Wildfire and smoke

The AQI numbers and what they actually mean for your lungs

The Air Quality Index (AQI) runs from 0 to 500. EPA defines six categories. The first two — Good (0–50) and Moderate (51–100) — require no special precautions for most people. At Unhealthy for Sensitive Groups (101–150), people with asthma, COPD, heart disease, diabetes, pregnancy, children, and elderly adults should limit prolonged outdoor exertion. At Unhealthy (151–200), everyone should limit extended outdoor exposure. At Very Unhealthy (201–300) and Hazardous (301–500), everyone should avoid all outdoor activity.

The particle that matters is PM2.5 — fine particulate matter smaller than 2.5 microns. These particles penetrate deep into the lungs and enter the bloodstream. Cloth masks and standard surgical masks do not filter PM2.5. The fibers in cloth are too widely spaced; surgical masks are not sealed to the face.

What works: an N95 or KN95 respirator, properly fitted with a seal around the nose and bridge. "Properly fitted" is the key phrase — gaps around the edges defeat the filter. EPA guidance and CDC both specify N95-or-better for meaningful PM2.5 protection outdoors during smoke events. For indoor protection, a HEPA air purifier is more practical than wearing a respirator all day.

Check AQI at AirNow.gov before going outside during any smoke event. Current readings, not yesterday's forecast, determine today's exposure.

One room well is better than all rooms poorly

EPA's indoor air quality guidance for wildfire smoke recommends the clean air room strategy when smoke infiltrates homes. The principle: concentrate resources in one room to achieve genuinely clean air there, rather than running one purifier in a large open home and achieving marginal improvement everywhere.

How to set it up: Choose an interior room, ideally one where vulnerable household members spend the most time (a bedroom for overnight, a living room during the day). Close all windows and doors to the room. Seal visible gaps under doors and around windows with damp towels, weather stripping tape, or plastic sheeting. Run a HEPA air purifier appropriate for the room's square footage on its highest setting. Check the purifier's rated coverage — a unit designed for 200 square feet running in a 600 square foot room is providing about one-third of rated filtration.

If you do not have a commercial air purifier, a box fan with a MERV-13 or higher furnace filter taped to the intake side can provide meaningful PM2.5 filtration. EPA has published instructions for this DIY approach (sometimes called a Corsi-Rosenthal box or a box fan filter). It is less effective than a commercial HEPA purifier but significantly better than no filtration.

During extended smoke events, the clean air room is where household members with asthma, COPD, heart disease, or pregnancy should spend most of their time. Brief excursions through the rest of the house for meals and bathroom use are acceptable; sustained time in unfiltered rooms during a Hazardous AQI event is not.

The fire arrives before the flames

The dominant mode of home ignition in wildfires is not direct flame contact. Cal Fire and USFS research has established that most homes that burn in wildfires are ignited by embers — firebrands carried by wind that land on or near the structure and ignite combustible material.

Embers can travel one mile or more ahead of the fire front under wind conditions common in wildfire weather. This means a house can begin igniting from ember spotting well before the fire's leading edge arrives. It also means that homes two miles from a fire, under strong wind, are in active ember range.

Where embers ignite: dry material in gutters and on roofs is the most common point of ignition. Next: wood decks, fences connected to the house, dry vegetation in the zone immediately around the structure, and vents without ember-resistant screening. A single ember landing in a clogged gutter full of pine needles can start a fire in the roof framing within minutes.

Before fire season: clear gutters, install ember-resistant vent covers, remove dead vegetation from the zone immediately around the house, and eliminate wood-to-structure connections (fences that run directly to wood siding, wood mulch touching the foundation). These are the actions that actually prevent ember ignition.

Cal Fire's three-stage system and what each stage demands

Cal Fire's Ready, Set, Go! program was designed to address the most common pattern in wildfire fatalities: households that waited for the GO stage before beginning to prepare, then ran out of time. The three stages are sequential but not equally spaced in time. SET can become GO in minutes.

READY is not a stage — it is a condition. Ready households have completed their preparations before any fire threatens: medications packed in the go-bag, important documents photographed and stored digitally, pets' carriers and food located and accessible, evacuation route decided in advance, fuel in the car maintained above half a tank.

SET means a fire is threatening your area. This is not a passive monitoring phase. At SET, you should: load the vehicle, move pets into carriers, retrieve the go-bag, fill the car with water and medications, be physically in the car or at the car. The SET stage may last 30 minutes or 30 seconds. It is preparation for departure, not time for reflection.

GO means leave now. There is no additional preparation at GO. The households that survive the GO stage are the ones who were ready to drive away at SET. The households that die at GO are often the ones who were still packing when GO was issued.

Cal Fire is direct in its messaging: leaving early, before GO is issued, is almost always the safer choice. The earlier you leave, the less congested the roads. The less congested the roads, the more quickly you are in a safe location.

The five feet that matter most

Cal Fire's defensible space regulations divide the area around a home into zones. Zone 1 runs from the structure out to 30 feet (or to the property line), requiring reduced fuel density. Zone 2 runs from 30 to 100 feet, requiring further fuel management.

But research from the Insurance Institute for Business and Home Safety and USFS fire behavior studies has identified Zone 0 — the 0 to 5 feet immediately adjacent to the structure — as the most critical zone for home survival. A house with a cleared Zone 0 and an unmanaged Zone 1 survives more often than a house with a managed Zone 1 and a combustible Zone 0.

What Zone 0 means in practice: no combustible material touching or directly adjacent to the structure. No wood mulch (use gravel, rock, or concrete). No dry or dead vegetation. No wood furniture, propane tanks, or stored materials within 5 feet of the walls. No wood decking attached directly to the structure without a noncombustible break. No vines or climbing plants on exterior walls or trellises connected to the house.

Zone 0 is achievable for most homeowners in a single weekend of work. It does not require professional landscaping or significant cost. It does require getting rid of the things most homeowners have immediately around their foundations: bark mulch, firewood stacked against the house, old furniture on the deck, and dead plants in the garden beds adjacent to the walls.