Wildfire smoke has become the Bay Area's biggest and least predictable air quality threat. From 2017 through 2021, the region went through repeated smoke episodes that closed schools, canceled events, emptied streets and sent people searching for masks and air purifiers. Some of that smoke came from fires close to home, such as the North Bay fires of 2017 and the lightning fires of 2020. Much of it came from far away, from fires in the Sierra Nevada, the northern mountains and even Oregon. Knowing how smoke travels, and what makes it settle over the region, helps you plan outdoor activities, protect your health and understand why the sky sometimes turns orange.
How Wildfire Smoke Reaches the Bay Area
A large wildfire can send its smoke thousands of feet into the air. Once it is up there, winds can carry it hundreds of miles. In late summer and fall, those winds often blow from the north or northeast, especially when high pressure builds over the interior West. That pattern carries smoke from fires in Northern California and Oregon south and west toward the Bay Area.
What happens next decides how bad the air gets. Over the Bay Area, a layer of warm air often sits above the cooler air near the ground, a pattern called a temperature inversion. That warm layer acts like a lid, keeping the air below it from mixing upward. If smoke gets trapped under the lid, it piles up near the ground where people breathe, and readings climb quickly.
Sometimes the lid works the other way. When smoke stays above the cool marine layer, the air at ground level can stay fairly clean even as the sky turns hazy or orange. That happened on September 9, 2020, when thick smoke high overhead turned the sky a deep orange while many monitors still showed moderate readings. As the marine layer mixed out later, more of that smoke reached the ground.
The Camp Fire: November 2018
The worst smoke episode in modern Bay Area history came from the Camp Fire in Butte County. The fire started on November 8, 2018, during strong, dry offshore winds and swept through the town of Paradise. It killed 85 people, destroyed more than 18,000 structures and eventually burned about 153,000 acres.
Northeast winds carried the smoke straight toward the Bay Area, more than 150 miles away. With calm weather and an inversion over the region, the smoke settled in and stayed. For nearly two weeks, air quality across the Bay Area ranged from unhealthy to very unhealthy, with AQI readings rising above 200 on several days.
Schools across the region closed, cable cars stopped running and many outdoor events were canceled. N95 masks sold out at hardware stores, since few residents owned them at the time. The episode was a turning point. It showed that smoke from a distant fire could disrupt daily life across the whole region for weeks. Afterward, many households bought air purifiers and began keeping masks on hand.
Deep Dive: Why Are Smoke Particles So Hard on the Body?
Wildfire smoke is dangerous mainly because of its size. Most of its particles are far smaller than a human hair, small enough to slip past the nose and throat and settle deep in the lungs. Research also suggests that smoke particles may be more harmful than the same amount of pollution from other sources.
How Small Are Smoke Particles?
PM2.5 includes every particle smaller than 2.5 micrometers across, about one thirtieth the width of a hair. Most of the particles in wildfire smoke are much smaller still, less than 1 micrometer. Particles that small follow the air as you breathe instead of sticking in the nose and throat.
Many of them reach the alveoli, the tiny air sacs where oxygen passes into the blood. From there, the smallest particles and the chemicals they carry can inflame the lungs and affect the heart and blood vessels. That is why smoke can trigger both asthma attacks and heart problems.
Particle size also explains why good filters work. Very small particles drift and bump into filter fibers, while larger ones crash into the fibers head on. The hardest size to catch sits in between, around 0.3 micrometers. HEPA filters are rated to remove at least 99.97 percent of particles of that size, so they catch the smaller and larger ones even better.
Is Smoke Worse Than Other Pollution?
A 2021 study in Nature Communications by Rosana Aguilera of the Scripps Institution of Oceanography and colleagues compared the two. They used 14 years of hospital admissions across nearly 700 ZIP codes in Southern California. They identified wildfire smoke days using strong Santa Ana winds with fire upwind.
An increase of 10 micrograms per cubic meter in wildfire PM2.5 was linked to a 1.3 to 10 percent rise in respiratory hospital admissions. The same increase in PM2.5 from other sources was linked to about a 1 percent rise. The authors could not say for certain why smoke appears more harmful, and scientists have not reached a consensus.
Smoke from burning homes, cars and other structures can add metals and other toxic compounds that ordinary vegetation smoke lacks. Whatever the reason, the study supports treating smoke days with extra caution, even when the AQI looks only moderately high.
Protecting Yourself During Smoke Events
The most important steps are simple, but they work best if you prepare ahead of time. During a smoke event, stay indoors with windows and doors closed. Run an air purifier with a HEPA filter, or make a low-cost version by strapping a furnace filter to a box fan. A purifier in the bedroom matters most, since that is where you spend the longest stretch of time.
Getting ready before fire season makes smoke days much easier. In early summer, buy a few N95 masks and a spare filter for your air purifier. If your home has central heating or cooling, add a high-efficiency filter rated MERV 13 or higher. Pick one room you can keep sealed and clean, ideally a bedroom with few windows and a purifier sized for it. When smoke arrives, supplies often sell out within a day, so having them on hand matters.
If you must go outside, wear a well-fitted N95 or KN95 mask, which filters most fine smoke particles. Surgical and cloth masks help far less. Check the AQI before exercising outdoors, because breathing hard pulls more smoke deep into your lungs. People with asthma, heart disease or other heart and lung problems, along with older adults and children, should be especially careful once the AQI rises above 100.
If you run your home's heating or air conditioning system, set it to recirculate indoor air rather than pulling in air from outside. In a car, use the recirculate setting too. A clean, high quality filter in your home system can also help cut indoor smoke levels.
Smoke affects health even when it does not look dramatic. Fine particles in wildfire smoke can irritate the eyes and throat, trigger asthma attacks and strain the heart. Symptoms such as coughing, a scratchy throat, headaches or tightness in the chest are signs to move indoors and reduce your exposure. Anyone with trouble breathing or chest pain should seek medical help right away.
Air quality can change quickly during a smoke event, sometimes by more than 100 AQI points within a few hours as the smoke shifts. It can also differ a lot from place to place. Coastal areas often get cleaner ocean air while inland valleys stay smoky. An inversion can also trap smoke in the East Bay flatlands while the Marin coast is clear.
The Bay Area Air Quality Management District and the AirNow website post real-time readings from monitors across the region. The AirNow Fire and Smoke Map also shows corrected readings from low-cost sensors, which fill gaps between official monitors. Checking the reading nearest you, rather than a regional average, gives the most useful picture of the air you are breathing.
