Online educational resource on achieving indoor environmental quality with radiant based HVAC systems


Dealing with smoke from wildfires

Copyright (C) 2023, Robert Bean, ASHRAE Fellow & Distinguished Lecturer, Free use is granted when credited.

See also:
How to set up an emergency isolation room inside a home or apartment for a suspected infected occupant.
Simplified and Idealized Illustration of a Portable Fan/Filter (CR) Box in a Classroom


2023.06.10 Version 1, original (based on my June 6th Twitter post)
2023.06.19 Version 2, added informative text and images.

This is a triage approach and by no means implied to substitute the combined advice from a building engineering professional. Typically wildfire emergencies last for short periods often subsiding or ending with sufficient time between fires before another emergency is declared. Take the safe times to prepare.

Note: due to the urgency of this bulletin, there may be spelling and grammatical errors...if butchering the English language drives you crazy to the extent that you won't read the advice - all I can say is - good luck.


What can you do when surrounded by wildfire smoke?  

  1. If you have the wherewithal - prepare your home for vacation & leave until it's safe to come back. Arrange to meet your homeowner's insurance requirements for an unoccupied home.

  2. If you can't leave, prevent outdoor air from entering your home; close windows, doors, and shut down any systems connected to the outdoors (ex. portable a/c units). You may have to be creative so use what resources you have or can buy or borrow. If you are unable to develop creative solutions ask a family member, friend, neighbour or hire a tradesperson.

  3. If there are outdoor air intakes/ducts (sometimes called fresh air ducts) connecting the outdoors to your air systems (furnace, a/c, HRV/ERV, central vac etc.) block them (don’t close any damper unless you know how to return the damper to its original positions)...same for wood burning fireplaces and stoves seal up/block so there is no connection between the indoors and outdoors....take pictures to remind yourself what you have you can return the system to its previous state when it's safe. If you don't know if you have an outdoor air duct(s), this would be a good time to hire an HVAC contractor to explain the system(s) you have.

  4. Those individuals living in spaces conditioned with electric, steam or hot water systems may or may not have separate ventilation systems. If you do live in spaces with these types of systems, and have an independent ventilation system follow the same advice for those with furnaces and outdoor air intakes/ducts. Your spaces may be of a vintage that had no ventilation system; that is no fans to exhaust indoor air from the home and no fans to bring outdoor air into the home. In these vintage homes typically windows and building enclosure leakage was relied upon for ventilation. Do your best to seal up leakage paths.

  5. If you live in an older condo or apartment there is a high probability that outdoor air could be introduced to the common hallways unless the building operator shuts off the air handling systems. Block the airflow under and around the door with a towel and tape.

  6. If you have an option to set your furnace fan to recirculation - do so...ensure the duct fitted air filter (located before the furnace intake) is MERV 13+ or the equivalent to the 3M 1900+ series...lower performing filters (<MERV 8) can recirculate smoke particles throughout your home eventually settling onto surfaces which will then be disturbed by the movement of occupants and pets. If you have the wherewithal, have a carbon based filter installed after the particulate filter to help remove some combustion related gases. Follow the manufacturers recommend guidance for replacement.

  7. Avoid sustained use of exhaust fans including kitchen, bath, clothes dryer, and central vacuum systems. These have the potential to pull smoke into the home through leakage pathways such as windows, doors, and duct work connected to the outdoors etc.

  8. Avoid if you can all heat generating devices and activities...exercising, cooking, cleaning, laundry plus computers, TV's etc... until it is safe to open windows or operate A/C and exhaust fans. Adding heat to home is never good when it's already hot and smoky outside and cooling and clean air is needed inside - but not available.

  9. In space filtration such as the Corsi-Rosenthal Box are your friend...use them. MERV 13 or 3M 1900 or better is the minimum. Have replacement filters on hand. Some have installed MERV 13+ filters in window openings to enable outdoor air to enter the home filtered (in part) before being drawn into the in-space filter units.
    Relocate to a basement. Basements are often better isolated from the outdoors than above grade rooms...and they will be cooler.

  10. Wear N95 respirators for added protection...don’t use the typical blue surgical masks handed out at hospitals - they do not provide a face seal...i.e. they leak. You can purchase N95 online or at DIY building supply store. If you are at risk due to exposure of smoke particles and gases follow the respirator guidance from 3M, see < >

  11. In a worsening situation, if possible, isolate yourself in a single smaller room in the basement...bring with you in-space filtration & masks. All things being equal, a Corsi-Rosenthal Box in a small room will be more effective than in a larger room.

  12. Stay hydrated!

  13. If you have CO2 monitors - keep an eye on them...don't panic if they run high <3500 ppm...use any opportunity of clean outdoor air to ventilate your rooms. This frequently happens when the wind changes direction or fire crews have redirected fires or fires have been controlled through intervention or natural suppression (rain).

  14. If you are sensitive to elevated CO2 >1000 ppm follow (if possible) a strategy of moving between two rooms (airlock technique c/o Thekla Richter). One room is being ventilated outdoors to drop CO2 down, then isolated and then cleaned with a combination of MERV 13+ (equal to 3M 1900+) and activated charcoal filters. Once the CO2 is below an ideal 800 ppm then move into the clean room and repeat cleaning on the vacant room. If you are not able to make this strategy work, wear specialized masks and cartridges recommended by 3M. See Figure 6 from our isolation page for an ideal case where two bedrooms share a common bathroom. Isolate one room and ventilate to outdoors using bath fan and open window, then reverse the process. Occupy the room that has been ventilated and cleaned.

  15. If you are at elevated risk (elderly, infirm, asthmatic etc.) let neighbours, family and friends know and call your family physician and let them know - before you become stressed.

  16. If you know someone who is high-risk do the right thing and help them out.

  17. There are several newer public places that may offer relief such as recreation centers, shopping malls, movie theatres etc.

  18. If there is a corresponding heat wave get below grade on the north side of a building and elevate the air speed with air filters, if necessary, wear saturated towels wetted with cold tap water...remain calm, reduce your exertion levels, wear light clothes and stay hydrated...follow guidelines for high-risk individuals.

  19. 98% of the fatalities in BC's recent heat wave died indoors...when emergency response was overwhelmed due to the number of calls...many could have survived had the above been initiated with help from emergency services, neighbours, friends and family...this includes taking cool baths or wearing saturated wet towels in elevated air speeds (fans – ceiling, floor, desk).

  20. Again, stay hydrated. The ability for one's body to cool is intimately tied to the heart's ability to pump blood and is based on the bloods viscosity (think honey vs. water). A dehydrated body puts stress on the cardiovascular system preventing adequate cooling and respiration.


Figure 1. Back to front, box fan, carbon filter, MERV 13 filter. Can be assembled into any number of Corsi-Rosenthal box configurations. Make sure the carbon filter is sandwiched between the fan and particle filter.


Figure 2. Some might think this is a dirty filter but what you are seeing is a clean carbon filter. The small carbon particles have tiny pores that capture some gases associated with smoke odours.


The Corsi Rosenthal box...for less than $200 CDN you can build a portable fan/filter assembly that outperforms  the most expensive retail products available (carbon filter extra). I've lost count on how many I have made for family and takes less than 20 minutes to need 1 box fan, 4 @ MERV13 Filters (equal to Filtrete 1900 or 2300), packing tape, scissors/knife...and your favourite beverage. Search online for videos and visit

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