From the LA Times:
The war on smog has been called one of America's greatest environmental successes. Decades of emissions-cutting regulations under a bipartisan law – the 1970 Clean Air Act – have eased the choking pollution that once shrouded U.S. cities. Cleaner air has saved lives and strengthened the lungs of Los Angeles children.
But now, air quality is slipping once again.
Bad air days are ticking up across the nation, and emissions reductions are slowing. The most notable setback has been with ozone, the lung-damaging gas in smog that builds up in warm, sunny weather and triggers asthma attacks and other health problems that can be deadly.
Health effects from ozone pollution have remained essentially unchanged over the last decade — "stubbornly high," according to a study published this year by scientists at New York University and the American Thoracic Society.