How do you protect your lungs from wildfire smoke?

Smoke carries many hazardous pollutants, mainly fine particles. The clearest harm is to the lungs; evidence for heart effects is mixed^1,2. The Air Quality Index (AQI) tells you when to act⁵:

AQI	Meaning
0–50	Good
51–100	Moderate
101–150	Unhealthy for sensitive groups
151–200	Unhealthy; some of the general public may be affected
201–300	Very unhealthy; risk rises for everyone
301 and up	Hazardous; emergency conditions

Indoors helps, if you work at it. With doors and windows shut and no air cleaner, indoor PM2.5 averages about 55 to 60% of outdoor levels, ranging from 30% to 100% by home. With windows open, indoors matches outdoors⁶. In 38 homes hit by wood or forest-fire smoke, a portable HEPA cleaner cut indoor fine particles by an average of 55% in winter and 65% in summer³.

Ranked fixes

1. Close windows and doors, set the AC to recirculate, run the HVAC fan continuously and fit a MERV 13 or better filter⁶.
2. Run a HEPA air cleaner on high in the room you use most. Choose a high CADR (clean air delivery rate) for tobacco smoke, at least about two-thirds of the floor area in square feet: 65 cfm for 100 sq ft, 130 for 200, 195 for 300 and 260 for 400⁷. More on air purifiers and filters.
3. No purifier? A box fan with a MERV 13 furnace filter taped on gave about 111 cfm in a lab test; a 4-inch filter raised that 123% and four filters in a Corsi-Rosenthal box 261%. Change filters when dirty, because smoke-loaded ones lose much of their output⁸.
4. Make a cleaner-air room with few windows and no fireplace, and skip candles, gas stoves, frying and vacuuming, which add particles⁶.
5. Skip ozone generators and ionizers: they can emit ozone, a lung irritant. Filtering gases needs a large activated carbon filter⁷.
6. Outdoors, a snug NIOSH-approved N95 filters over 95% of smoke particles; any gap raises exposure by 10% or more⁴.

When the smoke clears, open windows to air the home out⁶; a shut-up house builds carbon dioxide (see stale air and ventilation).

What the trials measured. Purifier trials measure particles reliably, and small short ones also found lower blood pressure and inflammation in healthy people^9,10. Asthma and illness outcomes are the weak spot¹¹.

Health effects of smoke. Reid and colleagues reviewed studies of mortality and of respiratory, cardiovascular, mental and perinatal health. They found consistent links with asthma and COPD flare-ups, growing evidence for respiratory infections and all-cause mortality, mixed evidence for cardiovascular outcomes, and too little research to say which subgroups are most susceptible¹. Cascio's review reached similar conclusions and named people with respiratory and possibly cardiovascular disease, middle-aged and older adults, children, pregnant women and the fetus as susceptible².

Does the home protect you? Barn and colleagues measured indoor and outdoor PM2.5 in 21 winter wood-smoke homes and 17 summer forest-fire homes. Without a filter, the share of outdoor particles reaching indoors averaged 0.27 in winter and 0.61 in summer; with a HEPA cleaner running it fell to 0.10 and 0.19. Mean air cleaner effectiveness was 55% (SD 38) and 65% (SD 35). Number of windows and season predicted infiltration, but no home feature predicted cleaner effectiveness³. This measures particles, not illness.

What purifier trials measured. Allen and colleagues ran a randomized crossover trial in 45 healthy adults in a woodsmoke-affected community: 7 days with filters, 7 without. Filters cut indoor fine particles by 60%, with a 9.4% rise in a blood-vessel function index (95% CI 0.9 to 18%) and a 32.6% fall in C-reactive protein (4.4 to 60.9%); oxidative stress markers did not change⁹. A systematic review of 10 trials (604 people, median 13.5 days) found home air filtration lowered systolic blood pressure by about 3.9 mmHg (95% CI −7.0 to −0.9) but not diastolic pressure (−0.95; −2.81 to 0.91), and study designs varied¹⁰. For asthma, Lanphear and colleagues randomized 225 children aged 6 to 12 who were exposed to secondhand tobacco smoke to two active or two inactive HEPA cleaners. The HEPA group had fewer unscheduled asthma visits (P = 0.043), but parent-reported symptoms and exhaled nitric oxide did not differ significantly¹¹. That was tobacco smoke, not wildfire.

What is assumed. No trial has randomized people to purifiers during a wildfire and counted heart attacks, admissions or deaths. Fisk and Chan modelled six home-filtration strategies over a 10-day Southern California smoke episode and projected 11 to 63% fewer smoke-attributable hospital admissions and 7 to 39% fewer deaths. They note that costs across all homes exceed the economic value of avoided admissions, and that targeting the homes of older adults is more cost-effective¹². These are projections built on assumed exposure-response relationships, not trial results.

Box-fan filters. Holder and colleagues tested designs in a chamber with simulated smoke. A box fan with a taped-on MERV 13 filter had a CADR of 111.2 ± 1.3 cfm. A shroud added 40%, a 4-inch-thick filter 123%, two filters in a wedge 137% and a four-filter Corsi-Rosenthal box 261%. Output fell sharply once filters were heavily loaded with smoke⁸. By the EPA's sizing chart (65 cfm per 100 square feet), the basic version suits a room of roughly 170 square feet⁷. That is a chamber result; real fans, filter fit and rooms vary.

Masks. The EPA says a correctly worn N95 will filter more than 95% of smoke-sized particles, but a poor seal can leave exposure as high as wearing nothing. Surgical and dust masks do not prevent smoke inhalation, and homemade masks and scarves are expected to protect less than an N95 or KN95⁴.

Where results conflict. Cardiovascular effects of smoke are mixed in the epidemiology^1,2. The biomarker findings come from healthy volunteers over days to two weeks, with one marker set moving and another not⁹. Diastolic pressure did not change¹⁰, and the asthma trial's symptom scores did not differ¹¹. None of these settings matches a smoke event in intensity or duration.

Bottom line (moderate confidence). Keeping smoke out, running a properly sized HEPA cleaner and wearing a snug N95 outdoors all reduce particle exposure, and that part is well documented. Fewer illnesses during a wildfire is plausible and partly supported by short trials, but not directly shown, so treat these steps as sensible precautions, especially for people with lung or heart disease.