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

PASADENA, CA - DECEMBER 1:  Morning commuters travel by car and train along the 210 freeway between Los Angeles and cities to the east on December 1, 2009 near Pasadena, California. President Barack Obama will attend the international climate negotiations in Copenhagen next week with a vow to reduce US greenhouse gas emissions to about 17 percent below 2005 levels by 2020, and 83 percent by 2050. Meanwhile, California, which has some of the toughest clean air laws after decades of fighting some of the worst smog in the nation, is in the final phase of building a cap-and-trade market to provide incentives to reduce greenhouse emissions.  More than 60 world leaders are expected to take part in the climate negotiations in Copenhagen.   (Photo by David McNew/Getty Images)
David McNew/Getty Images
Morning commuters travel by car and train along the 210 freeway. The South Coast Air Quality Management District is asking people to carpool and take public transit to help mitigate unhealthy ozone levels this weekend.

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."

Year to date Days above federal ozone standard
2017 44
2016 40
2015 29
2014 37

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.