As the sweltering heat settles into Southern California, so is one summertime pollutant: ground-level ozone.

Also known as smog.

Dr. Scott Epstein: “Unfortunately, Southern California happens at the worst ozone levels in the country,” Dr. Scott Epstein, Program Supervisor for South Coast Air Quality Management District, said. “On days that are especially hot like we have coming this week, ozone is extra high.”

So, what causes this high level of smog?

Dr. Epstein continued, “Ozone is a pollutant that’s made in the atmosphere. You need three main ingredients to make it, so one is you need sunlight. Another ingredient is oxides of nitrogen, or NOx. That comes from combustion. Then the third ingredient is volatile organic compounds, or VOCs. They come from the evaporation of organic compounds, things that you may smell, like cleaning compounds or gasoline. So, these VOCs and NOx react to the atmosphere and form the ozone. When the temperatures are higher, you have a lot more evaporation of those VOCs so you have more emissions of VOCs. They’re very complicated reactions, and a lot of them move faster at higher temperatures. So we have our highest ozone days on days with the highest temperatures.”

Ozone is an invisible pollutant but can sometimes cause haze if other pollutants are in the air.

This makes it all the more important to check the Air Quality Index (AQI) before going outside.

“So we look at air pollutants and ozone is one of them,” Dr. Epstein explained. “We report it as an Air Quality Index. You may have seen the colors green, yellow, orange, red, purple, maroon, and so on. We reported those colors and each one of those colors has an action that we recommend people take to protect their health. So it’s important to check the levels frequently because ozone changes throughout the day. It’s typically worse in the afternoon and also depends on your location.”

While the Coachella Valley usually has cleaner air than the LA basin, this ground-level ozone isn’t giving any region relief.

“Ozone is not bad all day. Generally in the mornings it’s fairly clean,” Dr. Epstein said. “On these high ozone days, you want to try and move your outdoor activities to the morning hours when the air quality is cleaner, much as you would do with a heat wave. In the Coachella Valley, we expect to reach orange levels, ‘Unhealthy for Sensitive Groups,’ at least over the coming week. When air quality is unhealthy for sensitive groups, we recommend that people with pre existing lung conditions, children or older adults minimize prolonged, heavy exertion outside. It is possible in the Coachella Valley to reach ‘Unhealthy Air Quality’ this coming week which would be in the red category. If we reach that, we recommend that everybody should reduce prolonged or heavy exertion outside and we recommend that those sensitive groups should avoid prolonged or heavy exertion outside as well.”

This advisory is expected to last until Sunday, September 4th.

For live updates on AQI near you, click here.

For more information on the ozone advisory, click here.

Coachella Valley Under “Ozone Advisory,” Causes and How to Prepare