Michigan Medicine DIY Box Fan Filter Cuts Air Particles 90% in Test

Michigan Medicine

Taping a MERV 13 furnace filter to an ordinary box fan eliminates 90% of fine airborne particles, delivering effective allergy relief at a fraction of the cost of an $800 commercial HEPA unit.

AI-assisted summary of Build a Do-It-Yourself Air Purifier for About $40 — original source. Check the original for full context.

Source published: 2011-06-27

Key ideas

A do-it-yourself air purifier constructed from a 20-inch box fan and a MERV 13 furnace filter reduces 0.3-micron airborne particulate matter by approximately 90%, cutting particle counts from 400,000 to 54,000 per cubic foot. (1:42)
While an $800 commercial HEPA unit achieves near-total particulate elimination down to zero particles, the DIY design offers an accessible, budget-friendly option for around $25 to $40. (0:00)
Proper DIY assembly requires aligning the airflow arrows on the furnace filter with the fan's exhaust and taping the perimeter to ensure an airtight seal across the fan housing. (1:42)
Health specialists recommend prioritizing purifier placement in bedrooms due to the extensive hours spent sleeping, with additional inexpensive units deployable in living areas as needed. (1:42)

Detailed notes

## Background Published on YouTube by Michigan Medicine on June 27, 2011, this educational video features Dr. Jeffrey E. Terrell demonstrating how to construct an affordable do-it-yourself air purifier. The video description explains that building an air cleaner using standard hardware store parts can protect against wildfire smoke, dust, and viruses, providing a cost-effective indoor filtration method when local Air Quality Index ratings rise.

## TL;DR Dr. Jeffrey E. Terrell conducts a side-by-side performance demonstration comparing a premium $800 commercial HEPA air purifier against an inexpensive homemade alternative. Using an optical particle counter, he reveals that while the commercial unit completely eliminates 0.3-micron particles from an ambient baseline of 400,000 per cubic foot down to zero, a DIY rig built from a 20-inch box fan and a MERV 13 furnace filter achieves an impressive 90% reduction down to 54,000 particles. He details how to properly attach and align the filter with the fan's airflow, highlighting the assembly as an accessible, high-impact air cleaning solution for bedrooms and high-use living areas.

## Atmosphere & style The presentation is practical, grounded, and methodically clinical, evoking the feel of an approachable science demonstration directly from a medical professional's workshop. Dr. Terrell delivers straightforward, unpretentious explanations while the audible cadence of the particle counter cycling every six seconds creates an engaging, empirical sense of discovery. Rather than pushing expensive medical hardware, the video feels empowering and accessible, validating a simple household hack with hard, real-time data.

## Bulletin - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) A do-it-yourself air purifier constructed from a 20-inch box fan and a MERV 13 furnace filter reduces 0.3-micron airborne particulate matter by approximately 90%, cutting particle counts from 400,000 to 54,000 per cubic foot. - [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) While an $800 commercial HEPA unit achieves near-total particulate elimination down to zero particles, the DIY design offers an accessible, budget-friendly option for around $25 to $40. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) Proper DIY assembly requires aligning the airflow arrows on the furnace filter with the fan's exhaust and taping the perimeter to ensure an airtight seal across the fan housing. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) Health specialists recommend prioritizing purifier placement in bedrooms due to the extensive hours spent sleeping, with additional inexpensive units deployable in living areas as needed.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) Airborne indoor allergens are a primary trigger for allergy flare-ups, but mechanical air filtration substantially reduces their concentration. - [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) Commercial room air cleaners range widely in price, from entry-level units around $100 up to high-end models costing $800 or more. - [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) Testing ambient indoor room air with an optical particle counter measured baseline concentrations of approximately 400,000 fine particles (0.3 microns) per cubic foot. - [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) A premium $800 commercial HEPA filter demonstrated the ability to scrub fine particulate counts down from 400,000 completely to zero. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) A functional homemade air purifier can be assembled using an inexpensive 20-inch box fan and a standard 20x20-inch furnace filter. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) Using a high-efficiency filter rating such as MERV 13 is critical because it is specifically rated to trap microscopic 0.3-micron particles. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) The filter must be placed against the fan's face matching the directional airflow arrows stamped on the filter frame and secured with tape along all edges. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) In direct testing, the DIY filter system reduced fine particle counts from 400,000 to 54,000 particles per cubic foot, representing roughly a 90% filtration efficiency. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) The total hardware cost for the DIY solution is approximately $25 to $40, making room-scale air cleaning financially accessible to households on a budget. - [01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) Placing air filters in bedrooms provides the greatest health benefit due to prolonged overnight exposure, though multiple DIY units can be distributed across a home.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=kH5APw_SLUU&t=0s) Commercial Air Cleaners and Particle Counter Testing

