Millions of people in Southern California were advised Thursday to limit their outdoor activities and avoid burning wood due to concerns about air quality.

The mandatory wood burning ban has been expanded to include the entire South Coast Air Basin, which includes Orange County and the non-desert areas of Los Angeles, Riverside and San Bernardino counties.

The ban prohibits the burning of wood, pellets and logs indoors and outdoors.

It does not apply to areas above 3,000 feet in elevation, to homes that rely on wood as their sole source of heat, or to low-income homes that do not have a natural gas connection.

The ban and air quality warning are in effect across the South Coast Air Basin until midnight Thursday.

The no-burn rule was enforced because smoke particles can enter people’s lungs and cause breathing problems.

“To protect public health, burn days are required when high particulate matter pollution is expected throughout the South Coast Air Basin,” a National Weather Service warning said.

It noted that burn ban days are implemented based on projected particulate matter levels over a 24-hour period, which may not be reflected in real-time air quality maps.

The agency suggests residents limit their outdoor activities and keep windows and doors closed when air quality is poor.

People are also advised to run air conditioners and air purifiers and avoid using devices that introduce outside air.

California suffers from some of the worst air quality problems in the country, due in part to its high population density, hot weather, frequent wildfires and high human-caused emissions.

The state has taken several measures to address air quality issues, such as imposing strict vehicle emissions standards and promoting clean energy alternatives.

Regulators approved changes to the low-carbon fuel standard last month that will require gas stations to carry a more expensive but cleaner blend of fuel.

Air quality is an ongoing problem for the state and more air quality warnings have been issued in California this year than in 2023.

Depending on the specific concern, air quality warnings can be accompanied by different calls to action. Sometimes residents are advised to avoid refueling their cars or are asked to avoid driving and carpooling instead.

Alarms are more common in the summer because hotter weather affects the formation of ground-level ozone, a harmful pollutant.