Wildfire Smoke and Health

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Introduction: Wildfire Smoke in San Francisco

Climate change increases the frequency and intensity of wildfires in the Western United States. Although San Francisco is less directly vulnerable to wildfires than other regions of the state, the city is indirectly affected by wildfire smoke. The health effects from exposure to wildfire smoke can range from the relatively minor (e.g., eye and respiratory tract irritation) to the serious (e.g., exacerbation of respiratory conditions, cardiovascular conditions). The health impacts of wildfire smoke are inequitable. The people and communities that are most likely to suffer the greatest impacts are disproportionately exposed, most likely to have pre-existing health conditions, and least likely to have the resources to prepare or respond.

What is wildfire smoke?

Wildfire smoke is comprised of both gaseous and hazardous pollutants, water vapor, and particulate matter (PM).1 These particles can be made up of acids, inorganic compounds, organic chemicals, soot, metals, soil or dust particles, or biological materials. Particulate matter under 2.5 micrometers in size (PM2.5) is particularly harmful if inhaled. 

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Climate Projections

Are wildfires actually getting more frequent and more extreme?

Yes. Since 2015, California has experienced:²

• 12 of the 20 largest wildfires in California's history.
• 7 out of the 20 deadliest wildfires in California's history.
• 15 out of the 20 most destructive wildfires in California’s history.

What is the connection between climate change and wildfire?

• A warmer and drier climate kills trees and vegetation which accelerates burning.³ Heat, drought, and invasive species such as the bark beetle have killed an estimated 150 million trees in the Sierra Nevada Mountains since 2012.⁴ Climate change, combined with forest management practices that site homes and infrastructure in the wildland urban interface, has driven recent wildfire severity.⁵

• The conditions that favor dry lightning, when the air is so dry that water vapor evaporates before it reaches the ground, are more likely to occur as the climate warms. 46% of all lightning strikes between May and October (1987-2020) in central and northern California occurred as dry lightning, and these strikes accounted for nearly 50% of California’s burned area.⁶ 

How will San Francisco be impacted by wildfires elsewhere in the state? 

While San Francisco is unlikely to have wildfires within our city boundaries, we expect to be affected by wildfire smoke. 

• In 2018, the Butte County Camp Fire in and around Paradise, California claimed 85 lives and destroyed 18,804 structures. The smoke from the Camp Fire was funneled south and west into the San Francisco Bay Area. San Francisco's Air Quality Index (AQ!) was over 150, "unhealthy" for 12 straight days, peaking at 250.⁷ 
• Fall wildfire smoke events are especially likely to impact the San Francisco Bay Area if they coincide with the Diablo Winds. The Diablo Winds are October and November northeasterly winds that bring hot and dry air from the Great Basin through the Central Valley and over the Coastal Range.⁸ Many of San Francisco’s most severe wildfire smoke events, including the 1991 Oakland Hills Fire, 2017 Napa/Sonoma Wildfires, and 2018 Butte County Camp Fire all occurred during the Diablo Winds. 

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Health Impacts

How does wildfire smoke impact public health?

While many health impacts of wildfire smoke are relatively mild, exposure to smoke increases the odds of non-traumatic mortality across all age groups. ⁹ The following represents some mild and severe health impacts of wildfire smoke.¹⁰ 

• Eye irritation, skin irritation, and allergies

• Short-term respiratory symptoms (coughing, phlegm, wheezing)
• Asthma and chronic obstructive pulmonary disease
• Cardiovascular disease
• Adverse birth outcomes (low birth weight, pre-term birth, gestational diabetes, gestational hypertension). 
• Behavioral and cognitive impacts (depression, anxiety, stress).

Does wildfire smoke have other impacts?

Wildfire smoke can also have social and economic impacts. Wildfire smoke is one of the leading causes of school closures in the Western United States.¹¹  Wildfire smoke from the Butte County Camp Fire forced San Francisco's schools to close as many of these facilities do not have adequate ventilation to protect against smoke intrusion.¹² This disruption has cascading impacts on families as parents are more likely to miss work to meet unexpected childcare needs. This disproportionately impacts lower income households.

How can I prepare for the health impacts of wildfire smoke? 

• Connect to the San Francisco emergency preparedness portal, SF72. 
• Check the air quality forecast.
• Learn about the health impacts of wildfire smoke.
• Learn about wildfire smoke and indoor air quality.

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Drivers of Health Impacts

While everyone is vulnerable to the health impacts of wildfire smoke, not everyone will be impacted evenly. The communities that suffer the most are the ones who already currently carry the heaviest health burden. This inequitable distribution of health impacts is referred to as the climate gap.

Certain communities will be particularly impacted based on:

• Their exposure to the hazard

• Their physical sensitivity to the hazard

• Their ability to adapt to the hazard—to have access to the economic, political, and social resources to be resilient.

Below we define these three categories. Please check out our Climate Change and Equity page for more information on the drivers of health impacts.

Exposure

Exposure refers to a person's physical proximity to the hazard. Exposure to wildfire smoke and air pollution varies from neighborhood to neighborhood, building to building, and community to community. Things that can modify exposure include:

• Access to ventilation.

• Being unhoused or marginally housed. 

• Living in a neighborhood with more air pollution because it is adjacent to freeways, highways, or industrial uses. 

• Living in a neighborhood with less trees or green space. 

Sensitivity

Sensitivity refers to a person's physiological reaction to wildfire smoke. Two people may be equally exposed to smoke, but one person may be more sensitive to that exposure. People that are particularly sensitive to wildfire smoke include:

• Older adults

• Children

• Pregnant individuals

• People with pre-existing health conditions such as cardiovascular illnesses, asthma, and diabetes. 

Adaptive Capacity

Adaptive capacity refers to the ability of a person to prepare for or respond to wildfire smoke. Two people may be equally exposed and equally sensitive, but one person may be more resilient because they have access to political, economic, or social resources. Adaptive capacity can be influenced by:

• Race and ethnicity

• Social isolation

• Income

• Disability

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