In short
- Staying indoors reduces your exposure to wildfire smoke—but doesn't eliminate it. Indoor PM2.5 can reach 50-75% of outdoor levels in a typical home with closed windows.
- The five-step protocol (seal, recirculate ventilation, HEPA purifier, clean room, mask during peak hours) is the only combination that genuinely reduces exposure.
- Vulnerable populations—children, the elderly, asthmatics, COPD, or heart patients—should treat indoor air quality during a smoke event as a medical priority, not a comfort issue.
The advice sounds reassuring: stay indoors. And it's not wrong—being inside does reduce your exposure to wildfire smoke. But "reduces" is not "safe." During the Fontainebleau fires in July 2026 near Paris, Airparif's Recloses station recorded hourly PM2.5 peaks of approximately 400 µg/m³—more than 26 times the WHO's 24-hour recommendation (15 µg/m³). People miles away, with closed windows, were still breathing smoke-laden air.
This guide is for anyone confined to their home during a smoke event. We explain precisely how smoke enters, what the data says about indoor air quality during a fire, and the actions that actually work.
The myth of "stay indoors"—why your home isn't a bubble
Here's the uncomfortable truth: a standard home is not an airtight refuge. It's a porous envelope through which smoke continuously infiltrates, simply slower than if you were outside.
How smoke infiltrates buildings
Smoke particles—mainly PM2.5, smaller than 2.5 micrometers—are extraordinarily fine. They don't need an open window to enter. They pass through:
- Gaps around doors and windows—especially in older buildings with worn seals
- Electrical outlets and plumbing penetrations—tiny but numerous
- Ventilation systems—any system drawing in outdoor air actively pumps smoke indoors
- Exhaust fans (bathroom, kitchen) which create negative pressure and draw outdoor air through any tiny crack
- Chimneys and fireplace flues, even when "closed"
A 2026 review of evidence published by the NCCEH found a median indoor/outdoor PM2.5 ratio of 0.75 in wildfire smoke studies—meaning three-quarters of the outdoor concentration can be found indoors. In older, poorly insulated homes, this ratio can approach 1.0.
Indoor and outdoor PM2.5 during a fire—the data
Studies consistently show that indoor PM2.5 levels rise sharply during a smoke event, even with closed windows:
| Situation | Outdoor PM2.5 | Indoor PM2.5 | Indoor/Outdoor Ratio |
|---|---|---|---|
| Typical home, windows closed | 49 µg/m³ | 8–20 µg/m³ | ~0.2–0.4 |
| Porous older home, no purifier | 49 µg/m³ | 25–37 µg/m³ | ~0.5–0.75 |
| Extreme event (Bordeaux, July 2026) | 300+ µg/m³ | 150–225 µg/m³ (estimated) | ~0.5–0.75 |
| Home with HEPA purifier running | 49 µg/m³ | 3–8 µg/m³ | ~0.06–0.16 |
Sources: NCCEH 2026 evidence review; EPA indoor air quality recommendations; Copernicus/CAMS data July 2026.
Even in the lower range, an event with 300 µg/m³ outdoors will put you at 150 µg/m³ indoors—ten times the WHO's 24-hour limit—while you sit on your couch, convinced you're protected.
How quickly does indoor air degrade?
Quickly. Smoke can begin to infiltrate within minutes after outdoor levels rise. In a home without filtration, indoor concentrations equalize with outdoor levels in 2 to 4 hours. Worse: once deposited on surfaces and furniture, particles can be resuspended and persist 8 to 10 hours after outdoor air has cleared—so you can breathe contaminated air long after the plume has passed.
Practical consequence: don't wait until you smell smoke indoors to act. By then, concentrations are already significant.
Who is most at risk indoors?
Not everyone faces the same risk. But several groups are at high risk.
Older, poorly insulated buildings
Buildings constructed before 1980—very common in France and Southern Europe—have air exchange rates 2 to 3 times higher than modern insulated buildings. More exchanges mean more smoke infiltration per hour. If your home has single-pane windows, gaps under doors, or no weatherstripping, your indoor/outdoor ratio during an event will approach 0.7–0.9.