Airborne allergens inside a home are a primary driver of allergy symptoms, but their concentrations can be substantially curtailed through mechanical filtration. Domestic air-cleaning hardware spans a broad spectrum of form factors and price points, ranging from in-line furnace filters mounted on a central air handler's blower unit to standalone portable systems designed for single rooms. Commercially manufactured room units typically run from $100 up to $800 for top-tier models. Dr. Jeffrey E. Terrell introduces a side-by-side performance comparison to show how an expensive, high-end commercial device measures up against an inexpensive homemade alternative built from off-the-shelf components.

To demonstrate commercial-grade performance, Dr. Terrell tests a high-end $800 unit loaned by Ann Arbor Vacuum. He pairs it with a specialized optical particle counter configured to measure microscopic particulate matter measuring 0.3 microns in diameter, which takes an air sample once every six seconds. Before filtration, the ambient indoor air measures approximately 400,000 particles per cubic foot. When the commercial HEPA purifier is engaged, the particle count drops rapidly over subsequent sampling cycles, bottoming out at zero. Dr. Terrell observes: “This is an incredibly effective, high-efficiency particulate air filter, or HEPA filter.” He underscores that by completely scrubbing the air from 400,000 particles down to zero, such systems eliminate the particulate triggers that irritate airways and cause allergy flares.

- **Baseline particle count**: Approximately 400,000 particles (0.3 microns in size) per cubic foot of ambient room air. - **Instrument sampling rate**: One particle count recorded every six seconds. - **Commercial unit cost & performance**: An $800 commercial system (loaned from Ann Arbor Vacuum) achieving essentially 0 particles per cubic foot.

---

[01:42](https://www.youtube.com/watch?v=kH5APw_SLUU&t=102s) Building and Testing the DIY Box Fan Filter

