Air pollution in metropolitan Los Angeles reached unhealthy levels in many areas on Monday.

Data from IQAir and Purple Air, both of which measure particulate matter pollution (PM2.5), showed readings of 100 micrograms per cubic metre or higher in downtown L.A., the San Fernando Valley, San Gabriel Valley, Riverside and San Bernardino counties.

Measurements from the South Coast Air Quality Management District showed slightly better air quality early in the afternoon.

PM2.5 refers to particles in the air that are less than 2.5 microns in diameter, which can include vehicle exhaust, smoke, power plant and factory emissions, and metal vapors.

PM2.5 readings between 101 to 150 are considered “Unhealthy for Sensitive Groups.” People with heart or lung disease, older adults, children, and people with diabetes are urged to avoid prolonged or heavy exertion outdoors.

Despite improvements over the past three decades, several California cities, including Los Angeles, still have the worst air quality in the nation, based on the American Lung Association’s annual ‘State of the Air’ report.