As more parts of the world are being forced to deal with wildfire smoke and COVID-19 simultaneously, it is important to discuss how these health threats interact, and how to maintain good indoor air quality (IAQ) in the face of these dual disasters.

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As summer rolls around, the dual crises of wildfire smoke and the ongoing COVID-19 pandemic are causing both indoor and outdoor air to be hazardous to human health. COVID-19 is a complex viral illness linked to a wide range of health problems, and it is primarily transmitted through airborne particles that spread especially easily indoors. As a result, much of the guidance for reducing COVID-19 infection risk has been based around increasing ventilation by flushing out indoor spaces with outside air, and/or socializing outdoors.

However, during times of poor air quality such as caused by wildfire smoke, the outdoor air has been rendered hazardous (as assessed by the Air Quality Index), and this advice must be altered. Instead of ventilating (flushing indoor air with outdoor air), we must compensate with air filtration (filtering out particles from the air using appropriate air purifiers, filters, and respirator masks). As more parts of the world are being forced to deal with wildfire smoke and COVID-19 simultaneously, it is important to discuss how these health threats interact, and how to maintain good indoor air quality (IAQ) in the face of these dual disasters.

The Dangers of Wildfire Smoke and COVID-19

Wildfire smoke is composed of gaseous pollutants (e.g., carbon monoxide), hazardous air pollutants (e.g., polycyclic aromatic hydrocarbons), water vapor, and particle pollution (also referred to as particulate matter, or PM), all of which can abruptly render outside air hazardous and unbreathable – as many communities across North America have experienced this summer–many for the first time. 

Although it is good advice to reduce exposure to wildfire smoke by sheltering indoors, it is important to be aware that the smoke can infiltrate indoor spaces through small openings in walls, doors, or windows, or be pumped in through air conditioners that source outside air. 

Smoke can accumulate indoors, and, considering the increased time we spend inside, it creates a potential hazard to health from breathing in toxic chemicals that can accumulate and cause a range of health issues including eye irritation, nasal congestion, respiratory problems as well as cause the development of or the worsening of chronic health conditions, such as asthma, heart disease, and lung disease. Many at-risk populations exist, including those with pre-existing health conditions, children, and the elderly have to be exceptionally careful as the smoke will have an even harsher effect on them. 

COVID-19 infections can also pose short-term and long-term risks to health. The same groups especially vulnerable to wildfire smoke are also at high risk for severe complications from COVID-19. Yet, because COVID-19 spreads most easily in closed indoor spaces where ventilation is limited, the same guidance that is advised to keep wildfire smoke out of homes and indoor spaces – such as turning off outside ventilation and sealing doors and windows – can actually create conditions that increase risks of COVID-19 transmission!

Steps to Protecting Yourself From COVID-19 During Wildfire Smoke:

1- Adjust HVAC settings in your home and vehicles: When outside air quality is poor, it is important to reduce ventilation in your home and vehicle(s). This is the opposite of standard COVID-19 risk reduction strategies, in which we increase ventilation with outdoor air. Yet, during times of poor outdoor air quality, it is important to prevent harmful outdoor particulates from getting inside. 

If you have central air, change your HVAC settings to the “ON” (not Auto) function so it is continuously running the air through filters. Do not use window air conditioners or window fans. Use “recirculate” settings on vehicles. 

Since ventilation is now reduced, it is important to compensate by increasing filtration to reduce both COVID-19 risks, as well as to clear out any smoke particulates that do get in. One simple way to increase filtration is to install upgraded HVAC filters that are at MERV 13 (or the highest compatible filter with your HVAC system). Another way is to add supplemental portable air cleaners (see section 3). It is important to have at least one “Smoke-free Room” with as good of air quality as possible, to spend the most time in.

2- Create a “Smoke-Free Room”: Designate at least one room that is large enough to accommodate all household members (such as a bedroom with an attached bathroom) as a smoke-free room, where you can retreat when outdoor air quality is poor. Close windows and doors, but don’t do anything that blocks emergency exits. Do your best to create a seal in areas that leak outside air (such as using caulk to fill cracks by windows). This room should also have good airflow– with fans on, central air running (see above section), and supplemental air filtration.

