LOS ANGELES, CA — Southern California smog worsened for a second straight year in the latest sign that progress in cleaning the nation's most polluted air is faltering, it was reported Wednesday.

The dive in air quality comes even though emissions are declining, forcing regulators to explain why returns are diminishing after years of progress battling smog, the Los Angeles Times reported.

Ozone, the harmful gas in smog that inflames the lungs and triggers asthma attacks and other health problems, has violated federal health standards 145 days this year across the sea-to-mountains South Coast air basin, The Times reported, citing data from state and local air regulators.

That's up from 132 ozone violation days last year and 113 in 2015 across Los Angeles, Orange, Riverside and San Bernardino counties, according to the newspaper.

The last time the region had more than 140 bad air days was in 2004, when there was more than twice as much smog-forming pollution spewing out of tailpipes and smokestacks, The Times reported.

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The South Coast Air Quality Management District, the smog-control agency for the region of 17 million people, believes the uptick is probably the result of more days with hotter temperatures and inversion layers -- weather patterns that trap pollution near the ground.

We're not denying that we've had two really bad ozone years, Philip Fine, deputy executive officer for the district, told The Times. But he added: "It's quite possible weather explains all of it."

Still, air quality officials are looking into other possibilities, including whether smog-forming pollution is actually declining as predicted in its emissions models, according to The Times.

City News Service; Photo by Jacob Repko on Unsplash