In this article, we discuss the basics of air cleaning and filtration, how to build your own portable air cleaner, and some performance enhancement considerations.
There are three main approaches for improving indoor air quality: (i) reducing sources of indoor air pollution, (ii) dilution ventilation, and (iii) air cleaning. Air cleaners are generally used to reduce contaminant concentrations in buildings. Different technologies are employed to target different types of indoor air contaminants, such as media filtration for removing particles, sorbents for reducing the indoor concentration of organic and inorganic gas-phase pollutants, and ultraviolet germicidal irradiation (UVGI) for inactivating bio-aerosols and microorganisms. Therefore, the types of air contaminants which an air cleaner can mitigate depends on the technologies that it uses.
Air cleaners relying on media filtration are commercially available and commonly used for reducing the concentration of airborne particulate matter in residential, commercial, and industrial environments. Particulate matter is known to cause a range of health issues, including irregular heartbeat, decreased lung function, exacerbation of asthma symptoms, coughing, and decreased cognitive function. Studies that examined particle filtration interventions reported improvements in health outcomes from the use of portable air cleaners. Even for those without underlying health conditions, portable air cleaners can be used daily to reduce adverse respiratory health effects by lowering the indoor concentrations of particulate matter that result from regular activities. For example, cooking generates large amounts of airborne particles, which is especially problematic if one does not have or operate a directly-exhausting range hood. A portable air cleaner can be an effective tool to lower particle concentrations and improve health outcomes while still allowing occupants to engage in regular activities. Moreover, residential heating, ventilation, and air conditioning (HVAC) systems typically run for only a small portion of the day — one study found a median runtime of 18%. Consequently, even if your building has an HVAC system with filtration, you will only benefit from improved indoor air quality during the limited time when the system is actively operating. A portable air cleaner allows you to decouple from the central HVAC system and provide targeted, on-demand air cleaning in the most regularly occupied spaces. Beyond everyday use, portable air cleaners can help reduce the transmission of infectious diseases, such as COVID-19. They also offer essential protection from extreme weather events such as wildfires, which increasingly threaten indoor air quality and public health. In this article we’ll discuss the basics of air cleaners and teach you how to make your own portable air cleaner.
What is CADR?
Portable air cleaner performance is commonly reported as a Clean Air Delivery Rate (CADR) in cubic feet per minute (cfm). The CADR is the product of 1) the fraction of airborne particles removed by the filter as air passes through it one time, which is called the “single-pass filtration efficiency,” and 2) how quickly air passes through the filter, which is called the “airflow rate.” This means that the CADR can be improved with higher efficiency filters and/or a higher airflow rate. A higher CADR means more particles can be removed from the air, or in other words, a higher CADR yields a better air cleaning effectiveness. When purchasing a commercially available/off-the-shelf portable air cleaner, the rated CADRs can be compared across units. If you’re building your own do-it-yourself (DIY) portable air cleaner, there are some steps that you can take to increase your air cleaner’s CADR.
Steps to improve the CADR of a DIY Air Cleaner
You can begin to improve your DIY air cleaner’s CADR by purchasing the right supplies for building it:
- Select a fan with a high airflow rate. The fan’s rated airflow is usually based on the its highest speed, so note that airflow rates will be lower in practice if the fan is run on a lower speed setting.
- Select high efficiency filters. Look for MERV 13 or higher.
About Air Filter Ratings
Three systems are typically used for rating the removal efficiency of air filters: the Minimum Efficiency Reporting Value (MERV, ranging from 1 to 16), Micro-particle Performance Rating (MPR, ranging from 100 to 2800), and Filter Performance Rating (FPR, ranging from 1 to 10). A higher rating indicates better performance in removing aerosols from the air. It is important to note that air filter removal efficiencies vary across particle sizes — some air filters might do well at removing larger particles, but much worse at removing smaller particles. High-efficiency particulate air (HEPA) filters have the best removal performance among air filters by removing more than 99% of all particle sizes.
If you’re interested in learning more about air filter rating systems, you can find additional information here.
Increase the filtration area by using more filters or deeper filters.
