How To Build Your Own Air Filter At Home

How To Build Your Own Air Filter At Home
Photo credit ByoungJoo/Getty Images

It is becoming more clear that proper air filtration has an important role to play in slowing the spread of COVID-19.

Scientists are increasingly emphasizing that the novel coronavirus can spread through tiny particles in the air known as aerosols, which can linger in rooms for hours or even days. The good news is that HEPA air filters can actually filter and remove these tiny particles from the air. 

On KCBS Radio’s "Ask An Expert" segment early Friday, indoor air expert Dr. Richard Corsi explained that portable air filters can do a good job of removing aerosols from the air and reduce risk, and there is a way for people to create their own HEPA filters at home.

"If you can afford it, a good off-the-shelf portable air cleaner with a known, tested CADR is better," said Dr. Corsi, Dean of the Maseeh College of Engineering & Computer Science at Portland State University.

A good filter is one with a high CADR (Clean Air Delivery Rate) that has been tested by the Association of Home Appliance Manufacturers, paired with filters that have a MERV (Minimum Efficiency Reporting Value) rating of 13 or higher. For a typical classroom size, you would want a CADR of 300 square feet per minute or better.

But portable air filters can cost hundreds of dollars, which may put them out of reach for many people, or business owners or school administrators who need one air filter for every room. 

DIY-ers will sometimes construct their own HEPA filters by simply attaching an air filter to the front of a box fan, but Dr. Corsi says that might not be as effective as it seems.

"Box fans oftentimes on high flow generate huge amounts of flow, so if you try to attach that filter directly to the box fan, the pressure pushing up against the filter will cause it to separate from the fan," he explained. "So around the edges of the filter you’ll get what’s called bypass, so a lot of the air doesn’t actually go through the filter."

1/ @BoilingPb's point below is correct. A better design is a filter designed to attach onto one side of a six-sided non-flexible box that is perhaps 300-400 L or so in volume. Assuming it sits on the ground, that leaves 4 sides for filters. https://t.co/0dc1EZZTX3

— Dr. Richard Corsi (@CorsIAQ) August 7, 2020

To solve for this issue, Dr. Corsi suggested using a rigid material such as plywood to construct a box with an open side.

Attach the fan securely to the opening.

"Usually I would say the exhaust side, but it could be the intake side as well...so that the fan is blowing out and sucking through the filter," Dr. Corsi said.

Then, cut out space in the three remaining sides and the top of the box to attach filters rated MERV 13 or higher. 

"That will significantly reduce the back pressure on the fan, which can hurt the fan but also be more effective at removing particles from air," he said. 

Experts contend air filters can help to lower the concentration of virus particles in the air and therefore reduce the risk of transmitting the coronavirus