– Southern Californians might remember the summer of 2018 for its sweltering heat waves, record ocean temperatures and destructive wildfires. But it also claimed another distinction: the summer when nearly three months passed without a day of clean air.

The region violated federal smog standards for 87 consecutive days, the longest stretch of bad air in at least 20 years, state monitoring data show. The streak is the latest sign that Southern California’s battle against smog is faltering after decades of dramatic improvement.

The ozone pollution spell began June 19 and continued through July and August, with every day exceeding the federal health standard of 70 parts per billion somewhere across Los Angeles, Orange, Riverside and San Bernardino counties. It didn’t relent until Sept. 14, when air pollution dipped to “moderate” levels within federal limits for ozone, the lung-damaging gas in smog that triggers asthma and other respiratory illnesses.

It’s not unusual for Southern California summers to go weeks without a break in the smog, especially in inland communities that have long suffered the nation’s worst ozone levels. But environmentalists and health experts say the persistence of dirty air this year is a troubling sign that demands action.

“The fact that we keep violating and having this many days should be a wake-up call,” said Michael Kleeman, a professor of civil and environmental engineering at UC Davis who studies air pollution.

The South Coast Air Quality Management District, which is responsible for cleaning pollution across the region of 17 million people, said that consecutive bad air days is the wrong way to gauge progress curbing ozone, that this smog season was not as severe as last year’s and had fewer “very unhealthy” days.

The U.S. Environmental Protection Agency judges whether the region meets Clean Air Act standards based on the highest pollution readings, not how long bad air persists. By federal metrics, air district officials argue, they are making strides. The highest ozone levels recorded this summer, they point out, were lower than the previous year, and the smog season began later.

“By all accounts this year is not great, but it’s a little better than last year,” said Philip Fine, deputy executive officer for the South Coast air district.

But Joseph Lyou, a South Coast air quality board member who heads the Coalition for Clean Air, said he’s concerned that although the intensity of Southern California’s air pollution has dropped, its longevity is increasing.

“It’s a disturbing trend no matter what the law says you’re accountable for,” said Lyou. “It’s telling us we have a persistent problem and that we still have a long way to go.”