The filters you select and how they are attached to the fan impact how much air is actually moved through the filter area, which affects the CADR. Filters resist airflow and induce a back pressure on the fan, which means the fan must work harder to move air through the filters. This typically results in a reduced airflow rate. In general, filters with higher particle removal efficiencies tend to have greater resistances to airflow (often described as a “higher pressure drop across the filter”). Since it is best to select the highest filter efficiency possible, additional steps can be taken to overcome the associated airflow reductions. Increasing the filter area is one way to help: when the force of the air is spread out over a larger area, the pressure decreases, which ultimately improves the airflow rate and CADR. Using more filters or a deeper filter to build your air cleaner will increase the filter area and improve the CADR.
Deeper filters: You may have noticed that the filter media is pleated, not flat. The filter area we typically refer to is the entire pleated area and not just the outer dimensions (e.g., 20” x 20”). If you were to stretch out the filter material so until it’s flat (not pleated and folded up in the filter frame), it would be larger than the frame that it is contained in. Therefore, a deeper/thicker filter compared to the standard 1” option provides a greater filter area, even if the outer filter dimensions between the thinner and thicker option are the same (e.g., a 20” x 20” filter that is 1” deep has less filter area than a 20” x 20” filter that is 2” deep). Consider a thicker/deeper filter if the brand you’ve selected offers that option. Note that there can be variation across filter brands and products (e.g., one brand’s thinner filter might be superior to another brand’s thicker filter). Using a deeper filter is particularly straightforward if you choose to build the Parallel air cleaner option that is described in the “Do-It-Yourself Instructions” section below.
More filters: In the “Do-It-Yourself Instructions” section below, we present three portable air cleaners that use different numbers of air filters.
Air Filter Performance
You might be wondering just how much airflow resistance and filter area will impact the performance of your unit. Without testing each unit, it is hard to say for certain, since there will be variation across filter types, brands, and DIY air cleaner configurations and construction. Nonetheless, one investigation found that installing a 1” MERV 13 filter to a box fan reduced the measured air speed by close to 60% compared to fan operation without a filter. When a single MERV 13 filter of greater depth was used, the air speed improved: a single 2” MERV 13 filter and a single 4” MERV 13 filter resulted in ~50% and ~40% reductions in measured air speed compared to the fan operation without a filter attached, respectively. Beyond filter depth, increasing the filter area by incorporating five filters resulted in a much lower reduction in air speed of ~25%. Adding air filters can improve airflow: as shown in the figure below, using two air filters in a prism configuration yields faster airflow than using one air filter in parallel configuration. Overall, increasing filter area, either through increasing filter depth or increasing the number of filters, should improve the airflow rate through the filters and thus improve the resulting CADR.
Do-It-Yourself Instructions
Now we’ll walk you through three DIY options that can cost less and be as or more effective than some commercially available air purifiers. The DIY options are also helpful during events when the availability of off-the-shelf air cleaners is limited, like the COVID-19 pandemic or the 2023 forest fires in Maui, Hawaii. All three DIY options involve some variation on attaching one or more pleated media air filters to a commercially available box fan. The DIY air cleaners can be used in a variety of indoor spaces, such as schools, offices, or personal residences, and provide targeted airborne particle removal. Note that since media filtration is the only technology implemented in these DIY air cleaners, they only remove airborne particles (including bio-aerosols, which can contain viruses). These devices will not remove gaseous pollutants and they will not inactivate microorganisms.
Across the three DIY options, there is a trade-off between (1) air cleaning effectiveness (i.e., CADR) and (2) the resources (e.g., time, money, and materials) required to build the unit. As shown below, options with a higher CADR require more effort and resources to build.
Once you’ve decided on which of the three DIY air cleaners you intend to build, you can follow the instructions below. We’ve included all the steps within each DIY air cleaner’s instructions, so don’t worry about reading all three sections if you already know which cleaner you’re going to build. If you’re unsure, you can always start with the Parallel option and then purchase more filters and upgrade to one of the higher CADR options at a later time. If you are interested in further increasing your unit’s performance, see the “Extra Considerations” section that follows the DIY instructions.
Option 1: Parallel
The simplest of all three DIY air cleaners is the Parallel air cleaner. It requires the fewest materials and is the easiest to build — it basically involves taping a filter to the back of a box fan. However, it has the lowest CADR of all three options, since it has the lowest filter area and thus the lowest airflow rate too.
Materials
- One box fan
- Duct tape and a liquid adhesive (e.g., super glue)
- One 20″ x 20″ particle filter (or whichever filter size best matches the size of your box fan)
Look for a filter with a high MERV rating (ideally MERV 13 at a minimum and as deep as possible) and a box fan with a high flow rate (look for something like the “Maximum Air Flow Capacity (CFM)” on the package or webpage) to build the most effective air cleaner possible.