3- Add Supplemental Filtration: Portable HEPA air purifiers and/or DIY air cleaners such as DIY Corsi-Rosenthal Boxes (CR Boxes) should be added and run on their highest setting to filter out harmful pollutants from wildfire smoke, as well as remove viruses from the air, reducing the risk of COVID-19 transmission. But how do you determine the appropriate size and number of purifiers and/or air cleaners for your space?

Covid-19 and wildfire smoke CC BY-ND 4.0 A Dual Disaster Plan There are 7 boxes with information Image has a box that says “Adjust HVAC settings Upgrade filters to MERV-13, keep HVAC units in "ON" (vs AUTO) mode, and use units/settings that recirculate indoor air instead of bringing in outdoor air.” To the right there is another box that says “add hepa purifiers and cr-boxes These units filter both wildfire smoke and SARS-CoV-2 from the air to reduce risks to your household's health. See our blog for more information.” safe indoor spaces Create at least one "Smoke-free" room. Add a separate "Isolation Room" for a confirmed or suspected COVID-19 positive household members. Both rooms should avoid ventilation with outdoor air and have supplemental air filtration. Mask when in common areas. Activities Avoid outdoor activity when you can. Change your masks and clothing more frequently. Reduce activities that create smoke, involve harsh chemicals, or increase COVID-19 risks. Playing board games, watching movies, and phone/video calls are examples of good activities. RESPIRATORS Such as N95, KF94, KN95 or better reduce the risks of both wildfire smoke and COVID-19. Wear outdoors, and indoors when with non-household or exposed household members. Respirators are available in pediatric sizes for the protection of children ages 2 and up. VENTILATE WHEN AIR QUALITY IMPROVES When outdoor air quality improves to safe levels, as can be assessed by the Air Quality Index (AQI), then we encourage you once again to ventilate your home by opening windows or using mechanical ventilation to exchange indoor and outdoor air. prepare ahead During disasters, supplies might run short. Prepare in advance by having N95 or better particulate respirators, MERV-13 HVAC filters, and portable filtration devices. Consult with your healthcare provider about medications to have on hand for COVID-19, and know what emergency symptoms to look out for.
Image 1: A Dual Disaster Plan– COVID-19 and Wildfire Smoke. Credit: Air Support Project | CC BY-ND 4.0

 When buying an air purifier, consider the following tips:

  • Avoid Ion Generators and Plasma Air Cleaners: Steer clear of air purifiers that use ion generators or plasma technology, as they can emit ozone, which poses respiratory hazards and can lead to serious health issues.
  • Stay Away from Photocatalytic Oxidation (PCO) Air Cleaners: Avoid air purifiers equipped with photocatalytic oxidation (PCO) technology, as they have been found to generate harmful substances like formaldehyde, acetaldehyde, nitrogen dioxide, and carbon monoxide.
  • HEPA air purifiers are certified to remove 99.97% of airborne respirable pollutants. If the words -“like”, -“type”, or -“ready” are added after “HEPA-” it may not be truly certified to remove those particles. Look for the words “True HEPA”, “HEPA” (with no hyphenated additions), and that it says it removes 99.97% particulates to ensure quality.
  • Check the CADR Rating: Clean Air Delivery Rate (CADR) is the most important metric when picking out your units. CADR for HEPA purifiers is listed by the manufacturer (use the smoke rating for COVID and wildfire smoke). If it is listed in cubic meters per hour (cmh), multiply it by 0.589 to get the CADR in cfm (cubic feet per minute). Note that a higher capacity purifier may be needed for COVID-19 and/or wildfire smoke than is typically specified by the manufacturer for your room size (see formula below to calculate the total CADR needed for your customized spaces).  

DIY CR Boxes are able to filter out smoke, allergens, and viruses when made with MERV-13 or higher filters and an appropriate fan.

  • A CR Box CADR varies depending on the components used when making it. The typical CR Box made with four 2-inch MERV-13 filters and a Lasko 20-inch box fan has a CADR of 600-800 cubic feet per minute (cfm)
  • Aim to get 12 Air Changes per Hour (ACH)–a common recommendation to reduce COVID-19 risks– by combining units with high CADR to achieve the total CADR needed for your space. Use the formula below to determine the amount of combined CADR needed for your specific space to achieve 12 ACH: 

Total CADR needed (in cubic feet per minute, or cfm) = 

[(LxWxH of room in feet) ✖  (12 ACH)]➗60

4- Isolation Guidance for Suspected, Exposed, or Known Positive COVID-19 Infections: If a household member is positive or suspected positive for COVID-19 during a wildfire event, create a second “smoke-free room” to use as an isolation room. Use the following guide for isolation when positive for COVID-19, but make sure to exclude ventilation with outdoor air and instead supplement with extra filtration from HEPA and/or CR Boxes (see Step 3, above). Mask when in shared zones.

