Air PollutionAir purifiersAir QualityCITY MWildfires

Strategies to Prepare for Wildfire Smoke Pollution, According to Air Quality Company Camfil

By June 9, 2023 No Comments
19–22 minutes to read

Wildfire smoke. Be prepared.

In the early 1900s, a young soldier coined the phrase “Be Prepared” which was later adopted as the motto for the Boy Scouts of America. The sentiment behind the phrase can be applied universally to many situations, but none more so than being prepared for wildfire smoke. 

Who should prepare for wildfire smoke?

While there are areas of the country and times during the year when wildfires are more likely to occur than others, they can appear suddenly and wherever there is fuel to burn. Since wildfire smoke can travel hundreds if not thousands of miles from the source, virtually anyone can be impacted. 

There are times when the danger from flames is so great, evacuation is necessary for survival. In 2022, North Texas wildfires destroyed almost the entire town of Carbon, while 7000 separate wildfires across the state of California killed nine people and demolished countless homes. 

This is a serious situation, particularly for hospitals and long-term care facilities that must deal with patients in critical condition. There are also times when the smoke from a nearby fire is heavy for days or weeks at a time, but evacuation isn’t ordered or is not practical. 

In those situations, steps can be taken to protect property and lessen the risk of the flames spreading. However, this article will focus on the steps to protect the lung health of those who remain behind in areas where wildfire smoke is present. These can be divided into two broad categories: steps to limit smoke entering a building and steps to protect the health of the occupants when it does. 

Does the building and HVAC equipment matter? 

Strategies to limit smoke from entering a building may differ based on its intended use. Apartment buildings, condominiums, commercial offices, or retail spaces follow different approaches than an individual residence might for example. This is often due to the type of HVAC system commonly used for each as well as specific code requirements for ventilation. Commercial spaces for example often have minimum requirements for how much outside air must be brought into the building based on size, intended use, or occupancy. But there also may be requirements on how much air to exhaust from certain spaces such as kitchens or restrooms. Commercial buildings also have much larger and more complicated HVAC systems and controls. 

The Importance of Air Pressure in Keeping Wildfire Smoke Out

If the air pressure inside a building or room is lower than the surrounding area, a pressure differential exists and air will naturally flow from higher pressure to lower pressure areas. Windows with poor weather stripping, gaps under doors, and even small cracks can let a surprising amount of air inside. When smoke is present in the air surrounding a building or room, an important strategy is to keep the air pressure inside higher (or at least equal) than the pressure outside, as this creates a barrier of air that keeps smoke out. 

How to Prevent Wildfire from Getting Inside Your Building

The first and most obvious step is to close windows all the way and make sure there are no gaps. Make sure spaces under any doorway to the outside are sealed. There are a surprising number of areas where air can move in and out of a building and it may take some time to locate them all. 

A building or home also has various pieces of equipment with fans that move air into or out of the building. It’s easy to overlook something like a clothes dryer, but air flows into the dryer from inside a house and then is exhausted outside which can alter the pressure inside. Becoming familiar with this equipment and how it alters air pressure inside a building is critical to limiting smoke from getting inside.

  • HVAC systems – Most commercial HVAC systems are designed to draw in a set amount of outside air continuously as part of an overall ventilation strategy. The outside air is mixed with recirculated air upstream of the air filters and follows ductwork into the buildings. However, when wildfire smoke is nearby, outside air intake should be closed off or significantly reduced. While outside air does enter upstream of the air filter, unless a series of high-efficiency filters that can control both particulate matter and gaseous chemicals at very high levels, wildfire smoke will penetrate the filters and enter the building.
  • Window AC units – Some window AC units bring in fresh air while others do not. Consult the operating manual to understand how a particular unit functions and how to close any outdoor air intake. Portable AC units generally only recirculate air from inside but there are models with dual hoses designed for one hose to bring in a small amount of outdoor air. Verify how each system works and shut off the outdoor air intake. 
  • Exhaust vents and fans – Some buildings and homes have exhaust vents and fans such as an attic fan. When these are running they exhaust inside air to the outside, but the consequence is the air pressure inside the building may then be lowered to less than the outside pressure. When this occurs, outside air can rush in with surprising force through small and almost invisible seams and cracks. 
  • Clothes dryers and range hoods – These act the same as exhaust vents. They exhaust indoor air outdoors, but like exhaust vents, they could potentially change the positive pressure inside the house to negative and draw smoke-filled air inside.  

How to Protect Your Lungs from Wildfire Smoke When It Gets Indoors

While strategies can be employed that limit smoke from entering a building, it’s inevitable that some smoke will find a way inside. Keeping in mind the guidelines mentioned previously regarding controlling outside air intake, setting the HVAC system’s fan to run continuously gives an opportunity for the air filters to remove hazardous particles found in wildfire smoke. However, to maximize this opportunity, the air filters themselves must be of high enough efficiency to make a difference. Otherwise, this strategy simply recirculates contaminated air through the facility. 

The smoke produced by a wildfire contains a high percentage of microscopic particles less than 2.5 microns in size, known as PM2.5. In fact, its estimated 90% of the total mass of wildfire smoke is comprised of these particle sizes. To filter the PM2.5 particles  out as air is passed through the HVAC system filters requires air filters with an efficiency rating designed to target the microscopic particles. 

