Some of the warmest weather of all time, compounded by stagnant winds, has worsened the problem, the report said. Ozone levels have topped federal standards on 91 days this year, compared to 67 days last year.
Policymakers in developed countries have implemented a number of regulations to improve air quality in recent decades, leading to a dramatic decline in air pollution. But that progress has slipped in some places in recent years for a variety of different reasons. Rising temperatures in Southern California—due to climate change, El Niño and other factors—has heated hydrocarbon and nitrogen oxide pollution from vehicles, leading to smog. Storms typically clear the air, but they have been in short supply during the state's drought.
California is not the only place with an air pollution problem. In many European cities, a rise in diesel vehicles has contributed to high levels of air pollution. Developing countries like India and China continue to experience high levels of air pollution as a result of rapid industrial growth and weak regulation.