Why air quality is deteriorating in the SF Bay Area on Friday

Air quality in the San Francisco Bay Area deteriorated Friday afternoon due to a prescribed burn in Marin County. Similar conditions occurred on Thursday, according to the Bay Area Air Quality Management District.

On Friday just before 3 p.m., air quality levels were moderate across a large swath of the region, according to the federally run AirNow website that tracks air quality across the country. The popular PurpleAir website showed the majority of the region with moderate air quality levels and some areas in the unhealthy for sensitive groups category.

Air quality is measured on a scale of 0 to 500: good (0 to 50), moderate (51 to 100), unhealthy for sensitive groups (101 to 150), unhealthy (151 to 200), very unhealthy (201 to 300) and hazardous (301 and higher).

AirNow’s readings are pulled from sensors monitored by the Environmental Protection Agency, while PurpleAir gets its data from community members’ sensors.

“It’s in a pocket between Novato and Lucas Valley,” said Marimar Ochoa, a spokesperson for Marin County Fire. “It’s planned to be over 100 acres.” 

Marin County Fire said in a news release that smoke would likely be visible today in Novato and the northern San Rafael areas of Lucas Valley, Marinwood, Terra Linda and Santa Venetia. Drivers on Highway 101 will also likely spot some smoke.

Prescribed burns are deliberately started in areas with overgrown vegetation and are closely managed by firefighters. They’re used to prevent large destructive fires. The burns are approved by the air quality management district.

There are no plans for any prescribed burns in Marin County this weekend, Ochoa said.