A Spare the Air day is an air quality alert from the Bay Area Air Quality Management District, called when pollution is forecast to reach unhealthy levels. In winter, from November through February, a Spare the Air alert makes it illegal to burn wood indoors or outdoors anywhere in the Bay Area. Weather decides when those alerts happen, because calm, cold, clear days trap smoke near the ground, while wind and the sea breeze clear it out.
What Is a Spare the Air Day?
A Spare the Air day is a day when the Air District forecasts unhealthy air and asks the public to cut pollution. Winter alerts target fine particles (PM2.5) from wood smoke and carry a legal burn ban. Summer alerts target ground-level ozone and ask people to drive less, but they do not ban burning.
Winter alerts follow the weather closely. The Air District calls them when light winds, cold nights and high pressure trap smoke at ground level like a lid (Bay Area Air Quality Management District). Alerts can be announced up to three days in advance.
What Can't You Do on a Winter Spare the Air Day?
On a winter Spare the Air day, burning wood, fire logs or pellets is illegal in fireplaces, wood stoves, pellet stoves and outdoor fire pits. The ban runs for the full alert day and applies across all nine Bay Area counties, though gas fireplaces and electric heat are allowed.
A first violation brings a $100 ticket, which first-time violators can replace by taking a wood smoke awareness class. A second violation brings a $500 ticket, and fines rise after that. A household can apply for an exemption only if a registered EPA-certified or pellet device is its sole source of heat. Homes with no natural gas or electric service can also qualify (UC Berkeley EHS), but open-hearth fireplaces never do.
To see whether burning is allowed today, visit sparetheair.org, call 1-877-4NO-BURN, or text START to 817-57 for alerts.
Alerts tend to come in streaks. A strong high-pressure system can park over Northern California for a week or more in December or January, bringing frosty nights, sunny afternoons and no wind. During those spells, smoke builds a little more each night, and the Air District may call alerts several days in a row. The streak usually ends when a storm arrives, bringing wind and rain that clean the air within hours.
Why Is Bay Area Air Quality Worse in Winter?
Bay Area air quality is worse in winter because cold, calm nights create temperature inversions that trap pollution near the ground. On clear winter nights, the ground loses heat to space and chills the air above it. That cold air is denser than the warmer air higher up, so it stays put and nothing mixes out. The same setup produces tule fog in the Central Valley.
Under the inversion, smoke from fireplaces, vehicle exhaust and industrial emissions build up through the morning, and PM2.5 can reach levels that are unhealthy for sensitive groups. Wood smoke is a major source of winter PM2.5 in the Bay Area, which is why the winter burn ban targets it. Concentrations stay high until sunshine, wind or an incoming storm breaks the inversion.
Wood smoke matters for health because it is mostly made of fine particles, the same PM2.5 found in wildfire smoke. A single fireplace can raise particle levels for the whole block on a still night, and the smoke seeps into nearby homes through windows and vents. Older adults, young children and people with asthma or heart disease feel it most. That is why the Air District treats winter wood smoke as a serious pollution source, even though each fire seems small.
Why Is Bay Area Air Cleaner in Summer?
Bay Area air is cleaner in summer, outside wildfire smoke events, because the afternoon sea breeze flushes pollution out every day. The westerly flow that cools the inland valleys replaces stagnant surface air with fresh marine air and pushes emissions downwind before they build up. Read more about how the Bay Area sea breeze works.
The marine layer is a sign of that flow at work. Under it, coastal areas usually breathe air that has just crossed hundreds of miles of open ocean. They often show lower particle counts than inland valleys on the same day.
Summer has its own air quality problem, which is ozone. On hot, still days, sunlight turns vehicle and industrial emissions into ground-level ozone, especially in the inland valleys, and those days bring summer Spare the Air alerts. Wildfire smoke can produce far worse air than any winter day, as described in wildfire smoke and Bay Area air quality.
Why Can Air Quality Differ So Much Across the Bay Area?
Air quality can differ sharply across the Bay Area at the same moment because terrain and the marine layer split the region into separate air basins. On a winter inversion day, inland valleys such as the Santa Clara Valley and the Tri-Valley pool cold, smoky air with nowhere to go. Coastal towns such as Half Moon Bay get cleaner ocean air. They can read Good on the AQI scale while downtown Oakland or San Jose reads Unhealthy for Sensitive Groups.
Check your own neighborhood before deciding whether to exercise outside, and check sparetheair.org before lighting a fire between November and February. The weather that makes a cozy fire tempting is often the same weather that makes burning illegal.
For the most local picture, use the Air District's monitor map or the AirNow Fire and Smoke Map, which also shows corrected readings from low-cost sensors. Many neighborhoods have a sensor within a mile or two, which can reveal a smoky pocket that the nearest official monitor misses.