Recognizing that premium commercial air purifiers remain out of financial reach for many households, Dr. Terrell demonstrates how to assemble a functional room filter using ordinary items sourced from a standard hardware store. As Dr. Terrell points out: “But not everyone can afford a 6- or 800-dollar HEPA system.” To bridge the gap, he presents an assembly built around a 20-inch box fan (costing approximately $12 to $15 at the time of recording) and a 20 by 20-inch pleated furnace filter. He highlights the critical role of filtration ratings, advising that consumers select a high-efficiency option like a MERV 13 filter, which is specifically rated to capture small, 0.3-micron particles.

```mermaid flowchart LR accTitle: DIY Air Purifier Assembly and Airflow accDescr: Flow diagram showing room air drawn into a 20-inch box fan and pushed through a taped MERV 13 furnace filter. subgraph Intake["Intake Stage"] A["Ambient Air<br/>(~400k particles/cu ft)"] --> B["20-Inch Box Fan<br/>($12–$15)"] end subgraph Filtration["Filtration Stage"] B -->|"Air blown outward"| C["MERV 13 Filter<br/>(20x20 in, taped to fan)"] C --> D["Cleaned Exhaust Air<br/>(~54k particles/cu ft / 90% drop)"] end ```

The construction process is straightforward: the furnace filter is placed directly against the front face of the box fan so that the fan blows air directly through the filter media. Dr. Terrell uses tape to seal and secure the edges of the filter to the fan housing, cautioning builders to check the printed arrows along the border of the filter frame to ensure the airflow direction matches the fan’s exhaust. When he switches on the fan and places the 0.3-micron particle counter in the exhaust stream, the count drops from an ambient baseline of 400,000 down to 54,000 particles per cubic foot across a few six-second cycles. This represents an approximate 90% reduction in fine particles. Dr. Terrell emphasizes: “So this $30 -- 25, $30 system is filtering 90% of the particles out of the air.” He recommends placing the unit in spaces like bedrooms, where occupants spend prolonged stretches of time sleeping, or setting up multiple units throughout the house for broader coverage.

- **DIY hardware components**: - One standard 20-inch box fan (approximately $12 to $15). - One 20x20-inch furnace filter rated at MERV 13 (or equivalent high-efficiency media). - Standard adhesive tape to seal the filter onto the fan casing. - Overall total assembly cost: approximately $25 to $30 (at original video production). - **DIY empirical performance**: Particulate count drops from ~400,000 to ~54,000 particles per cubic foot, yielding an approximate 90% reduction in 0.3-micron particulate matter. - **Assembly tip**: Observe the directional airflow arrows stamped on the cardboard perimeter of the furnace filter to make sure it aligns with the air blowing out from the front of the fan. - **Placement guidance**: Most beneficial in bedrooms due to the high volume of daily hours spent sleeping there, with additional units deployable in living areas as needed.

## Mentioned Jeffrey E. Terrell, Michigan Medicine, Michigan Sinus Center, Ann Arbor Vacuum, Environmental Protection Agency, Air Quality Index

## Video description **Current as of 2025: While the cost of materials may vary, the process and effectiveness of this option remain up-to-date.

You can build an air purifier with a HEPA filter for about $40 with parts from your local hardware store and protect yourself from dust, viruses, smoke from wildfires and other lung irritants.

The Environmental Protection Agency’s (EPA) Air Quality Index highlights the local air quality in your area. The higher the number, the worse the air pollution. When the AQI is over 100, using an air filter indoors, with HEPA filters or filters rated MERV 12 or higher, can help to clean the air.

Air filters can help protect against colds, flu, and COVID as well.

Jeffrey E. Terrell, former director of U-M Health’s Michigan Sinus Center, demonstrates how to build a DIY air purifier at home.

Read: "Wildfires: How to protect your lung health in poor air quality" and more news from Michigan Medicine’s Health Lab: https://www.michiganmedicine.org/health-lab/wildfires-how-protect-your-lung-health-poor-air-quality

*Note: When this video was made, the cost of materials was approximately $25. This has increased due to inflation.

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#AirQuality #AirPurifier #AirPollution #LungHealth

[Source](https://www.youtube.com/watch?v=kH5APw_SLUU)

中文简报

密歇根大学医疗中心测试：自制箱式风扇过滤器减少90%空气颗粒物将MERV 13级炉用滤网贴在普通箱式风扇上可清除90%的空气细微颗粒物，不仅成本仅为800美元商用HEPA净化器的一小部分，还能有效缓解过敏。 Jeffrey E. Terrell博士进行了一场性能对比演示，将一台价值800美元的高端商用HEPA空气净化器与廉价的自制替代方案进行了比较。借助光学粒子计数器，他展示了尽管商用设备能将每立方英尺40万个0.3微米颗粒物的环境基准彻底降至零，但由20英寸箱式风扇和MERV 13级炉用滤网组成的自制装置也能显著减少90%，降至54,000个颗粒物。他详细讲解了如何正确安装滤网并对准风扇气流方向，强调该装置是适用于卧室和高频使用生活区的一种易获取且高效的空气净化方案。由20英寸箱式风扇和MERV 13级炉用滤网组成的自制空气净化器可减少约90%的0.3微米空气颗粒物，使每立方英尺颗粒物数量从400,000个降至54,000个。 (1:42) 虽然800美元的商用HEPA设备几乎能完全清除颗粒物降至零，但成本约为25至40美元的自制方案提供了一种易于实现且经济实惠的选择。 (0:00) 正确的自制组装方法是将炉用滤网上的气流箭头与风扇排风方向对齐，并用胶带粘贴四周，确保风扇外壳处完全密封。 (1:42) 鉴于人们睡眠时间较长，健康专家建议优先将净化器置于卧室，并可根据需要在起居生活区域添置此类平价装置。 (1:42)

Reading summary