Assembly
- Check the side of the filter for an arrow that indicates airflow direction and line the filter up at the back of the fan accordingly. The filter arrow should be pointing towards/into the back of the fan.
- Tape the filter to the fan. Aim for a tight and secure fit with no gaps in order to ensure that the air flows through the filter instead of around it.
- If there’s a bit of a gap between the filter and the ground, try adjusting the height with something kicking around the house (e.g., an empty tape roll, a jar lid, etc.) as you would with a napkin under a wobbly table at a restaurant.
- Use additional tape and glue as needed to ensure the filter is securely attached.
Operation
You’re all set! Turn the fan on and locate it in the center of a frequently used room or close to someone who is especially vulnerable, at breathing height. Ensure there isn’t anything blocking the fan’s airflow path and try to run it at as high a speed as possible, noise permitting. Remember, you’re only cleaning the air when the fan is running!
Option 2: Prism
The Prism will take more time and resources to build than the Parallel unit, but it should also offer a higher effectiveness since it incorporates a second air filter and therefore has a higher filter area.
Materials
- One box fan
- Duct tape and liquid adhesive (e.g., super glue)
- Two 20” x 20” particle filters (or whichever filter sizes best match the size of your box fan)
- Some cardboard
- (Optional) A protractor and ruler
Look for filters with a high MERV rating (ideally MERV 13 at a minimum and as deep as possible) and a box fan with a high flow rate (look for something like “Maximum Air Flow Capacity (CFM)” on the package or webpage) to build the most effective air cleaner possible.
Assembly
- The filters will serve as the prism’s sides and two cardboard triangles will be the top and bottom. To make the triangles, you can use exact measurements if you have a protractor and ruler handy, or you can lay your cardboard down flat and place the box fan and two filters in the prism configuration (shown in the diagram) and trace around them on the cardboard. To do this, place one filter on each edge of the back of the fan, and angle them together to form a point away from the fan. (If you’re working alone and don’t have someone to help you hold everything in place, try taping the filters to the fan and taping them together where they meet.)
- Once you have the filters set up with the fan, trace around the outside of the filters and leave some extra cardboard at the base of the triangle (the side touching the box fan) to help with attaching to the fan later. Cut out your first triangle, following the tracing lines you just made. Use your first triangle as a guide to trace around and cut out your second triangle.
- Now it’s time to tape your two cardboard triangles to the filters as shown in the diagram. Before you tape, check the sides of the filters for an arrow that indicates airflow direction. When you form the prism shape with the filters, the airflow direction arrow should be pointing into the prism. When you tape, aim for a tight and secure fit with no gaps in order to ensure that the air flows through the filters instead of around them.
- Tape the prism to the box fan. Adjust the cardboard as needed — you can use the overlap to help securely tape the prism to the fan, or you can trim the cardboard if there is too much overlap and it gets in the way of a tight fit. If there’s a bit of a gap between the filter and the ground, try adjusting the height with something kicking around the house (e.g., an empty tape roll, a jar lid, etc.) as you would with a napkin under a wobbly table at a restaurant. Use additional tape and glue as needed to ensure the prism is securely attached.
Operation
You’re all set! Turn the fan on and locate it in the center of a frequently used room or close to someone who is especially vulnerable, at breathing height. Ensure there isn’t anything blocking the fan’s airflow path and try to run it at as high a speed as possible, noise permitting. Remember, you’re only cleaning the air when the fan is running!
Option 3: Cube (Corsi Box)
The final DIY option is the Cube, or the “Corsi box.” The Cube requires the most effort to build, but it should also have the highest CADR since it incorporates five air filters and thus has the largest filter area!
Materials
- One box fan
- Duct tape and a liquid adhesive (e.g., super glue)
- Five particle filters: one 20” x 20” filter and four 20” x 16” filters (or whichever filter sizes best match the size of your box fan).
Look for filters with a high MERV rating (ideally MERV 13 a minimum and as deep as possible) and a box fan with a high flow rate (look for something like “Maximum Air Flow Capacity (CFM)” on the package or webpage) to build the most effective air cleaner possible.
Assembly
- To assemble the cube, the 20” x 20” filter will be the back face and the four 16” x 20” filters will make a box/frame that goes in between the box fan and the 20” x 20” filter, as shown in the diagram above.
