Air quality in San Francisco

How's the air quality in San Francisco?

San Francisco’s air quality is generally rated “good,” meaning that the air poses little risk to health. In 2019, San Francisco averaged an annual PM2.5 level of 7.1 μg/m³, meeting the World Health Organization’s (WHO) guideline for annual exposure (< 10 μg/m³).

The ‘good’ air quality status in San Francisco can be attributed to the city’s coastal geography, meteorology, and sparse industrial activity.

Air pollution in San Francisco comes primarily from transportation emissions, such as from vehicles, planes, and ships.¹ Wildfires, which are becoming increasingly common in the area, give way to drastic air pollution spikes, usually in the summer and fall.

Discounting the influence of wildfires, winter months are commonly more polluted than summer months as a result of increased heating and wood burning. In 2019, November, January and December were the most polluted months for PM2.5 respectively (14.5, 10.7, and 8.2 μg/m³). November’s high average was partially attributable to the Ranch fire, which burned more than 2,534 acres.²

In the last 3 years, 5 of the most destructive California wildfires were in relatively close proximity to San Francisco, greatly impacting air quality across the region.³ In November 2018, the Camp fire burned 153,336 acres and drove San Francisco’s AQI for the month up to 137 (“unhealthy for sensitive groups”).

Numerous other fires in 2018, including the Kincade Fire, Carr Fire, and Mendocino Complex Fire, further elevated San Francisco’s air pollution levels. The city’s annual PM2.5 average for this year was 12.6 μg/m³ (“moderate”), exceeding the WHO annual target for safe PM2.5 exposure, and raising the city's ranking to 49 of 723 for worst air pollution in the United States.

Generally, air quality in San Francisco is best in the spring, as temperatures warm and forest undergrowth is still wet from the frequent rains in the winter months. March often experiences the cleanest monthly air quality of the year, as was the case in 2019.

While year-on-year air quality trends are fairly consistent, real-time air quality is subject to daily fluctuations based on weather events and emissions. Live air pollution data is therefore a valuable resource in understanding how to best protect one’s health and the health of loved ones. Refer to the top of this page for San Francisco’s forecast air quality data and real-time air quality data in order to better understand present conditions and necessary health advisories.

When will San Francisco air quality improve?

In recent years, San Francisco’s average annual air quality has been highly contingent on the wildfire season. In 2019, for example, San Francisco experienced a 43.7% decrease in PM2.5 from the year prior, though this reduction is attributed largely to the reduction in severe wildfires and not from emission reductions from other sources. 2018 experienced severe wildfires, and thus experienced a 26% increase in PM2.5 from 2017.

On a larger scale, despite a growing population and economy, air quality has improved significantly in the last 30 years since the 1990 amendments to the Clean Air Act. Increasingly tight regulations against industrial activity and related emission sources are to credit. The Hunters Point and Potrero Hill power plants were closed in 2006 and 2010 respectively, while other industrial businesses have since moved out of the city. The effect has been a reduction in “unhealthy” air quality. Yet there has not been a significant increase in days classified as “good” (AQI 0-50).⁴ This is because pollution levels on average have decreased, while peaks into “moderate” air quality levels are still common. In recent years, roughly 20-25% of calendar days average “moderate” or worse air quality.

In order to further reduce San Francisco’s air quality index, a shift from fossil fuel dependence, such as gas-powered transport, to cleaner, more sustainable energy is needed. San Francisco currently aims to transition to 100% renewable electrical power by 2030, while shifting to 100% greenhouse gas-free transportation by 2040.^5,6

Wildfires are a more difficult emission source to regulate, particularly in a warming climate. Prevention methods, such as creating fire lines and removing volatile forest undergrowth through low-intensity “prescribed fires,” offer ways to reduce the size and ecological impact of fires. The California Department of Forestry and Fire Prevention (CAL FIRE) estimates that 8-10 million acres urgently require thinning and ‘prescribed burning’ in order to prevent mega-fires, or future disastrous wildfires.⁷ In coordination with the U.S. Forest Service, they aim to thin a million acres a year, an ambitious target.

Is there smog in San Francisco?

Despite fewer power plants and industrial businesses as well as a growing mix of cleaner energy, San Francisco still experiences periods of unhealthy air quality. Primary factors for elevated pollution levels include transportation emissions from cars, trucks, planes, and ships as well as seasonal wildfires.

While transportation emissions are a constant source of air pollution, wildfires are generally the reason for extreme air quality events, such as air quality in San Francisco being described as “unhealthy” or worse.

City-wide emissions are frequently trapped near the ground as a result of a weather event described as marine inversion. Marine inversions are temperature inversions created by a city’s proximity to an ocean or large body of water. In the case of San Francisco, waters from the Pacific ocean are cold and reduce ground temperatures in surrounding areas. These temperatures are often significantly colder than the winds moving over the region from inland locations.

By viewing an air pollution map of San Francisco and present wind directions, it’s possible to get a sense of where polluted air is coming from - either inland emission sources, such as wildfires, regular emissions as a result of transport, or trapped air pollution due to a marine inversion (polluted San Francisco air, with wind blowing from towards the coast).

+ Article Resources

[1] San Francisco Planning Department - air quality element. (2020).
https://generalplan.sfplanning.org/I10_Air_Quality.htm
[2] Cal-Fire - Ranch Fire. (2020).
https://www.fire.ca.gov/incidents/2019/11/3/ranch-fire/
[3] Top 20 most destructive California wildfires. (2020).
https://www.fire.ca.gov/media/5511/top20_destruction.pdf
[4] Days with an EPA Air Quality Index Rating of "Good". (2020).
https://sfgov.org/scorecards/environment/days-epa-air-quality-index-rating-good
[5] SF Environment - Clean Fuels and Vehicles. (2020).
https://sfenvironment.org/clean-vehicles/overview/clean-fuels-and-vehicles
[6] SF Environment - Clean Fuels and Vehicles. (2020).
https://sfenvironment.org/electricmobilitysf
[7] Helvarg D. (2019, December 20). How will California prevent more mega-wildfire disasters?.
https://www.nationalgeographic.com/science/2019/12/how-will-california-prevent-more-mega-wildfire-disasters/#close