Wildfires and the air quality inside your home
Degraded outdoor air quality is one of the major ways wildfires impact us. Woodsmoke is a mixture of particulate and gaseous air pollutants that may be harmful to health. During a wildfire (Figure 1), our buildings can be protective of woodsmoke, reducing exposures and helping us to get through the event. The U.S. EPA recommends setting up a clean room in your home to increase the protective benefit of your home.¹ This post aims to provide actionable guidance to help you proactively prepare for the wildfire season by setting up a clean room in your home.
Summary: 5 steps to creating a “clean room” in your home
The goal is to create a “clean room” inside your home that:
- Substantially reduces indoor woodsmoke particle concentrations, aiming for >90% effectiveness
- Mitigate exposures to indoor sources of air pollution, including emissions from humans (metabolic bioeffluents)
The effectiveness of a clean room will primarily depend on the following five factors, which we will cover in detail subsequently. In short:
- Square footage of the zone: Identify a small area room (100–200 ft²) within your home
- Air cleaner sizing: Select an air cleaner with a clean air delivery rate of at least 2/3rd the floor area of the room
- Air exchange: Temporarily air seal points of outdoor air entry to the room to reduce infiltration
- Indoor sources: Remove sources of air pollution and manage human emissions, the latter by using the room’s door to control interzonal flow with the rest of the house and taking breaks from the space.
- Keep it simple: Use proven approaches to air cleaning, namely mechanical filtration.
Additional optional steps:
- Monitor performance. Every space is different — air monitoring can improve performance and confidence in your clean room.
- Consider gas-phase air cleaning. Woodsmoke is a complex mixture of particles and gases. Gases are also of concern for health, though can be more difficult to remove from air.
Step 1: Identify a small area room (100–200 ft²) within your home
Selecting a small room in your home, most likely a bedroom, is a logical choice for creating a clean room. Identify a bedroom (or another similarly sized zone in your home) and determine the floor area of the room (length x width) in square feet. A target floor area might be in the range of 100–200 ft² . The zone should also have a door which can be used to close off the space from the rest of the home.
During the extreme conditions of a wildfire, we want to maximize the air cleaning effect. Targeting a smaller zone within the home is important because the effectiveness of an air cleaner increases as the volume of the space being cleaned decreases. Remember, the goal is to create a clean room within your home — it would be more challenging and expensive to achieve the level of pollutant reductions were targeting (>90%) in the entire home.
Step 2: Select an air cleaner with a clean air delivery rate of at least 2/3rd the floor area of the room
Woodsmoke is a mixture of particle- and gas-phase air pollution. The most common air cleaning approach is filtration — passing air through a fibrous filter that removes particle-phase air pollution. Particle-phase air cleaning will be the primary focus of this guide. For gas-phase air pollutants, sorbents, like activated carbon, are a widely used approach. However, sorbent air cleaning tends to be more expensive, generally less effective (there are 100s -1000s of gaseous compounds elevated in woodsmoke that may differentially and competitively interact with the sorbent) and require frequent monitoring to ensure performance. See the optional step at the end of the post for more on gas-phase air cleaning.
For an air-cleaner to be effective, it must be sized appropriately for the space. The most common metric by which portable air-cleaners are compared and evaluated is the Clean Air Delivery Rate (CADR).² This is the amount of particle-free air emanating from a device in units of cubic feet per minute (cfm). A useful rule of thumb is the 2/3rds rule³: the CADR of the air cleaner (in cfm) should be 2/3 the floor area of your space (in ft²). In looking at specifications of an air cleaner, you may see three values of CADR: smoke, pollen, and dust. Use the data for “smoke”.
