San Francisco Air quality today

How does wildfire smoke reach San Francisco?

In November 2018, heavy smoke from the Camp Fire in Butte County moved into the Bay Area ^[5]. The air district said air quality was unhealthy through much of the region, because so much smoke was trapped at the surface ^[5]. It extended a Winter Spare the Air Alert through November 20, 2018, and asked people to limit outdoor activity as much as possible ^[5].

On September 9, 2020, dense smoke from several large wildfires in Northern California and Oregon turned the Bay Area sky orange and dark ^[4]. Smoke particles scatter blue light, so only yellow, orange and red light got through ^[4]. The North Complex Fire was one of the main sources of that smoke ^[4].

That morning, much of the smoke sat high in the air, so air near the ground was not as bad ^[4]. The air district reported mostly moderate air quality that morning, but warned it would get worse during the day ^[4].

Why is wood smoke a problem in winter?

The air district says that during winter months, wood smoke is the largest source of air pollution in the Bay Area ^[3]. It makes up more than one third of fine particle pollution ^[3]. Fine particles, called PM2.5, are generally 2.5 micrometers across or smaller ^[6]. An average human hair is about 30 times wider than the largest of them ^[6].

On cold, calm days, wood smoke can become trapped close to the ground by an inversion layer ^[2]. An inversion keeps the air below it from rising, so pollution builds up ^[2].

Fine particles can get past the body's defenses in the nose and throat and reach the lungs and bloodstream ^[3]. High levels can make breathing difficult and aggravate asthma ^[3].

When does summer smog form?

Between April and October, longer and hotter days mean ozone can build up to levels that break federal health standards ^[2]. Ground-level ozone, also called smog, forms when emissions from cars, trucks, construction equipment, gas stations, refineries and other sources react with sunlight ^[2].

In the American Lung Association's 2026 State of the Air report, based on 2022 to 2024 data, the San Francisco Bay Area ranked 14th among the most ozone-polluted places, with 20.2 unhealthy ozone days ^[1].

How does the Bay Area compare with other places?

The 2026 State of the Air report ranked the San Francisco Bay Area 13th on its list of places with the worst annual particle pollution ^[1]. It ranked 20th for short-term particle pollution, with 7.3 unhealthy particle days ^[1].

The same report also showed progress. The Bay Area reached its lowest annual particle pollution levels and its fewest ever unhealthy particle days ^[1].

What is being done about it?

In 2008, the Bay Area Air Quality Management District made it illegal to burn wood on days with a Spare the Air Alert for particle pollution ^[3]. The rule first applied only between November and February ^[3]. In 2019, it was extended to any day of the year with such an alert, for example during a wildfire ^[3].

During an alert, burning wood, fire logs, pellets or other solid fuels is illegal indoors and outdoors across the Bay Area ^[3]. Wood-burning devices are not allowed in new buildings in the region ^[3]. In the warmer months, an alert is also called when ozone is expected to reach unhealthy levels ^[2].

How to protect yourself in San Francisco

When wildfire smoke drifts in, US EPA advice is to stay inside with the doors and windows closed ^[7]. A portable air cleaner sized for the room can help, and it should make little or no ozone ^[7]. Avoid adding to indoor pollution: skip candles, incense, fireplaces and cooking at high heat ^[7].

If you must be outside in smoke, EPA says to wear a snug-fitting N95 respirator and not to rely on dust masks or bandanas ^[7]. Take it easier and avoid strenuous activity such as running while it is smoky ^[7]. The Bay Area air district adds that masks should not be used instead of staying indoors ^[5].

On Spare the Air days for particle pollution, do not burn wood indoors or outdoors ^[3].