- Before you tape any filters together, check the sides of each filter for an arrow that indicates airflow direction. The airflow arrow of each filter should be pointing into the cube.
- Tape the filters together to form the filter frame (step 1 in the above figure) and then attach the back face filter (20” x 20”) to form the open-faced cube. When you tape, aim for a tight and secure fit with no gaps in order to ensure that the air flows through the filters instead of around them.
- Slide the open face of the cube up to the back of the box fan and securely fasten it to the box fan using tape. If there’s a bit of a gap between the filter and the ground, try adjusting the height with something kicking around the house (e.g., an empty tape roll, a jar lid, etc.) as you would with a napkin under a wobbly table at a restaurant.
- Use additional tape and glue as needed to ensure the items are securely attached.
Operation
You’re all set! Turn the fan on and locate it in the center of a frequently used room or close to someone who is especially vulnerable, at breathing height. Ensure there isn’t anything blocking the fan’s airflow path and try to run it at as high a speed as possible, noise permitting. Remember, you’re only cleaning the air when the fan is running.
Extra Considerations
If you’re interested in improving the operation and CADR of your DIY air cleaner even further, there are a few more steps you can take.
Maintenance
Like filters in a central HVAC system, filters in an air cleaner also require maintenance and replacement. The filter you purchased should have replacement guidance on the package or manufacturer’s website. It would also be wise to keep an eye on the unit to see how “loaded” the filters are (i.e., how much dust has accumulated on the filters). A heavily loaded/soiled filter can negatively impact performance and should be replaced. It is good practice to use personal protective equipment, such as gloves and a mask, and to ensure the device is off when replacing the filters. Keep in mind that wildfires impact filter performance, so make sure to replace filters after wildfire events.
Acoustics
Regardless of which DIY air cleaner option you build, particles are only being removed from the air when the fan is on. Therefore, it’s important to understand how considerations of comfort, such as fan noise, could impact how often the air cleaner is actually used. The fan will typically be louder when operated at higher speeds, so it is also a good idea to review information about the fan’s noise (look for a “decibel (sound) rating” on the fan specifications). This is particularly important if the air cleaner will be used in a space where distractions could be especially problematic, or if individuals are highly sensitive to noise. Fortunately, sound produced by the DIY air cleaners can be comparable to or even quieter than some commercially available units. Note that like CADR, the noise produced by DIY units depends on the materials used and construction methods. Please refer to the images below and follow these links to see how the noise produced by Parallel and Cube DIY air cleaners compares to commercially available units.
Airflow
To further improve airflow through the filter area:
- Consider using weather-stripping in between connections (e.g., filter-filter, filter-cardboard, and/or filter-fan) before taping, and adding zip-ties to ensure a tighter seal. This will help reduce pathways for air to bypass the filter and ensure air flows through those high efficiency filters instead of around them.
- Consider incorporating a “fan shroud” to improve the airflow rate. A fan shroud is basically a frame in front of the box fan which prevents backwards airflow around the fan’s corners, improving the effectiveness of your unit. You could incorporate a fan shroud either by using a fan with a built-in shroud or by making a DIY fan shroud.
We hope this guide has been helpful in providing you with an overview of portable air cleaner basics and assisting you in building your own DIY air cleaner. If you’re interested in reading more about any of the topics discussed, see the hyperlinks throughout this blog or refer to the resources below.
References
ASHRAE. (2017). ANSI/ASHRAE Standard 52.2-2017: Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. https://www.ashrae.org/File%20Library/Technical%20Resources/COVID-19/52_2_2017_COVID-19_20200401.pdf
Butz AM, Matsui EC, Breysse P, et al. (2011). A Randomized Trial of Air Cleaners and a Health Coach to Improve Indoor Air Quality for Inner-City Children With Asthma and Secondhand Smoke Exposure. Arch Pediatr Adolesc Med. 165(8):741–748. doi:10.1001/archpediatrics.2011.111
Chao J. (2013). Pollution in the Home: Kitchens Can Produce Hazardous Levels of Indoor Pollutants. https://newscenter.lbl.gov/2013/07/23/kitchens-can-produce-hazardous-levels-of-indoor-pollutants/
Davison G, Barkjohn KK, Hagler GSW, Holder AL, Coefield S, Noonan C and Hassett-Sipple B. (2021). Creating Clean Air Spaces During Wildland Fire Smoke Episodes: Web Summit Summary. Front. Public Health 9:508971. doi: 10.3389/fpubh.2021.508971
Fisk, W.J. (2013). Health benefits of particle filtration. Indoor Air, 23: 357-368. https://doi.org/10.1111/ina.12036
Gao X., Coull B., Lin X. et al. (2021). Short-term air pollution, cognitive performance and nonsteroidal anti-inflammatory drug use in the Veterans Affairs Normative Aging Study. Nat Aging 1, 430–437. https://doi.org/10.1038/s43587-021-00060-4
Grundig T., Köksal N., Sing A. (2021). Is an $800 purifier best to clean your home’s air? Marketplace tested5 top brands and their claims. https://www.cbc.ca/news/business/portable-air-purifier-tests-marketplace-1.5900782
Harriman L., Stephens B., Brennan, T. (2019). New Guidance for Residential Air Cleaners. ASHRAE 61 (9): 14–23. www.ashrae.org.