An example: The floor area of your bedroom is 150 ft². To meet the ANSI/AHAM 2/3rd rule, the smoke CADR should be at least 100 cfm (100/150 = 2/3)
Higher CADR results in greater air-cleaning effectiveness. An oversized air cleaner (or multiple units) may enable running the air-cleaner on lower settings. Effective air cleaners with high CADR can be loud, and the psychological impact of the constant noise of an air cleaner is a factor many will want to consider. However, this up-sizing solution may not be universally accessible — while not uniformly true, higher CADR air cleaners typically cost more upfront, cost more to replace filters, and cost more to operate due to higher power draw. In Step 3, we will discuss an approach to increase the effectiveness of the air-cleaner through air-sealing. For a more detailed sizing calculation and other background, see the CU-Harvard sizing calculator. More guidance on selecting an air-cleaner, and what to avoid, can also be found in Step 5.
Let’s take a look at the impact of an air-cleaner that meets the 2/3rds rule for a 150 ft² bedroom during an extreme wildfire air pollution event (Figure 2). Outdoor PM2.5 is 300 µg/m³ (orange line), taken as constant during a 4-hour modeling period. The indoor concentrations are modeled for two cases: no air cleaner (yellow line) and with the air cleaner operating (green line). Other inputs to the model are noted in the figure caption and are held constant for the “no air cleaning” and “with air cleaning” scenarios.
Indoor levels of PM2.5 without the air cleaner are 120 µg/m³, a very high concentration, though demonstrating some protective benefit from staying indoors even absent air cleaning. With the air-cleaner operating for one hour, indoor concentrations stabilize at ~18 µg/m³ after 1 hour of operation. These data allow us to calculate the air cleaning effectiveness, or the extent of pollutant reduction in the space with the air cleaner compared to without:
The above calculation shows our intervention effectiveness is 85%. However, 18 µg/m³ is still a high level of indoor PM2.5 compared to normal! If this were an outdoor environment, this PM2.5 concentration would put us in a “moderate” air quality index, where guidance suggests that sensitive people and the elderly are at risk and should consider reducing exertion and activity. As noted above, further reductions can be achieved through more air cleaning (more units or units with higher CADR). In Step 3, we’ll discuss another option, air sealing, to improve the overall air cleaning effectiveness.
Step 3: Temporary air-sealing can reduce the outdoor air exchange rate.
Impact of air-sealing
Air sealing reduces the infiltration (or leakage) of outdoor air to an indoor space. All other things equal, this serves to reduce the indoor concentration of woodsmoke. This is because particle removal processes that occur in your home are “competing” against a smaller amount of woodsmoke entering the space. Let’s revisit our model to predict the impact of air-sealing on our air cleaning intervention, assuming the outdoor air exchange rate of the room is reduced from 0.5 /h to 0.3 /h.
Shown in Figure 3 is an additional model run (blue line) where we see the impact of outdoor air sealing on indoor PM2.5 concentration. With the same 100 cfm CADR air cleaner in the same space and the addition of air sealing, we now see an 11 µg/m³ PM2.5 concentration at steady state; to achieve the same indoor PM2.5 concentration via air-cleaning alone (i.e., without air sealing) would require a 70 cfm increase (i.e., increasing from 100 to 170 cfm) in air cleaning CADR.
Also shown in Figure 3 is that air sealing with the air cleaner results in a 91% intervention effectiveness (relative to the no air cleaning or sealing scenario, yellow line). Thus, the combination of air cleaning following the 2/3rds rule with modest, temporary air-sealing met the initial goal of a (combined) intervention effectiveness >90%.
Suggestions on where and how to air-seal
During a wildfire, windows and exterior doors to your home should be closed. If your home HVAC system has an outdoor air intake, it should also be closed. Temporary (during the wildfire) air-sealing should prioritize locations of outdoor air intrusion into the clean room. A logical place to focus is reducing infiltration is around exterior windows in the clean room. Especially with older windows, substantial leakage occurs here. There exist low-cost weatherization kits that create a temporary barrier via double-sided tape and a thin film of plastic. This air sealing can be done quickly and inexpensively, and easily taken down after the wildfire ends. These kits can be used to seal around the window frame or on the drywall that surrounds the window (see picture). Look through the selected room for other possible areas of outdoor air intrusion. The goal here is reduce, not fully eliminate, outdoor air infiltration. Targeting windows alone may be sufficient.
Things get more complex if your clean room is connected to the rest of the home via a central heating, ventilation, and air-conditioning system. If you know you have a system that includes a high-efficiency filter, recirculates high flowrates continuously throughout the home, and does not induce outdoor air infiltration into your clean room and/or home, then you might consider leaving the supply and/or return registers to the room open. If you are not sure of those things, then consider closing the dampers and use the air-sealing kit to create a plastic seal around the registers serving the room. This allows you to have control over one zone of your home by disconnecting it from the airflows that will be mixing the air throughout the rest of the home. Even with a central HVAC system operating with a filter, your home may have a high outdoor air infiltration rate and/or insufficient air cleaning to maintain low indoor PM levels throughout an entire home. Further, HVAC system operation is complex, and can change your home’s indoor-outdoor pressures and alter infiltration throughout the home. This is a good example of where a particle measurement device is valuable — one could run some tests under scenarios to get an idea of what approaches are more effective.
Step 4: Address indoor sources of air pollution, especially if temporarily air sealing.
Strategies for reducing exposures to indoor sources of air pollution during a wildfire
Indoor sources of air pollution should be eliminated or reduced, in general, but especially during a wildfire. For some sources, this is relatively straightforward. Common sources of indoor air pollution include activities like cooking and cleaning — take a break from these activities to the extent possible during the wildfire. Don’t burn candles or incense and don’t use air fresheners — particularly in the clean room.
More difficult to address are continuously emitting indoor sources of air pollution like yourself — the occupant. As a result of metabolism, we are constantly emitting carbon dioxide (CO₂)⁴ and a wide range of organic compounds,⁵ often referred to as metabolic bioeffluents. Addressing indoor sources like bioeffluents is especially important if you’ve taken steps to temporarily air seal. In an airtight space, bioeffluents can accumulate, potentially causing respiratory impacts.⁶ In general, low ventilation rates are associated with adverse health effects,⁷ though the literature on ventilation and health assumes outdoor air quality is generally superior to indoor air quality. During a wildfire event, elevated bioeffluent exposure is likely the lesser concern compared to woodsmoke exposure, and our targets for bioeffluent exposure are greater than would be desired under normal circumstances.
In this section, we’ll develop and evaluate some simple strategies for addressing bioeffluent exposure. The first strategy is simple: take periodic short breaks from the clean room, perhaps while donning an N95 respirator and getting out into the larger home or even outdoors. Breaks are also essential as it is important to consider psychological benefits of movement and changing location along with air pollution exposures.
A second approach is to use the door of the clean room as a “damper” to modulate interzonal mixing, or transfer of indoor air from the adjoining zone of the home with the clean room and vice versa. This interzonal flow can be used to exhaust accumulated bioeffluents in the room to the larger volume of the residence. This benefit must be balanced with the trade-off of entry of woodsmoke from the home into the room. Refer back to Figure 2 and 3 — the yellow line on those plots showed us the expected indoor concentration of PM2.5 absent indoor air cleaning. We’ll assume this is the PM2.5 concentration in the rest of the home. While still a high indoor PM2.5 level, 120 µg/m³ of PM2.5 is obviously preferred than the outdoor air concentration at 300 µg/m³.
Impact of strategies to reduce exposures to indoor air pollution during a wildfire
The next questions that follow are: how frequently should I open the door and how much? How frequently and for how long should I take a break from the space? We’ll explore these questions using a model similar to that shown in Figure 2 and 3, but now includes predictions of metabolic CO₂, a proxy for human bioeffluent emissions. We’ll also need to know the magnitude of the interzonal flow between a bedroom and the adjoining space — the extent of this interzonal flow will vary substantially depending on many factors specific to the space. One study⁸ in a test house measured bi-directional airflow rates across wide-open doors vs. closed doors, finding that wide open doors allowed 60 m³/h of bidirectional airflow while a closed door provided 1 m³/h of bidirectional airflow. Let’s assume that opening the door halfway provides 30 m³/h of bidirectional flow, or 15 m³/h of ventilation from the indoor environment to the clean room and 15 m³/h vice versa.
In general, there exists a trade-off between bioeffluent exposure and PM2.5 exposure in our clean room. Figure 4 shows one solution to balancing woodsmoke exposure (indicated by PM2.5) and bioeffluents (indicated by CO₂). We can see in the figure that when the door opens to 50%, the rate of CO₂ increase slows while PM2.5 accumulates. In the approach modeled, the two adult occupants also take 15-minute breaks from the room every two hours, keep the door open to the space halfway during the break. These two actions, in combination, hold the CO₂ concentration to 1850 ppm and the indoor PM2.5 level to 17 µg/m³ over an eight-hour period.
What about sleeping? When sleeping in this bedroom, one would obviously not want to wake every hour to open and close the door. You might consider leaving the door open at 50% for an entire 8-hour period. This will allow exchange of air between the bedroom and the rest of the home and strike a reasonable middle ground between PM2.5 and CO₂ exposure. Modeling this increase in interzonal exchange (not shown) results in a eight-hour average PM2.5 exposure of 20.7 µg/m³(slightly higher than the strategy shown in Figure 4), but lower eight-hour average CO₂ exposure (1666 ppm). Elevated levels of CO₂ (and associated bioeffluents) have been studied in sleeping environments. Levels of ~2400 ppm have been associated with lower scores on tests of mental performance and self-reported sleepiness.⁹ Not ideal, but during a wildfire, likely an acceptable outcome.
Step 5: Keep it simple.
Air cleaning is essential to creating a clean room. Rely on proven approaches that remove harmful pollutants from air. For particles, fibrous filters (sometimes called just “filters” or “mechanical filters”) have a long track record of testing and study.¹⁰ Filters remove particles from air by collecting them on the fibers of the filter. You can read more about filtration and other air cleaning approaches via ASHRAE’s air cleaning position statement.¹⁰ Electronic, ionizing, chemical fogging, radical generating, or other “additive” air cleaners have not (to the author’s knowledge) demonstrated CADR or efficacy under wildfire conditions that would enable the type of predictions regarding intervention effectiveness made here. If CADR-type metrics are available, evaluate them accordingly and compare to other air cleaners on the market. For additive air cleaners, concerns exist regarding potential for byproduct formation, especially under conditions of high levels of indoor particles and organic gases during a wildfire event. See the appendix to this article for more information about possible gas-phase air cleaning options.
Purchasing a commercial air cleaner. Look for the device’s specification sheet that includes a CADR for smoke and follow the 2/3rd rule discussed in Step 2. Many air cleaning manufacturer’s report this metric in a standardized “AHAM Verifide” seal. If they do not, consider contacting the company to inquire about their testing and sizing recommendations. Noise and cost are two other major considerations.
Do-it-yourself air cleaner.: The most well-known arrangement for a low-cost air-cleaner is to place a MERV13 filter or filters on the inlet side of a box fan. One simple arrangement is to seal a single filter on the inlet-side of a box fan. Slightly more complex builds using multiple filters, but with much higher airflow and clean air delivery rate, are the so-called Corsi-Rosenthal Box or Comparetto Cube. Other groups have published instructions on these types of air cleaners, including the Yale school of public health. These approaches to air cleaning are expected to have high CADR values based on measurements of flowrate, though actual CADR testing is forthcoming.
(Optional) Monitor performance.
Air monitoring can enable iteration to optimize your clean room and provide confirmation that it is effective by allowing you to determine the exposure concentration (i.e., the concentration of PM2.5) in your clean room. Monitoring can also help you manage exposure to indoor bioeffluents. For example, if you observe that PM2.5 concentrations are low in the rest of your home while CO₂ levels are high in your clean room, you may opt to leave the door open for longer periods.
For those interested, air monitoring can also help evaluate the effectiveness of your clean room. One can empirically estimate the intervention effectiveness by accounting for the potentially changing outdoor conditions via a slight modification to our prior effectiveness equation:
The effectiveness can be determined by running an experiment (one relatively simple example of a protocol):
- Measure PM2.5 while operating the air-cleaner in your clean room until levels stabilize, ~1 hour of operation for an air-cleaner sized as discussed in this post.
- Measure PM2.5 while NOT operating the air-cleaner in your room until levels stabilize, ~3 hours of operation.
- Ensure no indoor sources of PM (e.g., cooking, cleaning) are present in the clean room or home during the measurements for steps 1 and 2 and do not enter the test space during testing
- Visit https://www.purpleair.com/map?mylocation#11/45.4804/-122.5891 or your state’s air quality program to estimate PM2.5 outdoors.
- For greater accuracy, collect the corresponding outdoor air data averaged over the periods with and without the air cleaner operating and insert into the above equation according
There exist many particle measurement devices that range $50-$250 and may be suitable for the above activities. Look for an NDIR (non-dispersive infrared) CO2 monitor and an optical (light-scattering) particle sensor. Researchers at the Lawrence Berkeley National Laboratory studied several low-cost monitors that may be useful for measuring particles during wildfire periods. For CO2 monitoring, researchers have shown low-cost monitors can be effective.
(Optional) Consider gas-phase air cleaning.
Woodsmoke is a complex mixture of particles and gases that are harmful to human health. Removal of gas-phase species is in general, challenging compared to particle-phase compounds. Technologies to remove gases from air include the use of sorbents, to which harmful gases “attach” via physical and chemical bonds. Activated carbon is one common sorbent to address gas-phase compounds, though other sorbents exist that target specific types of compounds. Design of gas scrubbers is complex, and usually undertaken by a qualified engineer for a specific indoor space, who may be targeting a specific class of compounds and be designing with knowledge of many other site-specific variables. In such cases, air testing may be conducted to help evaluate how much sorbent is needed, with subsequent assessment to determine when the sorbent should be changed out or regenerated.
Given this complexity and additional cost, a blanket recommendation for everyone to pursue gas-phase air cleaning is more difficult to make at this time. Personally, I have pursued this option in air cleaners and if you choose to do so, look for air cleaners that contain sorbents like activated carbon, activated alumina, or potassium permanganate (or some combination). Each of these sorbents is selective (i.e. tuned) for a specific class of gas-phase compounds. A more effective implementation of these sorbents is usually “packed bed”, which contains a larger mass of sorbent than air cleaners that may have only a thin sheet or cloth of impregnated sorbent, which is ultimately a small mass.
Given the complexity, and lack of generalizable test data, rules of thumb are likely the only viable design approach. For sorbent systems, a rule of thumb for design is 4.5 lb of sorbent/1000 ft³ of room volume per year. though this resource notes this may overstate the amount of carbon needed). So, for our hypothetical bedroom that is 10 ft x 15 ft by 8 ft = 1200 ft³, we would be looking for an air-cleaner with a 5.4 lb packed bed of sorbent, and plan to replace it yearly. There exist several options of air cleaners that combine this mass of carbon with a HEPA filter for particle and gas-phase air cleaning, though again, with substantial additional upfront cost and maintenance cost.
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(2) ANSI/AHAM-. AC-1–2015: Method for Measuring Performance of Portable Household Electric Room Air Cleaners; 2015.
(3) Air Filtration Standards — AHAM Verifide.
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(10) ASHRAE. ASHRAE Position Document on Filtration and Air Cleaning; ASHRAE Position Document, 2015.
(11) Spengler and Samet, Indoor Air Quality Handbook, Chapter 10, Removal of Gases and Vapors ISBN: 9780074455494 Publication Date & Copyright:2001 McGraw-Hill Education