The Cleaner-Air Boundary
Wildfire smoke protection is a design problem: how can a person reduce the dose they actually receive when outdoor air is changing, buildings leak, heat is present, and no single mask or machine removes every pollutant? This chapter focuses on the boundary between smoky air and the spaces where people live, work, and rest. It explains how to build that boundary with source control, cooling, ventilation, filtration, and cleaner-air spaces, then shows when a fitted respirator or community support is useful—and where each option stops being reliable.
The important shift is from a slogan to a condition. “Stay indoors” is often sensible, but indoors is not automatically clean, cool, or safe. A protection plan has to account for the building, the person, the pollutant, and the duration of the smoke event. It should also work for people who cannot purchase equipment, leave work, drive to a shelter, or control the ventilation in their home.
Indoor air is a boundary you have to build
During wildfire smoke events, indoor PM2.5 concentrations are usually lower than outdoor concentrations, but they are often higher than during ordinary periods. The difference between those two facts matters. A person who moves inside may reduce exposure, yet still inhale substantial smoke if the building is leaky, doors and windows are open, or an air-conditioning system draws in unfiltered outdoor air.
The federal Wildfire Smoke Guide summarizes studies in which indoor PM2.5 was about 55–60% of outdoor concentrations when doors and windows were closed and no portable air cleaner was operating. That is an average, not a promise. Across homes, the indoor fraction ranged from roughly 30% to 100%. Newer, tighter homes and homes with central air conditioning generally limited infiltration more effectively than older homes. If windows and doors are left open, indoor and outdoor concentrations can become similar.
This makes the building envelope the first part of the boundary. During a smoke episode, close windows and doors when it is safe to do so. Check obvious gaps around doors and window-mounted equipment. If a central system has both fresh-air and recirculation settings, recirculation is generally the appropriate setting during smoke. Systems that intentionally bring in outdoor air, such as some whole-home ventilation systems, may need to be adjusted or temporarily turned off. Exhaust fans, whole-house fans, bathroom fans, range hoods, and clothes dryers can create negative pressure and draw smoky air through cracks; use them sparingly when outdoor smoke is high.
The same advice does not apply identically to every cooling system. A ductless mini-split generally conditions indoor air without bringing a large volume of outdoor air into the room. A single-hose portable air conditioner can place a room under slight negative pressure and pull unfiltered air inside. A dual-hose unit keeps the supply and exhaust paths separate and does not create the same pressure problem. Evaporative, or swamp, coolers bring in large amounts of outdoor air and may be difficult to use during heavy smoke. These distinctions are not trivia: the machine that keeps a room cool may also change how much smoke enters it.
Heat is the major boundary condition. Closing windows and doors can reduce smoke entry, but a sealed, uncooled home can become dangerous in hot weather, especially for older adults, young children, and people with health conditions. If indoor temperature is rising, cooling has to be solved rather than ignored. That may mean opening windows briefly when the outdoor air is cooler, using a suitable cooling system, going to a friend or relative’s filtered and air-conditioned home, or using a public cleaner-air and cooling space. If those options are unavailable and the home cannot remain cool, leaving the smoky area may be safer than following an indoor rule that creates heat stress.
Indoor sources matter because a home can add particles to the air even while outdoor smoke is falling. Smoking and vaping, frying or broiling, candles, incense, aerosol products, some cleaning activities, and gas- or wood-burning appliances can raise indoor pollutant concentrations.