Learn more: Air Filter Performance Test Standards

Here is another instance where the difference between equipment commonly found in commercial buildings versus what’s often used in residences matters. Commercial buildings typically have larger HVAC equipment with a greater footprint and higher fan capacity. The size difference may allow the units to accommodate larger air filters with more filtering area. While someunits only come standard with one- or two-inch filter-holding frames or tracks, many can hold filters with an even greater depth or can be retrofitted to hold larger filters. The value of deeper air filters with more filtering area is it allows for high particle capture efficiency ratings (MERV values) but with much lower pressure drops.  

For example, a commonly used MERV 8A two-inch pleated filter has a rated initial pressure drop of 0.31″ at 2000 cfm. However, a commonly used 12-inch deep MERV 14A V-bank style air filter has an initial pressure drop of 0.29″ at 2000 cfm which is lower than the MERV 8 filter despite a much higher MERV value. To achieve a MERV 8A value, a filter must have an average particle capture efficiency greater than 20% but less than 35% on particles between 1.0 and 3.0 microns in size. However, a MERV 14A filter is rated to capture an average of 90% of those same particles; 55% to 70% higher than a MERV 8A. When protecting lung health, the difference between MERV 8A and MERV 14A is significant. 

Most residential HVAC systems do not have the space (depth) to hold a 12-inch deep air filter. Typically these home units can only hold one- or two-inch deep filters. That limits air filter selection to standard, pleated-panel type air filters. While on paper, MERV 13 air filters with reasonable pressure drops are available, the reality is the air filters are only able to achieve a MERV 13 value by relying on a temporary electrostatic charge which inflates the actual performance. After a period of time, the filters lose MERV value, often dropping to as low as a MERV 8. 

Technical Bulletin – MERV vs MERV-A Explained 

What other air filter options are available for wildfire smoke?

If a home’s HVAC system is only capable of holding one- or two-inch air filters, steps can still be taken to improve the air. Yes, an electrostatically charged, standard MERV 13 pleated panel filter will lose efficiency as it loads with fine dirt particles, but the level of that drop and the timeframe that loss  occurs is dependent on dirt load and airflow. One strategy is to continuously run the fan in the HVAC system (following cautions about bringing in outside air), but replace the filter far more often than normal. And yes, following this strategy will cost more, but the filter will perform closer to its rated MERV value and the cost to provide clean air is far less than treatment for damaged lungs. 

The other option is to use room air purifiers.  Air purifiers became commonplace during COVID, particularly for parents of school-aged children who needed to make arrangements for in-home learning. There are several benefits to air purifiers which make them a good option when HVAC air filters are limited. 

Generally speaking, most air purifiers have filters with high particle capture efficiency, up to and including HEPA filtration. A HEPA filter has a minimum particle capture efficiency of 99.97% on 0.3-micron particles.  Ideally, the recommended air purifier should come equipped with  a test report certifying that the filter has undergone thorough testing to confirmHEPA-level filtration efficiency prior to being shipped from the factory. Otherwise, while it may be a better filter than most, there’s no guarantee it is an actual HEPA filter. 

Another benefit is that air purifiers are usually portable which provides flexibility to select key areas within the home and to create a zone of cleaner air. This may be a bedroom or a larger family room where several occupants gather. There are established calculations that offer guidance when determining if more than one air purifier is needed based on the size of the space in question. 

One other benefit is air purifiers can have activated carbon filters installed.  Wildfires release gaseous hydrocarbons and nitrogen oxides (that form ground-level ozone) as well as other harmful gases and volatile organic compounds. These are molecule-sized particles that are up to 1000 times smaller than what the best HEPA filters can capture. While some HVAC air filters include activated carbon, air purifiers have the capability of holding a wider variety and greater volume of carbon media which increases the number of chemicals that can be targeted and extends the filter’s longevity. 

Living with wildfire smoke

Ultimately there are areas of the country where, given enough time, wildfire smoke is a virtual certainty. When evacuation orders are issued, occupants should follow the instructions and get away from the area. In those situations where evacuation isn’t ordered or practical, understanding how to limit the amount of smoke entering a building, what to do if it does, and being prepared with the knowledge and equipment to protect the lung health of yourself as well as other occupants, will be an invaluable tool. 

About Camfil Clean Air Solutions

For more than half a century, Camfil has been helping people breathe cleaner air. As a leading manufacturer of premium clean air solutions, we provide commercial and industrial systems for air filtration and air pollution control that improve worker and equipment productivity, minimize energy use, and benefit human health and the environment. We firmly believe that the best solutions for our customers are the best solutions for our planet, too. That’s why every step of the way – from design to delivery and across the product life cycle – we consider the impact of what we do on people and on the world around us. Through a fresh approach to problem-solving, innovative design, precise process control, and a strong customer focus we aim to conserve more, use less and find better ways – so we can all breathe easier.

The Camfil Group is headquartered in Stockholm, Sweden, and has 30​ manufacturing sites, six R&D centers, local sales offices in 35+ countries, and about 5,600 employees and growing. We proudly serve and support customers in a wide variety of industries and in communities across the world. To discover how Camfil USA can help you to protect people, processes and the environment, visit us at 




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Lynne Laake 

Camfil USA Air Filters 

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