Homes near the fire perimeter
Distance matters greatly. During the Gironde fires in July 2026, Bordeaux residents reported coughing all night with closed windows. At outdoor concentrations above 200 µg/m³, even an infiltration rate of 0.3 results in 60 µg/m³ indoors—well within the "poor" zone. Within 10 km of an active front, confinement alone is insufficient without active filtration.
Individuals with respiratory or cardiovascular conditions
Pulmonologist Bruno Crestani, president of the Fondation du Souffle, was direct in July 2026: smoke particles "can enter the circulatory system" and potentially trigger diabetes, stroke, or heart attack. ANSES specifically identifies asthma, COPD, chronic respiratory failure, heart failure, and coronary artery disease as conditions making exposure particularly dangerous. For these populations, indoor air quality during a fire is a medical issue, not a comfort one.
Children and pregnant women also fall into this high-risk category. Their developing or physiologically stressed bodies are more vulnerable to the oxidative and inflammatory effects of PM2.5.
The indoor protection protocol—step by step
Here is the five-step combination consistently recommended by the EPA, ANSES, and public health agencies. No single step is sufficient alone. Together, they can reduce your indoor PM2.5 by 80 to 90% compared to doing nothing.
| Step | Action | Priority |
|---|---|---|
| 1 | Seal the home | High — do first |
| 2 | Turn off ventilation or set to recirculation | High — immediate |
| 3 | Run a HEPA + carbon air purifier | High — continuously |
| 4 | Create a clean room | Medium — for severe events |
| 5 | Wear a mask during peak hours | High for vulnerable populations |
Step 1—Seal the home
Close every window and exterior door. Then go further:
- Block gaps under doors with a draft stopper or rolled towel if you don't have seals
- Cover window cracks with tape or damp cloths (ANSES recommends placing wet cloths in front of air vents)
- Close the chimney damper—stuff it with a towel if necessary
- Turn off bathroom and kitchen exhaust fans—they create negative pressure that sucks smoke in faster
This single step can reduce your infiltration rate from 0.7 to 0.3–0.4 in a porous home.
Step 2—Turn off ventilation or set to recirculation
This is the step most people forget. If your ventilation system has a fresh air intake, it actively draws outdoor smoke in every time it operates. Switch it to recirculation mode (often indicated by a circular arrow). If you can't find this setting, turn the system off entirely.
If your system has a MERV-13 filter or higher, you can run it on recirculation: it will filter particles from the indoor air. A standard MERV-8 filter (most common) does almost nothing against PM2.5.
Do not open windows to "air out" during an event, even briefly. It takes hours to recover the air quality lost in minutes.
Step 3—Use a HEPA + activated carbon air purifier
A true HEPA filter captures 99.97% of particles 0.3 microns and larger—which effectively covers the PM2.5 range. Studies show that HEPA purifiers reduce indoor PM2.5 by 50 to 85% during smoke events.
But HEPA alone doesn't address gases and VOCs from smoke—benzene, formaldehyde, acrolein. For these, you need activated carbon. Look for a purifier with a substantial amount of carbon media (not just a thin sheet).
Sizing matters. The EPA recommends a CADR (clean air delivery rate) at least equal to the room's square footage. For a 25 m² room, aim for a smoke CADR of at least 270 m³/h. During severe events, allow for extra capacity and run the unit at maximum.
Do not add indoor pollution sources while operating: no candles, incense, high-temperature frying, or vacuum cleaners without HEPA filters.
Step 4—Create a clean room
If outdoor levels are extreme (above 150 µg/m³) or if you have vulnerable family members, designate one room as a clean room. Choose one with:
- The fewest windows and exterior walls possible
- A door that closes properly
- Space to run your purifier continuously
Seal the bottom of the door with a towel. Gather everyone there during the most critical hours. This concentrates your filtration capacity where it matters most.
Step 5—Wear a mask indoors during peaks
Yes, indoors. This is the surprising step—and the one that matters most when outdoor PM2.5 is extreme and your home isn't perfectly sealed.
