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Wildfire smoke and portable air cleaners: Improving air quality in schools and child care facilities

This blog was written by the National Collaborating Centre for Environmental Health (NCCEH) in partnership with CPCHE.  

By: Ryan D. Huff, MSc, PhD, Environmental Health and Knowledge Translation Scientist, National Collaborating Centre for Environmental Health 

Wildfires are a growing environmental and public health concern across Canada. So far, approximately 2.5 times more land has burned during the 2025 season than during an average season over the past 10 years. This is close to 7 million hectares, which is about the size of New Brunswick. Frequent, severe and extensive wildfires driven by climate change release tonnes of smoke into the atmosphere, which can degrade air quality across North America and beyond. Wildfire smoke is harmful to the physical and mental health of those exposed, and can have broader impacts, such as negatively affecting education outcomes. This post provides an overview of the health risks of wildfire smoke, particularly for children, and outlines best practices for using portable air cleaners (PACs) as a protective strategy in school and child care settings. 

How can breathing wildfire smoke affect health? 

Wildfire smoke is made up of small particles called particulate matter (PM) and many different gases, including volatile organic compounds. Breathing in these pollutants, especially small particles under 2.5µm in diameter (PM2.5), or getting them on your skin can cause a range of acute and chronic health effects. Short-term exposure to smoke can induce dizziness, headaches, nausea, and irritation of the eyes, nose, throat and lungs. Less common but more severe symptoms include wheezing, severe cough, asthma attacks, and in adults, chest pains, heart palpitations (i.e., irregular heartbeat), stroke, and heart attack. Long-term exposure to smoke has also been linked to the development of chronic respiratory, cardiovascular, and neurological diseases. People who are at higher risk of experiencing health effects include those with chronic health conditions, seniors, those who are pregnant, infants, and children.  

Children are especially susceptible to smoke because they are growing, they breathe more air relative to their body weight than adults and usually breathe faster from being more active. Increased respiratory symptoms in children, such as sore throat, cough, sneezing, and wheezing, as well as increased pediatric asthma emergency room visits have been reported during wildfire smoke events. Smoke exposures can also increase a child’s risk of developing respiratory infections, impact their neuropsychological development, and negatively affect growth.  

Does wildfire smoke disrupt childhood learning? 

Wildfire smoke can negatively affect educational outcomes by causing respiratory symptoms, headaches, and difficulty concentrating in class, affecting their ability to learn. A recent study of 11,700 US school districts found that wildfire smoke was linked to a drop in annual test scores, particularly for younger children. Wildfire smoke can lead to school closures or restrictions on outdoor activities, disrupting learning opportunities as well as the physical, mental, and social benefits of spending time outdoors. In California, wildfire-related school closures have been associated with lowered academic performance. 

Portable Air Cleaners (PACs) and wildfire smoke 

PACs can help reduce exposure to wildfire smoke in small indoor spaces (e.g., classrooms), regardless of the heating, ventilation, and air conditioning (HVAC) system. For example, PACs were tested in one room at a public library in Port Macquarie, Australia, which also had an HVAC system with relatively coarse filters (equivalent to MERV 8, see box below) installed. When the PACs were in use on heavy smoke days, the PM2.5 concentrations in the room were 72% lower than when the PACs were not in use, highlighting the ability of PACs to complement the existing HVAC. Overall, the indoor PM2.5 concentrations with HVAC and PACs running were 76% lower than the outdoor concentrations, with PACs contributing an estimated 40% of the reduction. In other settings, such as homes and public buildings, PACs have reduced levels of PM2.5 by an average of 57% during wildfire smoke episodes. 

