Heat wave means unhealthy air this weekend across parts of Southern California

As temperatures head north of 90 degrees this weekend, smog levels are expected to reach unhealthful levels across much of Southern California.

Air quality will be worst in the Santa Clarita, San Fernando and San Gabriel Valleys, and throughout the Inland Empire and the mountains. People in those areas should stay inside when ozone levels are highest at midday and avoid exercising outdoors.

When air quality index levels reach unhealthy levels (indicated by the color red on the South Coast Air Quality Management District's real-time map), the air is bad enough to affect everyone, even healthy people.

Smog, also known as ozone, forms when pollutants from cars, trucks, industry and naturally-occurring sources are baked by sunlight. The LA Basin has the worst ozone problem in the country, and polluted air causes 2,000 premature deaths here every year. 

Breathing high levels of ozone can aggravate asthma, cause throat irritation, chest pains and shortness of breath, and over the long term, can decrease lung function and increase susceptibility to respiratory infections. 

Hanging out in the shade at the city park in the Riverside County city of Corona on Friday, Vinny Limas said he didn't realize the air was so bad here.

"I never really paid attention to air pollution, but now that you’re like talking about it, the sky’s all blue and you go back there, and it gets all like, gray," he said, pointing towards the San Gabriel Mountains, which were barely visible through the smog.

Heat waves can exacerbate ozone by causing inversions, which trap pollutants close to the ground. Coastal breezes can help disrupt the stagnant air, which is why areas along the ocean usually have the best air quality. In Southern California, pollutants get trapped inland against the San Gabriel and San Bernardino mountains, making ozone levels higher in Riverside and San Bernardino counties than almost anywhere else.

Ozone levels typically peak around 2 p.m. in downtown Los Angeles, later in the afternoon further inland.

It was just before noon when Jorge Gonzalez and his family were picnicking in the park in Corona to escape their hot apartment. They live near the 91 freeway, and Gonzalez said he is acutely aware of the particulate pollution they are exposed to by being so close to traffic.

"You can clearly see it right there when you turn on the air conditioner, or the fan, it gets filled with black dirt," he said. "Imagine what we’re all breathing in."

2017 is already shaping up to be a bad air year: ozone levels have violated the federal Clean Air Act standard on 44 days so far, more than in any of the previous three years to date. The standard is 0.70 parts per million.