverside, Calif., with the highest concentration at 104ppb, had a risk 50% greater than an area without ozone.
Ozone is a secondary pollutant, formed from a reaction of fossil fuel emissions in the presence of sunlight, so Southern California’s sunny climate worsens its ozone problem. Federal limits for ozone are measured only in the short term, but even by that measure, 100 million residents of 345 counties are in an unsafe range. (Read more ozone stories.)