Many Southern California residents were urged to limit their outdoor activity on Tuesday as air pollution reached unhealthy levels across the region.

Based on South Coast Air Quality Management District’s map, the Los Angeles Basin, San Gabriel Valley and Orange County were seeing fine particulate levels considered to be unhealthy for sensitive groups as of mid-morning. This means people with heart disease, pregnant women, children and older adults, and those with lung disease should “limit prolonged or heavy outdoor exertion,” AQMD says.

Purple Air, which gathers data from dozens of both official and unofficial air monitors, shows higher readings considered to be unhealthy for everyone.

Many factors impact air quality, including particulate pollution from vehicles, factories, refineries, household combustibles and wood burning. Other common sources include dust and wildfire smoke.

A No-Burn Alert remains in effect for Tuesday and has been extended to Wednesday for all residents of the AQMD’s basin, which includes Orange County and non-desert portions of Los Angeles, Riverside and San Bernardino counties.

No wood burning is allowed except in communities above 3,000 feet, the Coachella Valley and High Desert, or if a home relies solely on wood for heating.

“[Particulate matter] can get deep into the lungs and cause respiratory problems (including asthma attacks), increases in emergency room visits and hospitalizations,” air quality managers said.