Regional air quality officials are warning people to avoid strenuous activities this weekend because much of Southern California will experience “very unhealthy” air pollution as the heat wave takes hold of the weather.

The areas of most concern include the Santa Clarita Valley, San Gabriel Mountains, portions of the San Fernando and San Gabriel valleys, Inland Empire and the San Bernardino Mountains, said an advisory from the South Coast Air Quality Management District.

The hot weather is coming with a predicted atmospheric inversion layer that will trap air pollution near the surface and cause unusually high and persistent levels of poor air quality.

The heat also increases the production of the ozone, the hallmark pollutant of smog that forms when different kinds of emissions react with each other in the atmosphere.

Ozone irritates the respiratory system and reduces lung function by inflaming and damaging cells that line the lungs. It aggravates asthma and other chronic lung diseases such as emphysema and chronic bronchitis.

The air district is recommending that people check air quality forecasts. When the air is expected to be unhealthy, avoid strenuous outdoor activities, especially when peaks in the mid-afternoon to early evening hours.

The district also advised that people use public transportation, carpool or reduce driving to help reduce emissions.

So far this year, the four-county air basin of Los Angeles County, San Bernardino County, Riverside County and Orange County has had 43 days that exceeded the federal health standard for ozone, making it the worst start of a smog season since 2008.

And the worst day of the year was Wednesday, June 14, with the worst ozone levels recorded in Redlands. The federal health standard is no more than 70 parts per billion averaged over eight hours. Redlands experienced readings of 100 parts per billion of ozone averaged over eight hours Wednesday.

How to find out when air quality is poor: