A DIY Box Fan Air Filter for the Room Your Animals Are In: The Build, the Measured Numbers, and Where to Put It

Build a box fan air filter only when a commercial portable air cleaner is unavailable or unaffordable, which is the condition EPA attaches to it. If you build one, use a box fan made since 2012 with a UL or ETL mark, attach MERV 13 filters, add a cardboard shroud, keep the unit where your animal cannot lean on it or topple it, check the filter every day, and watch the temperature of the room the animal is in. The build traps particles only. It does not remove gases or odors, and it does not cool the room.

Almost everything published about this build was written about a room with people in it, not a room with a dog in it. Where a source stops and our own reasoning starts, this page says so.

For the smoke thresholds that tell you whether to keep the dog in at all, see what AQI is safe to walk my dog. If buying is still an option, wildfire smoke and pets has the commercial air cleaner comparison and the room-size selector. If heat is the real problem, a heat emergency with no air conditioning applies. The fan on this page is a filtration device, not a cooling device.

Before You Build: EPA’s Order of Preference

EPA’s Wildfires and Indoor Air Quality page (last updated August 6, 2026) recommends DIY air cleaners only when products of known performance, such as commercial portable air cleaners, are not available or affordable. It says they are “not recommended as a permanent alternative to these products.” The reason is variability: well-built DIY units can perform similarly to commercial ones, but each can perform differently, even after you reassemble the same unit to change filters. Commercial units are more standardized, so their testing is more reliable.

EPA’s Indoor Air Filtration fact sheet (EPA-452/F-26-006) gives the same ranking in its comparison table: the DIY option is a good choice when portable air cleaners are not available or affordable. EPA’s cleaner air room fact sheet (EPA-452/F-26-005) opens its DIY bullets with “Get a portable air cleaner if you can.” It adds that DIY performance varies with how the unit is built and maintained, and that DIY units are often louder. It also calls the build an effective, less expensive option when portable air cleaners are not available or affordable.

EPA’s pet fact sheet (EPA-452/F-26-007) tells owners to buy a portable air cleaner and high-efficiency HVAC filters compatible with their central air system, and to think about a cleaner air room. It does not mention box fans or do-it-yourself air cleaners anywhere. So EPA does not recommend this build for animals. What it publishes is general household guidance, written for people, that a household with animals can use when the recommended option is not on the shelf.

The Build

Everything in this section comes from EPA’s two-page infographic, EPA’s fact sheets, or the California Air Resources Board (CARB). Nothing here is a design we invented.

The basic design

EPA’s Basic Design page lists a 20 by 20 inch box fan, a 20 by 20 inch air filter 1 or 4 inches deep with a suggested rating of MERV 13, and clamps, duct tape or bungee cords. The fan must be a certified fan with a UL or ETL marking, 2012 model or newer.

The steps:

1. Attach the filter to the back of the fan.
2. Check the arrow on the filter side for the direction of airflow, and point it the way the air is going, which means toward the fan. CARB states the arrow rule from the other side, and the two agree.
3. Replace filters when dirty.

EPA’s cleaner air room fact sheet adds that you should use a high-efficiency filter, preferably MERV 13 or higher, and get a good seal between the fan and the filter. CARB adds that you should close all windows and doors while the unit runs. EPA’s research page explains the MERV 13 recommendation: the filter rating strongly affects CADR, and MERV 13 is what removes the very small particles in wildfire smoke.

Good, Better and Best

EPA’s Beyond the Basic page has three columns:

• Good: the basic supplies plus a 20 by 20 inch cardboard shroud with a cutout the size of the fan blades.
• Better: two MERV 13 filters and a triangle cardboard cutout for the base on top.
• Best: four or five MERV 13 filters. If you use the five-filter design, use leg supports such as blocks so air can flow through the bottom.

EPA’s Ways to Improve Effectiveness list is a summary of its research: add a cardboard shroud (no cost), use thicker filters (4 inch rather than 1 inch), and use multiple filters (two to five). Its Key Reminders repeat the fan condition, “Keep extra filters on hand,” and replace filters when dirty. The four or five filter cube is the design known as the Corsi-Rosenthal box, first proposed by Richard Corsi with Jim Rosenthal making the first prototype.

The shroud is the free upgrade

EPA’s research page says the cardboard shroud raises CADR by 40% with no change in cost or footprint. A shroud is cardboard covering the corners of the fan’s outlet, leaving an opening about the size of the fan blades. EPA’s Science Matters item says it blocks recirculating air past the blade tips. In the cube design the fan blows out of the filter box, which creates slight negative pressure that may help seal the box, though leaks from holes in filters or tape are independent of flow direction.

What Each Design Delivers

EPA tested these designs in laboratories in Research Triangle Park, NC, using simulated wildfire smoke made from smoldering pine needles in a room-sized chamber. CADR is a standard measure of how well an air cleaner removes particles of a specific size from a standardized room. Row labels and figures below are EPA’s, as written.

Design	Wildfire smoke CADR
Box fan with one 1” MERV13 filter	111 ± 1
Box fan with one 1” MERV13 filter and cardboard shroud	156 ± 4
Box fan with one 4” MERV13 filter and cardboard shroud	248 ± 15
Box fan with two 1” MERV13 filters and cardboard shroud	263 ± 22
Corsi-Rosenthal Box with four 1” MERV 13 filters and cardboard shroud	401 ± 31

Source: US EPA Office of Research and Development, Research on DIY Air Cleaners to Reduce Wildfire Smoke Indoors (last updated July 6, 2026). The underlying study is Holder, Halliday and Virtaranta, Indoor Air, volume 32, issue 11, November 2022, as EPA cites it.

A cardboard shroud adds more than a third to the bare filter. A four-filter cube with a shroud delivers roughly three and a half times the bare single filter. More filters cost more, take more space and are harder to assemble, but EPA says they are much more effective and can clean larger rooms.

If you lack floor space for a cube, EPA says a single 4 inch MERV 13 filter is also highly effective (a 123% increase in CADR) and may suit smaller areas. It also says using multiple single-filter units in the same room is worth considering. For a household where a cube on the floor is a hazard because of what lives on that floor, a slimmer single-filter unit with a shroud is a published design with a published number, not a downgrade.

A second set of numbers, kept separate

Dal Porto, Kunz, Pistochini, Corsi and Cappa characterized a five-panel Corsi-Rosenthal air cleaner (Aerosol Science and Technology, volume 56, issue 6). Their abstract reports a size-integrated effective CADR of about 600 to about 850 cubic feet per minute, rising with fan speed. Four differences separate that from EPA’s 401:

• Different aerosol. EPA used simulated wildfire smoke. The UC Davis team used salt particles from a nebulizer and reports results for particles larger than 0.5 microns.
• Different build. EPA’s row is a four-filter cube with 1 inch filters. The paper’s unit is a five-panel build with 2 inch filters.
• Different setting. EPA used a room-sized chamber. The paper measured in a furnished but unoccupied 5926 cubic foot classroom and a furnished but unoccupied 1277 cubic foot home office.
• Different method ceiling. The paper says its method can determine CADR values above the upper limit of the standard test method, 450 cubic feet per minute.

These are two measurements of related objects, not two estimates of one quantity. Do not merge them into a single range.

ASHRAE’s Position Document on Filtration and Air Cleaning (approved June 26, 2024, expiring June 26, 2027) gives the general caution both sets of numbers deserve: performance in a specific application “should not be assumed equivalent to performance determined using controlled laboratory tests under specific conditions.” That document contains no guidance on box fans or Corsi-Rosenthal boxes.

Sizing It to the Room

Three groups publish room-size rules, and they answer slightly different questions.

• AHAM says an air cleaner’s CADR should be at least two-thirds of the room’s area as a general rule of thumb. For wildfire smoke it recommends a smoke CADR equal to the room’s size in square feet. Ceilings above 8 feet need an air cleaner rated for a larger room.
• EPA’s two fact sheets give the two-thirds rule for portable air cleaners. Neither attaches a room-size rule to the DIY option.
• EPA’s research page mentions the square-footage rule only as an AHAM suggestion, immediately before its CADR table.

Our own reasoning: holding EPA’s chamber figures against AHAM’s smoke rule puts the 401 cube in a room in the low hundreds of square feet and the bare single filter at something much smaller. We do not publish a per-design square-footage table. It would stack three assumptions: that a chamber figure transfers to your room, that a rule for tested commercial units transfers to a home build, and that your build performs like EPA’s.

What holds up:

• More filters and a shroud beat fewer filters and no shroud, by a wide margin, in the one controlled comparison a federal agency has published.
• The smoke rule is stricter than the everyday rule. Use it when smoke is why you are building.
• A tall room needs more than its floor area suggests.
• Two units in one room is a published option. EPA says you may benefit from more than one air cleaner if smoke is very thick or your home is not well sealed.

Which room gets it

EPA’s research page says that if you can designate one room that people spend time in when it is smoky, one air cleaner may be sufficient, and that some people move one air cleaner from room to room. EPA’s first step is to pick a room big enough for everyone in the household, and it suggests a bedroom with an attached bathroom.

In many pet households the dog is crated in the utility room, the cat is in the spare bedroom, the parrot’s cage is in the living room, and the people are in a fourth room. EPA’s guidance does not address that split. One unit cleans one room, so you are choosing which room gets the filtration, and it is better to choose before the smoke arrives. EPA does tell owners to bring outdoor pets into a less smoky, cooler place such as a utility room, garage or bathroom. A garage carries a carbon monoxide hazard a bedroom does not, covered in generators, carbon monoxide and pets.

The 2012 Fan Condition

This is the most repeated claim about the build and the one most often stated wrongly.

EPA’s cleaner air room fact sheet says: “Use a newer box fan (made since 2012) with a UL or ETL logo.” It says these fans have thermal safety fuses that reduce the risk of fire or burns from an overheated motor and outer surfaces, and to look for an electrical safety listing and a mark from a certifying organization such as UL or Intertek. Then comes the sentence that drops out of summaries: “If you must use an older fan, never leave the fan running unattended while you are away or sleeping.” This is a recommendation with a fallback, not a prohibition.

EPA’s Indoor Air Filtration fact sheet compresses it to one line in the DIY column: use a box fan made since 2012 with a UL or ETL logo to reduce the risk of overheating.

EPA’s research page words the older-fan half more strongly. It says fans built before 2012 “pose known fire risks.” It adds that you should use fans verified by an accredited third party to meet the UL 507 electric fan safety standard or equivalent, identified by a UL or ETL marking. It does not recommend DIY air cleaners built with pre-2012 fans, and says if they are used they should not be unattended or used while sleeping.

EPA’s Wildfires and Indoor Air Quality page says fans built before 2012 “may pose fire risks,” with the same advice not to leave one unattended or use it while sleeping. The two EPA phrasings differ, so we quote each to the page it came from.

CARB publishes the date independently (fact sheet dated August 20, 2021). It says only fans made in or after 2012 should be used, because they have a fused plug, “which will prevent electrical fires if the device is knocked over.” It carries the same fallback for older fans. EPA’s Science Matters item names fused plugs and thermal cutoffs as the safety features of a 2012-or-newer fan.

A recommendation, not a mandate

Two agencies publish the 2012 date. Neither publishes it as a legal requirement, and no cited source points to a regulation, standard revision or recall that sets a 2012 mandate. The UL Research Institutes report EPA relies on never mentions 2012. Call it a recommendation.

The Missoula City-County Public Health Department, a partner in EPA’s Wildfire ASPIRE Study and the how-to link on EPA’s research page, says to use a newer fan with a fused plug, not 3 prong, just slightly larger, and that “All fans sold in the US after 2012 will have this.” That is a health department’s statement about what is on the shelves, not a citation of a standard. It also says older box fans are more likely to catch fire.

Why the mechanism matters with animals

Only CARB names the scenario the fused plug is for: the device being knocked over. That is not a claim that a dog knocking over a fan will burn the house down. It shows the date exists for a scenario in which the device ends up on the floor on its side, and with a large dog, a cat that jumps onto things or a bird out of its cage, that is an ordinary Tuesday.

Placement With an Animal in the Room

What the agencies publish

EPA’s Indoor Air Filtration fact sheet gives three instructions for a portable or DIY unit: use it where you spend the most time, run it continuously on the highest fan setting, and “Make sure air can flow freely around the air cleaner.” EPA’s research page says DIY air cleaners made with newer model box fans are unlikely to pose a fire or burn risk but should be kept clear of obstructions and operated with common sense precautions.

EPA’s Wildfires and Indoor Air Quality page relays the box fan manufacturer’s rules: do not leave children unattended when the fan is in use, do not use an extension cord, do not use a damaged or malfunctioning fan, and have working smoke detectors throughout the home. In a pet household the extension cord rule matters. People reach for a cord to put the unit somewhere other than next to the outlet, and a cord across the floor is something animals chew and trip over.

What UL measured when the fan was blocked and tipped

UL Research Institutes’ 2021 report, conducted with support from EPA’s Office of Research and Development, tested five box fan models with a single filter taped to the back. It chose a single filter deliberately, because it puts the largest stress on the fan. One scenario was a two-sided obstruction: back completely blocked and the fan tipped onto its face. It ran for at least seven hours. The findings:

• “Fire ignition was not achieved with any of the filter/fan test scenarios.”
• Exterior surfaces that can touch people stayed below 36 °C, below the 47.5 °C first-degree burn potential.
• In the two-sided obstruction tests temperatures rose at the guard, switch and output air, but the parts stayed below burn-hazard thresholds. The highest reading, a 57 °C rise above ambient at the winding, came on fans with a dust-laden filter.
• Attaching a filter cut airflow by up to 29% for a clean filter, 31% for a smoke or soot-laden filter and 70% for a dust-laden filter, relative to the bare fan. The report notes large standard deviations in these measurements.

So a pet that lies against the unit, sits on it or knocks it over is a real problem, but the problem measured is that the room stops being cleaned, not that the house catches fire. An obstructed or dirty unit leaves the animal breathing air nobody is cleaning.

Two limits travel with this. The report says its results do not imply the fans tested meet UL 507, which only compliant third-party testing can confirm. EPA says it cannot assure the results represent all scenarios and that operating any electrical device carries inherent risks.

The gap UL names

The report says it assessed DIY air cleaners at room temperatures near 20 °C (about 68 °F), and that hotter rooms, for example 40 °C, may need further analysis. It also names a scenario it did not cover: if the blade turning is impeded, further heating is possible. It points to fans that meet UL 507 as the ones also required to pass a test for that hazard.

A tail, a paw, a nose, a mouthful of fur or a toy pushed through the guard is blade impedance. This is our reading, not an agency’s instruction: for a household with animals, the UL or ETL marking is non-negotiable. The certification is what EPA, CARB and UL each point to, and UL ties it to exactly the failure a curious animal creates.

The inverted build

The UC Davis team built a variant with the fan pointing at the floor, expecting it to be more robust against foreign objects dropped into the fan. They measured lower CADR than the standard upright orientation (for example 481 on low against roughly 600), suggested two possible explanations (short-circuiting of exhausted air back into the filters, or shear forces resuspending settled particles), and advise against pointing the exhaust at the floor. In a room with a shedding animal, blowing settled dust back into the air is a reason that gets stronger. Solve the problem with the arrangement of the room, not by modifying the unit.

Our reasoning, marked as ours

No cited source addresses an animal sharing a room with a DIY air cleaner. Applying the sources to a dog that leans on things, a cat that seeks warm surfaces and a bird or small mammal that could be out of its enclosure is our reasoning, not an agency’s instruction:

• Stand it where the animal cannot lean on it, sleep against it or topple it, and where the exhaust is not aimed at a bed, crate or cage.
• Route the cord where nothing can chew or catch it, and do not use an extension cord.
• Keep the filter faces clear. On a cube build the filter faces are vertical walls at animal height, and a bed, blanket or body against one is an obstruction.
• If you cannot place it out of reach, put a barrier around it rather than modifying it.
• Check it when you check the animal. A unit nudged or leaned on all afternoon is not cleaning the air.

Filters: What Decides Whether It Works

The build is not the hard part. Keeping the filter clean is.

EPA’s research page says DIY air cleaners were “almost completely ineffective with dirty filters,” and that filter lifespan varies with use and conditions. EPA’s instruction is specific: “Check the filter daily during a wildfire smoke event.” Change it when it is “dark brown or grey or smells like smoke.” EPA’s Wildfires and Indoor Air Quality page says to have extra filters on hand, and that during smoke events filters may need to be changed every few weeks or days. EPA’s guide to air cleaners says every filter needs regular replacement. Buy spares before smoke season, because filtration becomes hard to get exactly when everyone wants it.

The pet hair question

A common claim says animal hair loads a MERV 13 filter faster than dust, so pet households should replace filters much sooner. It sounds right, but none of the sources cited on this page measures animal hair as a filter-loading agent or gives a figure for it, and we do not publish one.

The one controlled comparison that exists runs the other way. UL loaded filters cleanly, with smoke and with a standardized dust it describes as a worst-case household blockage. Dust caused the largest airflow penalty, up to 70%. That is a finding about dust, not hair. The operating rule does not depend on settling it: look at the filter every day and change it when it is dark or smells like smoke, whatever loaded it.

The screen prefilter

Clean Air Crew, a volunteer aggregator and not an authority, describes adding fiberglass screen material as a way to keep pet hair out of filters. It reproduces a social media post dated November 14, 2021, reporting a 1.27% airflow loss with an 18 by 16 per inch mesh. That is one person’s bench measurement of his own build, not an agency or peer-reviewed figure. The claim that a screen prolongs filter life is an assertion with no measurement behind it. If you add a screen, keep doing EPA’s daily filter check. EPA’s own instruction, cutting the shroud opening the size of the fan blades, is the one we recommend.

Noise and Power

Only one source measures both. Dal Porto and colleagues used a decibel monitor 5 feet from the unit, perpendicular to the exhaust, against 40 dB of background noise, in unoccupied rooms. They report: “The CR Box loudness varied from 58 dB (low speed) to 67 dB (high speed).” For scale, they note a modern refrigerator is about 50 dB and a LEED certified vacuum must be under 70 dB. Two commercial HEPA units measured 59 dB and 54 dB. Matching the CR Box’s CADR on low would take about two of the first or four of the second, at 62 dB and 60 dB.

Power: “The power draw for the CR Box varied from 67 W (low speed) to 98 W (high speed).” The commercial HEPA units drew 89 W and 43 W. That is for a 20 inch box fan with MERV-13 filters. Your own fan’s data plate governs your own arithmetic.

EPA names noise and power as known drawbacks. Its cleaner air room fact sheet says DIY units are often louder. Its research page lists increased noise and heat from the fan motor, and its Science Matters item says the DIY units were as effective as commercial ones but louder and needing more power.

No source publishes a sound level at which a dog, cat, bird or small mammal is harmed or distressed, so no animal cutoff appears here. The behavioral readout is the one you have: if an animal will not enter the room, hides, paces or will not settle once the unit runs, that is information no specification contains. Ask your veterinarian about a noise-averse animal before smoke season. Quiet generator versus battery power station covers noise aversion in dogs, and the honest review of dog smoke masks has measured decibel figures for a commercial purifier.

For a battery power station, 67 to 98 watts is in the same range as the box fan load portable power stations for pets does runtime arithmetic around. We do not publish a runtime, because the arithmetic starts at your own fan’s data plate and your own station’s capacity; the watt-hour budget page shows where to find both. EPA writes a power outage into its reasons to leave a cleaner air room, not into a battery plan.

What the Build Does Not Do

Gases, odors and ozone

A pleated particle filter traps particles. CARB says HEPA filters are very useful for removing particles but cannot remove gases, which pass through. EPA’s guide says no air cleaner or filter eliminates all pollutants and that most filters handle either particles or gases. To filter gases, it says, choose a portable air cleaner with an activated carbon filter or another filter designed for gases, and it notes that the CADR rating system is for particles only. Every number in the table above is a particle number.

Neither source names ozone alongside a MERV 13 filter. That ozone is a gas the build does not remove is our own inference.

The build does not make ozone either. EPA’s Indoor Air Filtration fact sheet says mechanical filters, especially higher-efficiency ones, effectively trap smoke particles and do not produce ozone. It does not recommend electronic air cleaners for smoke removal, because it is unclear they remove smoke particles and they may produce ozone. It says the same of photocatalytic oxidation, hydroxyl generators and devices with ultraviolet light components. CARB certifies air cleaners sold in California to an ozone limit of 0.050 parts per million, but says certification does not reflect how effective a cleaner is. ASHRAE says electrically powered devices should be tested and labeled for ozone emission under UL 2998 or an equivalent, which a device assembled on the kitchen floor cannot satisfy.

It does not replace ventilation

EPA says portable air cleaners, DIY ones included, are only part of a comprehensive indoor air quality strategy and do not replace ventilation. In its cleaner air room routine, when air quality improves even temporarily, you air out the room. The other half is not adding particles: no smoking or vaping, candles, aerosols or vacuuming unless the vacuum has a HEPA filter. EPA’s pet fact sheet adds avoiding cooking, especially frying or broiling, and not using a fireplace or woodstove. For a pet household the vacuuming line matters most. A shut-in animal and a shedding coat is exactly when you want to vacuum.

It does not cool the room

EPA’s research page lists heat generation from the fan motor as a drawback of a DIY unit. It adds heat rather than removing it. A heat emergency with no air conditioning covers what published fan guidance says about cooling a body, including that four bodies publish four different temperature cutoffs and none was written about an animal.

Heat Outranks the Filter

Smoke season and heat season are the same season, and this build asks you to close a room.

EPA’s Indoor Air Filtration fact sheet says to use the air cleaner in a closed room “when it is safe to do so.” Its cleaner air room fact sheet keeps the exit open (close windows and doors, but do not make it hard to leave the room in an emergency) and tells you to stay cool with fans, window air conditioners or central air conditioning. The unit on this page is not one of those three things. It is a filter.

EPA also publishes an abandon condition: if you can’t stay cool at home, the power goes out, or too much smoke still gets in after you follow the steps, EPA says “staying in a cleaner air room may not be the best option for you.” It suggests staying with friends or family, going to a public cleaner air shelter, or finding another location with air conditioning and good air filtration. Its research page adds that you should keep an eye on temperature and have a way to stay cool, even if that means going somewhere else.

The UL safety testing was done at room temperatures near 20 °C. None of these sentences is written about an animal. EPA’s pet fact sheet is. Its opening paragraph says evidence in humans indicates smoke and heat together are much worse than smoke alone, and that this is also likely true for pets. It says animals that are older, younger or pregnant, or have heart or lung disease, are especially at risk from smoke or heat and should be closely watched. It tells owners to consult their veterinarian about managing pets during a smoke event or a combined smoke and heat event, and about making an evacuation plan. If your household holds such an animal, have that conversation before smoke season.

Our added step: the animals may not be in the room the household is judging. Put a thermometer in the room the animal is in, read it whenever you check the filter, and apply EPA’s reason for leaving a cleaner air room to that room. An animal behind a closed door cannot leave a room that has become too hot, and it is the one occupant whose discomfort nobody in the house will feel. Indoor temperature decay and species thresholds shows how to turn thermometer readings into a rate for your own building.

If leaving becomes the answer, settle beforehand whether the place you would go admits animals. That is set locally and is never safe to assume. Pets at cooling and warming centers covers how to ask by phone before you need it.

The Rest of the Household

Birds. EPA’s pet fact sheet says pet birds are particularly susceptible to smoke and should be kept inside, in a cleaner air room if possible. No source addresses a DIY air cleaner and a bird together. A bird’s respiratory system is not a mammal’s, so take the question to an avian veterinarian. Bird and parrot emergency preparedness covers the enclosure and transport side.

Cats and dogs. EPA’s instructions for pets as a group: keep them indoors with doors and windows closed if you can stay cool, keep outdoor bathroom breaks short, make water available at all times, and move dangerous products such as pesticides out of reach. That last one matters on a build day. Duct tape, blades or scissors, cardboard offcuts and packaging end up on the floor of the room the animal is shut in.

Short-nosed breeds. EPA says short-nosed dogs, such as pugs and bulldogs, can have trouble breathing if there is smoke or heat, and to use extra caution. Neither this nor the sentence about older, younger or pregnant animals says anything about a filter. Both are reasons to have the veterinary conversation before a smoke event.

Signs to call the veterinarian. EPA lists excessive coughing or gagging, trouble breathing, red or watery eyes, nasal discharge, excessive panting, fatigue or weakness, and reduced appetite or thirst. For cats it adds restlessness, searching for a cool spot, panting, drooling and sweaty paws. Call your veterinarian before you adjust the fan.

Reptiles, fish and small mammals. No source addresses an enclosure and a DIY air cleaner together. The exhaust is a directed airstream, and an enclosure depends on stable temperature and humidity, so do not aim the unit at a terrarium or tank. That is our caution, not a published rule. The temperature and humidity question belongs with the exotics practice that sees your animal.

Animals in more than one room. Deciding which room gets the filtration is a household decision. One room with incompatible animals covers partitioning when animals that cannot share a space end up sharing one.

The Build and Operate Checklist

• Try to buy a portable air cleaner first. EPA recommends the DIY build only when products of known performance are not available or affordable.
• Use a box fan made since 2012 carrying a UL or ETL mark. Look for the certifying organization’s mark on the label.
• If the only fan you have is older, follow the fallback both agencies publish: never leave it running unattended while you are away or sleeping.
• Buy MERV 13 filters, and buy spares at the same time.
• Align the arrows on the filter with the direction of airflow through the fan, and get a good seal between fan and filter.
• Add the cardboard shroud, cut the size of the fan blades. EPA measured a 40 percent increase in CADR at no cost.
• Add filters if you have floor space, or use a single thicker filter if you do not.
• If you build the five-filter design, use leg supports so air can flow through the bottom.
• Do not point the fan exhaust at the floor. The team that measured it found lower CADR and advises against it.
• Stand it where the animal cannot lean on it, sleep against it or topple it, with the exhaust away from any bed, crate, cage, tank or terrarium.
• Do not run it on an extension cord.
• Run it continuously on the highest fan setting, with doors and windows closed when it is safe to do so.
• Look at the filter every day during a smoke event. Change it when it is dark brown or grey or smells like smoke.
• Do not add particles to the room: no candles, aerosols, frying or broiling, woodstove, or vacuuming without a HEPA vacuum.
• Check that working smoke detectors are installed throughout the home.
• Watch the temperature, not just the air quality. EPA’s trigger to leave a cleaner air room includes not being able to stay cool and the power going out.
• Put a thermometer in the animal’s room and read it whenever you check the filter.
• Phone ahead to confirm whether any cooling center or cleaner air shelter you might use admits animals.
• Treat the warning signs above as a reason to call your veterinarian, not a reason to adjust the fan.
• If any animal is older, younger, pregnant, or has heart or lung disease, talk to your veterinarian about managing it during smoke and about an evacuation plan.

Do It on a Quiet Afternoon

Buy the fan now and check the label in the shop. The UL or ETL mark and the manufacture date are on the fan, not on a listing. Checking in a lit aisle in October beats checking by phone light during an evacuation warning.

Buy filters, then more filters. EPA says filters may need changing every few weeks or days in a smoke event and that a dirty filter makes the unit almost completely ineffective. The spares are not an accessory. They are the build.

Decide the room now, with the animals in mind. Work out where the crate goes, where the cord runs, where the unit stands so nothing can lean on it, and whether the door stays shut with an animal behind it. Then run the unit for an hour on a clear day with the animals in the room and see what they do about the noise while it costs you nothing to learn.

Where to Go Next

This page is the build-and-operate spoke of the pet emergency playbooks hub.

• What AQI is safe to walk my dog: the veterinary band table, and the page that decides whether you need this one.
• Wildfire smoke and pets: the commercial purifier comparison and room-size selector.
• Indoor enrichment for smoke days: gear that tires a dog without raising its breathing rate.
• Dust storms, haboobs and pets: the differences when the hazard is dust rather than smoke.
• Pets and power outages and a heat emergency with no air conditioning: when the power is out or the heat is the real problem.

Then do the one thing that costs nothing today: look at the box fan you already own and find out whether it carries a UL or ETL mark and when it was made. If it does not, you have learned that on a clear afternoon with time to fix it.