Dr. Jeffrey E. Terrell of Michigan Medicine demonstrates that while premium commercial HEPA air cleaners completely eliminate fine airborne particles, a budget-friendly alternative built from a box fan and a MERV 13 furnace filter captures approximately 90% of particulate triggers. Commercial HEPA Performance and Particle Baselines (0:00) Indoor airborne allergens act as primary triggers for allergy symptoms, though their presence can be curtailed through mechanical air filtration. Testing ambient room air with an optical particle counter calibrated for 0.3-micron particulate matter establishes an indoor baseline of approximately 400,000 particles per cubic foot sampled every six seconds. When Dr. Terrell tests a high-end $800 commercial unit loaned by Ann Arbor Vacuum, the device completely clears the air stream, reducing particle counts to zero. While commercial systems are exceptionally effective, standard standalone room cleaners typically retail between $100 and $800. These price points often present a financial barrier for households needing air filtration across multiple rooms or spaces. Assembling a MERV 13 DIY Alternative (1:42) To provide an accessible substitute for costly commercial systems, Dr. Terrell details a simple homemade design using hardware store materials totaling about $25 to $30 at the time of demonstration. The setup combines a standard 20-inch box fan (costing roughly $12 to $15) with a 20-by-20-inch pleated furnace filter secured to the casing with standard adhesive tape. Material selection and alignment are essential to performance. Dr. Terrell recommends a filter rated at MERV 13 to trap microscopic 0.3-micron particles. Builders must tape the filter directly against the front exhaust face of the fan while verifying that the directional airflow arrows printed on the cardboard perimeter align with the forward direction of the fan's airflow. Empirical Efficiency and In-Home Placement (1:42) When evaluated with the 0.3-micron particle counter, the DIY box fan assembly reduces the particulate count from the 400,000 baseline down to 54,000 particles per cubic foot over several six-second sampling intervals. This performance equates to roughly a 90% reduction in fine airborne irritants. Although it does not reach the zero-count mark of an $800 commercial HEPA unit, Dr. Terrell emphasizes that eliminating 90% of particulates provides substantial relief at a fraction of the cost. He advises deploying the homemade filter in bedrooms to protect individuals during extended hours of sleep, or assembling multiple units to provide cost-effective coverage throughout an entire home. The demonstration shows that a low-cost DIY box fan filter offers roughly 90% particle reduction compared to the complete particle clearance of an $800 commercial HEPA unit. The provided source material does not address potential long-term fan motor strain, noise differences, or durability over extended continuous use.

Original transcript

[0:00] [ Music ] [0:08] >> Airborne allergens can cause symptoms, and you can reduce the amount of airborne allergens [0:13] in your house by using filtration systems. [0:16] There's a whole array of filtration systems that can be used, from furnace filters that go [0:21] on the main furnace blower to systems that you can use in a room. [0:26] They range from $100 to $800 for the high-end systems. [0:33] And today I'd like to show you the high-end system, and then also show you a homemade system [0:38] that should work fairly well at home, for a fraction of the cost. [0:43] This high-end system that's been on loan to us from Ann Arbor Vacuum is an $800 system [0:49] that will filter virtually every particle out of the air. [0:53] This particle counter is measuring particles that are .3 microns in size, [0:57] and it samples every six seconds. [0:59] With a system like this, you can filter out the particles and therefore -- [1:04] and thereby reduce the allergy symptoms. [1:06] So you can see we started off, I believe it was about 400,000 particles. [1:10] And with this counting system, we can see that it goes down to essentially zero [1:18] after a few seconds, after a few measurements. [1:20] And now you can see we're 400, and down to zero. [1:24] This is an incredibly effective, high-efficiency particulate air filter, or HEPA filter. [1:30] And you can see that it'll scrub the air from about 400,000 particles per cubic foot [1:35] down to zero particles per cubic foot. [1:39] You've seen the high-end filter system now and what it can do. [1:42] It's incredibly powerful. [1:43] But not everyone can afford a 6- or 800-dollar HEPA system. [1:47] So there is something that you can do to filter your air, in a bedroom, for instance. [1:52] And this is something that you can put together with pieces from your hardware store. [1:56] And what I've got here is about a 12- to 15-dollar, 20-inch box fan; [2:01] and a HEPA filter that you can buy for a furnace. [2:05] So, this is a 20 by 20-inch HEPA filter. [2:08] And it's important that you recognize that it comes in different levels of filtration. [2:13] So, a MERV 13 filter is a very highly-rated filter that'll get a lot of very small -- [2:18] those 0.3 micron -- particles out of the air. [2:21] So this can simply be done by putting the filter in front of the box fan, [2:25] so the air is blowing through the filter. [2:28] And I'll just tape this filter here. [2:31] You can see there's a direction for the air flow marked on the side of the filter. [2:34] You'll see, if you look at yours carefully. [2:37] And now I'll take the same counter and count the particles coming through the system. [2:42] So, our ambient count, again, is about 400,000. [2:47] And I'll turn the fan on and count the particles. [2:51] And we'll see how they drop. [2:53] Typically, this MERV filter will cause about a 90% reduction [2:56] in those small .3-micron particles. [3:01] It measures every six seconds. [3:03] And you can see I've gone down from 400,000 to 54,000. [3:09] It'll probably hover around that point. [3:11] So this $30 -- 25, $30 system is filtering 90% of the particles out of the air. [3:19] You'll probably want to put something like this in a bedroom, for instance, where you sleep [3:23] and spend a good part of your day, to filter those particles out of the air. [3:27] If you want, you can put them in other areas of the house, as well. [3:30] [ Music ]