HVAC systems, MERV ratings, and wildfire smoke.
HVAC systems in schools and child care facilities can reduce exposure to wildfire smoke indoors by filtering the outside air before it is circulated inside the building. Health Canada and the US EPA recommend using filters with a minimum efficiency reporting value (MERV) of 13 or higher in HVAC systems during smoke events. A MERV 13 rating means that at least 50% of the particles between 0.3 – 1.0 µm in size (the same size range of smoke particles) are removed from the air with each pass through the filter. However, not all HVAC systems will be capable of running with MERV 13 filters depending on their design specifications. As outlined in ASHRAE Guideline 44, best practice is to work with HVAC professionals to develop a building-specific smoke readiness plan. 

What types of PACs are used for indoor spaces? 

There are two general types of air cleaners that remove PM indoors — mechanical fibrous media filters that physically block or capture particles, and electrostatic precipitators (ESPs) and ionizers that cause particles to settle out of the air. Most studies that have assessed the use of PACs during wildfire smoke events have focused on mechanical filters. 

  • Mechanical filters draw air through a material to block or capture particles. Those equipped with high-efficiency particulate air (HEPA) filters are equivalent to a MERV 16 or higher, meaning they can remove 99.97% of particles in the 0.3-µm range.  
  • ESPs electrically charge particles and then capture them on a charged plate; ionizers electrically charge air molecules that cause pollutant particles to settle onto surfaces more quickly. These devices can create harmful gases such as ozone as a byproduct, so only those independently tested and certified by the Canadian Standards Association (CSA, standard CSA 22.2 187-20) to produce little to no ozone should be used.  

Mechanical filters, ESPs, and ionizers can also be combined with sorbent media such as activated charcoal to filter the gases present in wildfire smoke.  

Caution: Ozone generators pose risks to health 
Ozone generators are PACs that purport to use ozone to clean or disinfect the air. Exposure to ozone in indoor air can be harmful to health. Health Canada advises against the use of ozone generators indoors.  

Considerations for choosing PACs to reduce wildfire smoke indoors 

Commercial vs. do-it-yourself (DIY) 

  • There are many commercial manufacturers, and more being added all the time. Tested by the Association of Home Appliance Manufacturers (AHAM) (pro), proprietary filters (con).  
  • DIY can be just as effective, noisier (con), can always get filters (pro). More information on DIY in next section. 

Capacity of commercial PACs 

  • Choose a PAC certified by AHAM. 
  • Find the clean air delivery rates (CADRs) and suggested room size on the AHAM label. CADRs in cubic feet per minute (cfm) are measured for tobacco smoke, dust, and pollen. The suggested room size is based on meeting a standard of 4.8 air changes per hour (ACH). Certified models and CADRs are listed online by AHAM. Note that the CADR is reported for the PAC running on its highest fan setting.  
  • For wildfire smoke, the minimum CADR required for a room should be based on the tobacco CADR rating and should be approximately two-thirds the room area in square feet (sqft). For example, the minimum CADR for a 1000 sqft room would be 1000 x 0.66 = 660 cfm. These calculations assume a standard 8 ft ceiling height. For different ceiling heights, the minimum CADR can be calculated based on room volume. 
  • AHAM capacity verification is based on PAC particle removal only. PACs containing activated charcoal filters can help remove gases such as volatile organic compounds (VOCs) present in wildfire smoke, but there is currently no performance rating system to certify effectiveness. 

 
Commercial mechanical filtration PACs 

  • Use PACs equipped with certified true HEPA filters, indicated to capture 99.97% of 0.3 µm particles in size.  
  • Avoid using uncertified filters labelled “HEPA-like”, “HEPA-type”, or “99% effective” as these may not filter the smallest particles present in wildfire smoke. 
  • MERV scale-rated filters should be rated at least MERV 13 or higher to capture smoke particles effectively. 
  • PACs equipped with activated charcoal filters may help remove wildfire smoke gases and odours from the air. 

