Wildfire Smoke and Outdoor Air Events

Why This Is a Building Question

Outdoor air events are usually treated as a personal health topic — check an app, stay inside, wear a mask. The building side is where the leverage actually is. A house is a filter with a pressure relationship to the outdoors, and how it is built and operated determines how much of an outdoor event arrives indoors.

The events this page covers share one structure: outdoor air is temporarily worse than usual, for hours or days, and the building either keeps it out or does not.

• Wildfire smoke
• Agricultural or municipal spraying, including mosquito abatement
• Neighbouring construction, demolition, roofing or paving
• Neighbour activity — lawn treatment, exterior painting, wood smoke, dryer vents carrying fragrance
• Temperature inversions concentrating ordinary traffic and combustion pollution

Reading Air Quality Data Honestly

AirNow publishes the official US AQI from regulatory monitors. Accurate and well-calibrated, but sparse — a monitor may be many miles away and report conditions that do not match your street.

PurpleAir is a dense network of low-cost sensors. Far better spatial resolution, less accurate individually. Two things to know when reading it: the raw values read high in smoke, so select a smoke-corrected conversion (commonly the US EPA conversion) when one is offered; and a single sensor can be sited badly or failing, so look for agreement between several nearby sensors rather than trusting one.

What neither measures. Both track particulate matter. Neither measures the gaseous fraction — the VOCs, aldehydes and acrolein in wildfire smoke, or pesticide drift, or solvent vapour from a neighbour's project. This is the practical reason a room can show a low particulate reading and still smell strongly of smoke, and why people who are chemically sensitive often report symptoms while the public dashboard reads acceptable. The number is not wrong; it is measuring something else.

On thresholds

Public AQI categories are designed around population-level outcomes. Individual thresholds for sensitive occupants are commonly lower, and this site takes no position on where any individual's threshold should sit — that is a medical question. What the building can do is stated plainly: the interventions below are worth taking whenever outdoor conditions are meaningfully worse than indoor, and the cost of taking them early is low.

Sealing the Building

In order of effect per unit of effort:

Close windows and doors, and stop using them. Repeated door openings are a significant infiltration path during a multi-day event. Designate one entry.

Close the outdoor air intake if the HVAC system has one. Many newer systems have a ducted outdoor air inlet on a controller or damper. During a smoke event it should be closed, and it is regularly overlooked because nobody knows it is there. If a filtered make-up air system is installed per the filtration protocol, it can usually keep running — that is the case it was built for — provided the carbon bed is not near the end of its life.

Stop exhausting air. Kitchen hoods, bathroom fans, whole-house fans, clothes dryers and unbalanced ERVs all pull the building negative, which draws smoke in through every gap in the envelope. A dryer run during a smoke event is a smoke intake.

Seal the obvious leaks temporarily. Weatherstrip gaps, unused window sashes, pet doors, mail slots, window air conditioner surrounds. Painter's tape and plastic sheeting are adequate and remove cleanly.

Address the attached garage. A garage is often the single largest source of indoor contamination during any event, and negative pressure makes it worse. Keep the connecting door shut and sealed; do not run vehicles inside it.

Combustion caution. If a fireplace or wood stove is not in use, close and seal the damper. During heavy smoke, an open chimney is a large open window.

The Prepared Room

Sealing and filtering an entire house to a high standard during an event is rarely achievable. One room done properly is achievable and is where the effort should go. In most households this is a bedroom, because it covers the longest continuous occupancy.

Pick the room with the fewest exterior walls and windows, no direct connection to the garage, and no fireplace.

Size the filtration generously. Aim for four to five air changes per hour for the room's volume, not the manufacturer's room rating. A unit intended for a much larger room, run at a low speed, is both quieter and better at gas removal than a small unit at maximum.

Include real carbon. Particle-only filtration in a smoke event removes the visible half and leaves the fraction that most people actually react to. See the filtration protocol for carbon quantities.

Seal the room's own leaks — door undercut, window frame, any wall penetration.

Prepare it before the season. Filters and carbon sell out regionally within a day of a major fire starting. A room assembled in advance costs the same and is available when needed.

On DIY box-fan filters

A box fan with MERV 13 filters — the Corsi-Rosenthal design — is a legitimate, well-tested particulate cleaner and delivers a great deal of particle removal per dollar. Two honest limits: it does essentially nothing for gases, and box fan motors are not all rated for continuous unattended operation, so use a fan with a thermal cutoff and do not run it unattended overnight. Treat it as a supplement to a carbon-bearing unit, not a replacement for one.

The Window Decision

Opening windows is the cheapest ventilation available and is genuinely valuable — it is the only thing that clears accumulated CO₂, odours and humidity that recirculating filtration cannot touch. The decision is about timing, not principle.

Two practical notes. Indoor air lags outdoor air in both directions — indoor levels keep climbing for hours after outdoor peaks, and stay elevated after outdoor clears, so flush deliberately once conditions improve rather than assuming the house recovered on its own. And where spraying is scheduled by an agency or a grower, many jurisdictions operate advance notification registries; where one exists, being on it converts a reactive problem into a scheduled one.

What the Building Can Do in Advance

Everything above is operational. These are the design decisions that make events easier to handle, and they are inexpensive at construction and expensive later.

• A tight envelope with mechanical ventilation. A leaky house cannot be sealed during an event at any price. Air sealing is the foundation for every other measure here.
• A closable, filtered outdoor air path — see the filtration protocol. It lets the building keep ventilating through an event rather than choosing between fresh air and clean air.
• Make-up air for large exhaust appliances, so using a kitchen hood does not depressurise the house.
• A garage that is genuinely isolated — sealed connecting wall, self-closing gasketed door, garage exhaust.
• Filter housings sized for deeper media, so a higher-grade filter can be fitted seasonally without restricting the system.
• Detached or well-separated garages and mechanical spaces — see HVAC Equipment Placement.

After the Event

Smoke deposits residue on surfaces, and that residue continues to release odour and VOCs for weeks. Filtration alone will not resolve it because the source is now inside the room.

Flush the building with outdoor air once conditions are genuinely better. Replace HVAC and portable unit filters — a loaded filter is a smoke source in the airstream, and carbon that saturated during an event should be replaced rather than trusted. Wet-wipe hard surfaces, launder soft goods, and HEPA vacuum textiles that cannot be laundered. Check window tracks and sills, where deposits concentrate.

If the building was wetted during fire suppression or flooding, that is a different problem with a shorter clock — see Mold in a Home You Already Live In. Materials wet for more than roughly 48 hours should be assumed to have started growing.

Evidence Basis

EPA and USFS Wildfire Smoke: A Guide for Public Health Officials, including guidance on cleaner air spaces and portable air cleaner use. AirNow AQI methodology and reporting. EPA and independent evaluations of low-cost particulate sensor performance and the US-wide smoke correction factor applied to PurpleAir data. ASHRAE guidance on building pressurisation and outdoor air control during outdoor pollution episodes. EPA Indoor Air Quality guidance on infiltration and post-event cleaning. Published testing of low-cost box-fan filter assemblies for particulate clean air delivery rate. CDC guidance on post-flood and post-fire building cleanup.