Air quality and wildfire smoke

On a typical day Palo Alto's air is clean. Breezes off the bay and the Pacific keep pollution moving and the city has no heavy industry. The bad air days come from three sources. In winter, smoke from fireplaces gets trapped near the ground on cold, still nights. In summer, sun and heat cook car exhaust into ozone smog. And from late summer into fall, smoke from wildfires anywhere in Northern California can drift over the Peninsula for days.

This guide explains who measures the air, how to read the numbers and what official agencies advise. It draws on the Bay Area Air Quality Management District, the EPA's AirNow site and the EPA's indoor air guidance. For wildfire risk in the hills above town, see our emergency preparedness guide.

Who measures the air

The Bay Area Air Quality Management District, usually called the Air District, regulates air pollution across the nine Bay Area counties, including Santa Clara County. It forecasts air quality, issues Spare the Air Alerts and enforces the wood burning rule. Its main line is (415) 749-5000.

The EPA runs AirNow, which turns readings from official monitors into the Air Quality Index you see in weather apps. During fires, AirNow's Fire and Smoke Map adds data from thousands of low-cost sensors, which gives a much finer picture of smoke street by street. AirNow recommends that map when wildfire smoke is affecting your area.

Reading the Air Quality Index

The AQI runs from 0 to 500. Higher numbers mean more pollution and more health concern. These are the categories on AirNow's AQI basics page:

The number that matters most during smoke is fine particulate matter, called PM2.5. These particles are small enough to reach deep into the lungs. In summer smog, ozone is usually the pollutant driving the index.

Spare the Air Alerts

The Air District calls a Spare the Air Alert when it forecasts unhealthy air. There are two kinds, according to its Spare the Air page.

Summer smog

Ozone alerts usually fall between April and October on hot, sunny days. The district asks people to drive less and cut back on other polluting activities. People sensitive to unhealthy air should limit time outdoors, especially in the afternoon, when ozone peaks. These days often line up with heat waves. Our heat waves guide covers the heat side.

Winter wood smoke and wildfire smoke

On cold, still winter days, temperature inversions trap wood smoke close to the ground. When fine particle pollution is forecast to exceed the district's threshold, it calls an alert that makes burning wood illegal everywhere in the Bay Area, indoors and outdoors. That covers fireplaces, wood stoves, fire pits and outdoor wood-burning devices. Since 2019 the ban also applies on any day of the year when an alert is called for fine particles, which includes wildfire smoke days.

The wood smoke rule has a narrow exemption. A household whose sole source of heat is a wood-burning device must use an EPA-certified device registered with the Air District. An open hearth fireplace never qualifies. The district also runs a program called Clean HEET that helps pay to replace wood-burning devices.

Getting alerts

• Check sparetheair.org, baaqmd.gov or the Spare the Air app.
• Call 1-800-HELP-AIR to hear whether an alert is in effect, or 1-877-4NO-BURN for wood burning bans.
• Text START to 817-57 for text alerts.
• Sign up for the district's email alerts, which arrive the day before an alert is called.

Alerts often fall on holidays. The district called wood burning bans for Thanksgiving Day and New Year's Day in recent seasons, so check before you light a holiday fire.

When wildfire smoke arrives

Smoke from fires far away can blanket Palo Alto even when no fire is burning nearby. Smoke levels change a lot during a single day, so check the Fire and Smoke Map often. AirNow's guidance on when smoke is in the air boils down to a few steps.

1. Reduce exertion. Skip running, cycling hard and yard work. Lower effort means less smoke breathed in. Our running routes guide will still be there when the air clears.
2. Shift outdoor tasks. Move them to a time when air quality improves. If you must work outside, take frequent breaks indoors in clean air.
3. Use the right mask. Cloth and paper dust masks do not protect against smoke. N95 respirators can, if they fit well.
4. Watch pets. Limit their time outside and watch for coughing or trouble breathing.
5. Stay ready to move. Follow local news in case a nearby fire leads to evacuations. Our emergency alerts guide explains AlertSCC.

Schools, sports leagues and Stanford may cancel outdoor events on heavy smoke days. Check with them directly. If you have asthma or heart disease, talk with your doctor before fire season about a plan for smoke days. Our healthcare guide lists local clinics and hospitals.

Cleaner air indoors

Most people spend smoke days at home, so the indoor setup matters. The EPA's guidance on wildfires and indoor air quality centers on a clean room.

Set up a clean room

• Choose a room that fits everyone in the household, ideally with a bathroom attached.
• Keep its windows and doors closed.
• Run a portable air cleaner sized for the room. Pick one with a known clean air delivery rate, called CADR and make sure it does not produce ozone.
• AirNow notes that portable cleaners work best run continuously with doors and windows shut.

Use your HVAC system well

If you have central heating or air conditioning, use a high-efficiency filter rated MERV 13 or higher if the system can take it and replace it often. The filter should fit snugly with no air leaking around it. AirNow suggests setting the fan to on rather than auto and closing any fresh-air intake during smoke. Ask an HVAC technician if you are unsure what your system can handle.

Do-it-yourself fan filters

If you cannot get a commercial cleaner, the EPA describes a fallback: one or more furnace filters attached to a box fan. Commercial units with a known CADR are preferred. AirNow warns never to leave an older-style box fan cleaner running unattended.

Do not add pollution

Avoid frying or broiling food, burning candles or incense, smoking, vaping and using gas, propane or wood stoves. Sweeping and ordinary vacuuming stir up particles, so use a HEPA vacuum if you must clean. When the outside air improves, even briefly, open the windows and run kitchen and bathroom exhaust fans to air the house out.

Smoke and heat at the same time

The worst days combine smoke with heat. Closing every window keeps smoke out but can make an older home without air conditioning dangerously hot. AirNow advises keeping the clean room cool with air conditioning, heat pumps, fans and shades. If that is not possible, go to a clean air shelter, a large air-conditioned building with good filtration, or stay with friends or family. Palo Alto's libraries, listed in our heat guide, are a practical choice on those days.

Gas stoves are a smaller everyday source of indoor pollution too. The city's electrification program, covered in our utilities guide, offers help switching to induction cooking and heat pumps, which also cool the house.

Frequently asked questions