Air quality officials warned Thursday of "very unhealthy" smog levels in the coming days as a heat wave envelops Southern California and primes the region for a bout of unusually high and widespread pollution.
Levels of ozone — the lung-damaging gas in smog — are likely to reach "unhealthy to very unhealthy" levels in the Santa Clarita, San Fernando and San Gabriel valleys, Inland Empire and the San Bernardino and San Gabriel mountains, according to the South Coast Air Quality Management District.
Scorching temperatures are expected to combine with atmospheric inversions that trap pollution near the ground and "may cause unusually high and persistent levels of poor air quality," according to the district. Smoggy conditions are expected to last through early next week or as long as the high heat lingers over the U.S. Southwest.
It's not uncommon for air quality to reach unhealthy levels, especially in the Inland Empire, where the air violates federal health standards on most summer days.
"What's unusual is we will potentially have very high levels across several regions all at once," AQMD spokesman Patrick Chandler said.
Ozone forms when emissions from cars, trucks, factories and other sources bake in the heat and sunlight. The invisible gas inflames and damages the lungs, worsens asthma and other respiratory illnesses and contributes to early deaths from heart and lung disease.
Officials advised residents to check the air quality forecast during hot weather and avoid strenuous outdoor activity in the mid-afternoon to early evening when ozone levels are highest.
The health warnings come as Southern California has experienced an increase in bad air days following decades of improving air quality.
Last year the region experienced its worst smog season in years, logging 132 bad air days and ozone concentrations not seen since 2009.
So far this year the South Coast basin, the coast-to-mountains expanse of 17 million people across Los Angeles, Orange, Riverside and San Bernardino counties, has violated federal ozone standards on 44 days, according to state regulators.
The air district in March adopted a plan to clean smog to federal standards by 2031 that, to be successful, will require boosting local, state and federal spending on cleaner vehicles by more than tenfold to $1 billion a year.