Protect your home from smoke as much as possible with the previous steps, but during peaks—outdoor PM2.5 above 150 µg/m³, visible haze indoors, or if you are in a high-risk group—a properly fitted respirator is your last line of defense.
Do air purifiers really work against wildfire smoke?
Yes—with important nuances.
HEPA filtration—effective against particles
True HEPA filtration is the gold standard for indoor air quality during a fire. It captures PM2.5, PM10, and even ultrafine particles down to 0.1 microns with high efficiency. A 2024 study conducted in public buildings during smoke events measured average indoor PM2.5 of 8.42 µg/m³ compared to 49.22 µg/m³ outdoors when HEPA systems were operating—an indoor/outdoor ratio of less than 0.2.
The keyword is true HEPA. "HEPA-type" or "HEPA-like" claims are marketing terms that do not meet the 99.97% standard. Check the specifications.
Activated carbon—essential for gases and VOCs
Wildfire smoke is not just particles. It contains carbon monoxide, benzene, formaldehyde, and acrolein—gases that pass through a HEPA filter. Activated carbon adsorbs these compounds, but quantity matters. A simple carbon pre-filter does little: look for purifiers with at least 1 to 2 kg of activated carbon media.
Harvard's Healthy Buildings program recommends, in case of strong smoke odors or a high-VOC environment, to prioritize a respirator with an organic vapor cartridge rather than relying solely on a purifier.
What to look for in a purifier for wildfire smoke
- ✅ True HEPA certification (not "HEPA-type")
- ✅ Substantial activated carbon (check the weight, not just the mention "carbon filter")
- ✅ Smoke CADR appropriate for your room size
- ✅ Continuous operation—it must run 24/7 during the event
- ✅ Auto mode or PM2.5 sensor—to ramp up when levels rise
- ❌ Avoid ionizers and ozone generators—they add indoor pollutants
When to wear a mask indoors?
Most people associate masks with outdoors. But during severe events, wearing a mask indoors is justified—and for vulnerable populations, it is recommended.
PM2.5 thresholds that justify wearing a mask indoors
| Outdoor PM2.5 | Index (US AQI) | Indoor action |
|---|---|---|
| < 35 µg/m³ | < 100 (good to moderate) | No mask needed indoors |
| 35–75 µg/m³ | 100–150 (unhealthy for sensitive groups) | Vulnerable populations: consider mask |
| 75–150 µg/m³ | 150–200 (unhealthy) | All vulnerable populations: mask indoors |
| > 150 µg/m³ | > 200 (very unhealthy to hazardous) | Anyone in a poorly sealed home: mask |
The French information threshold for PM2.5 is 50 µg/m³ on a daily average, and the alert threshold is 80 µg/m³. During the Gironde fires in July 2026, both these thresholds were simultaneously exceeded in four departments.
Which mask standard for indoor use
Surgical and cloth masks do nothing significant against PM2.5. Their filtration efficiency against fine particles is less than 30%, and they are not sealed to the face.
The minimum effective standard against wildfire smoke:
- FFP2 / N95: filters ≥ 94–95% of particles. Sufficient for moderate events. ANSES recommends FFP2 for sensitive populations.
- FFP3 / N99–N100: filters ≥ 99% of particles. The appropriate choice for severe events, individuals with respiratory issues, or anyone living in a poorly sealed home during extreme peaks.
For the most complete protection—particles and gases—an integrated activated carbon mask (FFP3+ standard with nanofiltration membrane) covers both particulate matter and VOCs. This is the relevant level when outdoor PM2.5 exceeds 150 µg/m³ and you cannot completely seal your home.
Fit matters as much as the standard. A perfectly rated mask worn loosely is no better than a surgical mask. Pinch the nose bar, check the seal, and perform an inhalation test to confirm no air leaks around the edges.
Vulnerable populations: systematic mask-wearing during events
ANSES is explicit: children, the elderly, pregnant women, and individuals suffering from asthma, COPD, heart failure, or coronary artery disease must wear a filtering mask (at least FFP2) during any significant smoke event—indoors and outdoors. The government distributed 1.5 million FFP2 masks in Gironde during the July 2026 fires precisely for this reason.