5- Masks: Wearing respirator masks (N-95, KF94, KN-95, or better) indoors when people from different households are present, or when a person in your household has a known or suspected case, can reduce the spread of COVID-19 (child-sized KN95 masks are available for children ages 2 and above). When going outside during a wildfire, it is especially important to use a respirator mask (such as an N-95), as it can help filter out both smoke and viral particles. Surgical or fabric masks, on the other hand, will not provide adequate protection against smoke inhalation or COVID-19, and should only be used if N-95 masks are not available. Half-face respirators, elastomeric masks, and PAPRs can be ordered with filters that can remove even more harmful substances from wildfire smoke, such as formaldehyde gases. For more information on different respirators, view Suggested Respirators for the Different Phases of a Wildfire and Mask Guidelines for COVID-19.

6- Choose Wise Indoor Activities, Reduce Outdoor Activities, and Practice Good Hygiene: When the outdoor air quality is poor, and indoor ventilation is reduced, it is important to minimize additional sources of indoor pollution, such as smoking, burning candles, and using harsh chemical cleaners which can be respiratory irritants. See Effectiveness of Cleaning and Disinfection for more information. Favor non-toxic, fragrance-free products and limit smoke-generating activities (like stovetop cooking or using fireplaces). 

When indoors with others outside of your household, space out, wear respirator masks, and reduce activities such as singing, yelling, and exercising indoors, as these produce more exhaled aerosols, and increase COVID-19 transmission risks. 

Prepare to work from home, and for online school classes when possible/necessary. Avoid outdoor activities, gatherings, and exercises, and take care to wear an N95 respirator mask or better when outside. Taking a shower after being outdoors, as well as changing clothes, washing sheets, and changing out masks more frequently will reduce the number of harmful particulates you breathe in. Vacuuming with vacuum cleaners that have HEPA filters is helpful, but ones that do not might spur up more particulates from wildfire smoke, so avoid using those. Recommended activities include playing board games, watching movies, or doing light stretching.

7- Ventilate When Air Quality Improves: When outdoor air quality improves to safe levels, as can be assessed by the Air Quality Index (AQI), then we encourage you to once again ventilate your home by opening windows or using mechanical ventilation to exchange indoor and outdoor air. This step can dilute potential virus particles and lingering smoke pollution in your home, and improve overall indoor air quality. It can also prevent carbon dioxide (CO2) from building up inside the home. CO2 should ideally be kept below 800 ppm, or below 1000 ppm if additional filtration measures are applied (which can be measured using a CO2 Monitor such as an Aranet 4). 

Image 2: A flock of birds flying in a hazy orange sky


8- Stay informed and be prepared. Keep track of local air quality reports, wildfire updates, and public health advisories. Have a disaster preparedness plan, such as a Wildfire Action Plan, adding in COVID-19 preparedness. For instance: stock N95, equivalent, or better particulate respirator masks, write down numbers to your healthcare providers and emergency phone numbers, and talk in advance with your healthcare provider about telehealth options, test-to-treat centers, what medications to have on hand at home, and what emergency symptoms to look out for.  

By considering both wildfire smoke and COVID-19 risks in your safety plans, you can navigate these challenges and keep your household healthier during these trying times. Stay safe, stay aware, and stay prepared, and you will be able to protect yourself and your family from these dual threats. di

Breath is the bridge between body and mind. Every time you breathe in, you are bringing in new life. Every time you breathe out, you are letting go of the old.



Creator of COVID-conscious advocacy works: writings, art, and social networking. BA degree in psychology, with over 11 years of continued education studies in allergy and autoimmune research, and continued research the past 3 years into airborne irritants and COVID-19. Parent, disabled rights advocate.

This content is published under a creative commons — attribution/no derivatives license.

Wildfire Precautions

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