Militello-Hourigan R.E., Miller S.L. (2018). The impacts of cooking and an assessment of indoor air quality in Colorado passive and tightly constructed homes.
Building and Environment. 144: 573-582. https://doi.org/10.1016/j.buildenv.2018.08.044
Ray K. (2019). Filters in Our Lives, Decreasing Our Exposure to Wildland Smoke. https://www.epa.gov/sites/production/files/2019-07/documents/ray_filters_in_our_lives_decreasing_our_exposure_ray_tagged.pdf.
Rosenthal J. (2020). A Variation on the ‘Box Fan with MERV 13 Filter’ Air Cleaner. https://www.texairfilters.com/a-variation-on-the-box-fan-with-merv-13-filter-air-cleaner/.
Rosenthal J. (2020). Comparing Filters for Resistance and Efficiency. https://www.texairfilters.com/comparing-filters-for-resistance-and-efficiency/
Sacks JD., Wichers Stanek L., Luben TJ., Johns DO., Buckley BJ., Brown JS., Ross M. Particulate Matter-Induced Health Effects: Who Is Susceptible? Environmental Health Perspectives. 119(4): 446-454. doi:10.1289/ehp.1002255
Salimifard P., Jones E., Allen J. (2020). Portable Air Cleaners: Selection and Application Considerations for COVID-19 Risk Reduction. https://schools.forhealth.org/wp-content/uploads/sites/19/2020/08/Harvard-Healthy-Buildings-Program-Portable-Air-Cleaners.pdf#page=4.
Snel, P. (2020). Air filtration a COVID-19: Indoor air quality expert explains how to keep you and your building safe. https://civmin.utoronto.ca/air-filtration-and-covid-19-indoor-air-quality-expert-explains-how-to-keep-you-and-your-building-safe/
Siegel JA. (2016). Primary and secondary consequences of indoor air cleaners. Indoor Air. 26(1):88-96. doi: 10.1111/ina.12194. Epub 2015 Mar 14. PMID: 25689321.
Stephens B., Novoselac A., Siegel JA. (2010). The Effects of Filtration on Pressure Drop and Energy Consumption in Residential HVAC Systems (RP-1299). HVAC&R Research, 16:3, 273-294, DOI: 10.1080/10789669.2010.10390905
Talheim T. (2020). $70 DIY Purifier Effectiveness Better than $1,000 IQAir. Smart Air. https://smartairfilters.com/en/blog/diy-purifier-effectivness/
Talheim T. (2013). Measuring Air Purifier Noise. Smart Air. https://smartairfilters.com/en/blog/china-english-noise/
Touchie, MF, Siegel, JA. (2018) Residential HVAC runtime from smart thermostats: characterization, comparison, and impacts. Indoor Air. 28: 905– 915. https://doi.org/10.1111/ina.12496
US. Environmental Protection Agency. Health and Environmental Effects of Particulate Matter (PM). https://www.epa.gov/pm-pollution/health-and-environmental-effects-particulate-matter-pm.
U.S. Environmental Protection Agency. (2018). Residential Air Cleaners – A Technical Summary. https://www.epa.gov/sites/production/files/2018-07/documents/residential_air_cleaners_- _a_technical_summary_3rd_edition.pdf.
Zhang J., Huntley D., Fox A., Cerhardt B., Vatine A., Cherne J. (2020). Study of Viral Filtration Performance of Residential HVAC Filters. https://www.ashrae.org/file%20library/technical%20resources/covid-19/zhang_digital-first.pdf