If you belong to these groups, do not wait for symptoms to appear. Damage related to PM2.5 occurs before you feel it.
When to evacuate rather than shelter in place
Sheltering in place has its limits. Evacuate if:
- Authorities issue an evacuation order—this is non-negotiable
- You are 2 to 3 km from an active fire front and the wind is blowing towards your home
- Outdoor PM2.5 levels exceed 300 µg/m³ and your home has neither an air purifier nor good sealing
- Someone in the household develops breathing difficulties, chest pain, confusion, or loss of consciousness — call 15 (SAMU) immediately
- Your home is without electricity and temperatures are extreme — heat stress exacerbates smoke exposure
If you must drive, close all windows and set the air conditioning to recirculation mode. Car cabins filter smoke reasonably well in recirculation mode, provided you don't open the windows.
If you have to choose between a leaky, unfiltered home and a well-sealed public building (school, community hall, hotel with air treatment), the public building is safer.
FAQ
Does closing windows really reduce indoor PM2.5?
Yes, significantly — but not entirely. With windows closed, the indoor/outdoor PM2.5 ratio generally drops from 0.7–0.9 to 0.2–0.4 in most homes. This is significant, but at outdoor levels of 300 µg/m³ and above, you'll still have 60 to 120 µg/m³ indoors. Sealing gaps and running a HEPA air purifier are necessary to get below 35 µg/m³.
Can I use my air conditioning during a smoke event?
Only in recirculation mode (without fresh air intake). If your air conditioning draws in outside air, running it will inject smoke directly into your home. In recirculation mode with a MERV-13 filter or higher, it can, on the contrary, help to clean indoor air. If in doubt about your system, turn it off and use a portable HEPA air purifier.
How long does smoke linger in the house after outdoor air quality improves?
Longer than you might imagine. Particles can persist indoors for 8 to 10 hours after outdoor levels decrease, as they settle on surfaces and then become re-suspended with the slightest movement. Keep your air purifier running for several hours after conditions return to normal.
Is an anti-fire smoke mask useful indoors?
Yes, during peaks. If outdoor PM2.5 exceeds 150 µg/m³ and your home is neither well-sealed nor filtered, a properly fitted FFP2 or FFP3 mask worn indoors provides real protection — especially for vulnerable individuals. It is not the first line of defense (sealing and purification are), but it is a valid layer.
What is the difference between PM2.5 from wildfire smoke and from urban pollution?
Wildfire PM2.5 is considered more toxic per microgram than urban PM2.5 from traffic or industry. It is richer in organic carbon, PAHs, and reactive compounds that cause greater oxidative stress in lung tissue. A 2021 study published in Nature Communications showed higher cardiorespiratory impacts at equivalent concentrations. This is why standard urban air quality thresholds may underestimate the risk during wildfire events.
The last layer of protection, when all else fails
Sealing and purifying remain your first two lines of defense. But when outdoor PM2.5 levels soar and your home cannot be made airtight, a properly fitted mask makes a difference. The R-PUR Nano range filters down to 0.05 µm and includes an activated carbon layer against benzene, formaldehyde, and smoke odors — for peak hours, both indoors and outdoors.
Useful Resources
- EPA — Wildfires and Indoor Air Quality (IAQ)
- EPA — Wildfire Guide for Public Health Officials (June 2026)
- ANSES — Indoor Air Quality Guidelines (IAQGs)
- Airparif — Air Quality Report, Fontainebleau Wildfires (July 2026)
- ARS Nouvelle-Aquitaine — Wildfires 2026 Press Release, Air Quality Update and Recommendations
- Copernicus CAMS — Extreme Wildfires in Western Europe Expose Millions to Dangerous Air Pollution Levels (2026)
- Harvard Healthy Buildings — Choosing the Right Respirator After a Wildfire (Feb. 2025)
- NCCEH — Wildfire Smoke Indoor/Outdoor Evidence Review (Feb. 2026)
- France 24 — Risks from Smoke as Wildfires Grip France and Spain (July 2026)
- WHO — Air Quality Guidelines 2021