Placement 

  • Follow manufacturer instructions for placement of PACs. For example, do not block the outflow by placing the unit too close to furniture or walls. 
  • Avoid positioning PACs to blow directly on to people using the room. 
  • The position of a PAC in a classroom does not strongly affect filtration effectiveness. Research in an elementary school classroom found that positioning only changed effectiveness by approximately 10%. 
  • Consider tripping and electrical hazards due to PAC cords. 
  • Consider whether moving parts such as fan blades are accessible to clothing, body parts (e.g., fingers or hair), or other objects (e.g., toys, pencils, crayons, etc.) when placing PACs in classrooms and child care settings. Placing PACs on elevated surfaces may help reduce these hazards. 

Noise 

  • Many PACs can be noisy and may disrupt the learning environment — check the manufacturer’s specifications for noise levels. 
  • To reduce noise disturbance, consider using PACs with a higher CADR value than necessary at a lower setting.  

Other considerations 

  • Keep spare filters available, and replace filters as recommended by the manufacturer or indicators built into the PAC. Overly clogged filters can reduce overall performance. Extended operation of PACs during wildfire smoke episodes may require frequent filter changes. If the amount of air coming out of the device drops or the filter noticeably begins changing color, the filter should be replaced. 
  • Consider using low cost PM2.5 monitors during wildfire smoke episodes to measure indoor concentrations and gauge how well air filtration strategies are working. Some PACs are equipped with monitors that can be used to gauge effectiveness. 
  • Outer windows and doors must be closed for PACs to be effective, but this can lead to indoor overheating in spaces without mechanical cooling. In rooms without central mechanical cooling, PACs should be used in combination with portable air conditioning to ensure that indoor temperatures remain <26C. Sustained indoor temperatures of less than 26°C are generally safe for everyone, whereas indoor temperature over 31°C for long periods can be dangerous, especially for infants and young children. 

DIY (Do-It-Yourself) PACs 

DIY PACs can be a cost-effective way of reducing indoor air pollution in schools and other learning settings. DIY PACs are constructed by attaching 20 by 20 inch MERV 13 or higher furnace filters to box fans. The US EPA tested several configurations for their effectiveness at filtering wildfire smoke and found them to be comparable with commercially available PACs. Altering the intended use of a fan can put stress on the motor and potentially cause overheating, presenting a fire risk. In light of this, the US EPA supported a study testing five different electric box fan models in a variety of DIY PAC configurations and found that modern fans meeting current electrical standards never exceeded maximum temperature thresholds for electric fans (US UL 507 standard). In this study, exterior surfaces were found to remain below 36°C, which is safe to touch. 
 

Designing and building a DIY PAC can also provide an empowering learning experience for students. By learning about the design of PACs and participating in building them, students can gain a deeper understanding of how the outdoor environment interacts with the indoor spaces we occupy. Specifically, students can learn about air pollutants, air filter materials, engineering considerations, and ways to measure particles in the air. Several resources are available from the BCCDC and the BC Lung Foundation on methods and considerations for building DIY PACs. 

Summary 

Wildfire smoke in schools, child care facilities and other educational settings can pose a health risk for children and interfere with learning. Commercial PACs equipped with quality certified, high-efficiency filters and appropriately rated for the room size are an effective way to reduce indoor smoke exposure. DIY PACs offer a practical and cost-effective alternative to commercial PACs, while also providing additional educational value in school settings. 

Useful resources on wildfires, PACs, and schools 

Blog written by Ryan D. Huff.  

Ryan is an interdisciplinary scientist with a strong interest in understanding how environmental exposures impact human health. He has extensive experience in basic science and clinical research related to air pollution, including wood, cigarette, and cannabis smoke, as well as ambient and traffic-related air pollution. At the NCCEH, Ryan’s work is currently focused on the environmental health impacts of wildfires and interventions. 

Citation: Huff R.D. Wildfire smoke and portable air cleaners: Improving air quality in schools and child care facilities [Blog]. Vancouver, BC. National Collaborating Centre for Environmental Health. 2025 Aug. Available from: https://ncceh.ca/resources/blog/wildfire-smoke-and-portable-air-cleaners-improving-air-quality-schools-and-child