Physicist, Non-Profits Build and Test Air Purifiers That Remove COVID Particles

With simple, inexpensive supplies available at any general department or home improvement store, Hugh Churchill is building and testing portable air filters that help remove infectious airborne particles — including the respiratory droplets that carry coronavirus — from indoor spaces.

And he wants to show you how to build one yourself. All you need is duct tape, a basic box fan and commercially available air filters.

“While masks and vaccines are polarizing topics,” said Churchill, associate professor of physics, “there shouldn’t be anything controversial about clean air. These devices facilitate that. They provide an additional layer of protection that could be widely deployed to make our K-12 and university indoor spaces healthier during this wave of the pandemic. And they’re easy and inexpensive to build. My nine-year-old built one.”

Churchill, a member of the Arkansas Research Alliance Academy of Scholars and Fellows, studies condensed matter and quantum materials to develop or improve devices that help people and the environment. For example, researchers in his lab focus on spiraling chains of selenium and tellurium, two materials that, when used in nanowires, show promise in the next generation of digital technology, solar energy and quantum computing.

“Improving filtration and ventilation in classrooms is a common recommendation to help fight the spread of Covid-19 and improve indoor air quality,” Churchill said. “There are commercial products that do this, but they can be cost-prohibitive. Our teachers and school districts have worked extremely hard and used many tools to keep our kids’ classrooms safe. This is one more tool to help them.”

PURIFYING AIR – HOW IT WORKS

Within the facilities and HVAC communities, there is a burgeoning movement nationwide to make indoor spaces as safe as possible. Relying on open-source and simple do-it-yourself designs by air-quality experts, citizen scientists and amateurs have joined this movement, fashioning their own homemade air purifiers.

Churchill brings a higher level of scientific credibility to the fabrication and testing of an essentially homemade box-fan filter. Part of his work with nanoscale electronic devices focuses on eliminating air particles, which can ruin devices. And, for 15 years, Churchill has worked in so-called “clean rooms” with extremely well-filtered air.

Churchill measures the success of the purifier with a particle counter, which helps researchers in his lab evaluate different types of filters. They also have special microscopes that enable them to see microscopic holes in filter material, holes that particles can pass through. For example, the filter they tested from one popular manufacturer had many such holes. Consequently, its particle removal efficiency was measured to be much lower than expected.

Churchill’s purifier is an augmented version of the “Corsi-Rosenthal cube,” a concept and design by Richard Corsi at Portland State University. Corsi is an internationally recognized expert on indoor air quality. Four or five filters with a minimum efficiency reporting value — known as “MERV” in the HVAC industry — of 13 and a basic fan form the sides of the cube. The corners and seams are sealed and held together with duct tape. Using multiple filters means clean air delivery rate is higher and the cube will last longer, possibly six months.

With help from HVAC expert Ben Doudna in Facilities Management, Churchill measured the particle filtration efficiency and air flow of the filters. These cubes can deliver an estimated clean-air delivery rate of about 470 cubic feet per minute, which means one cube could produce four air changes per hour for a 900-square-foot classroom with 8-foot ceilings. This is within the range of 4 to 6 air changes per hour recommended to reduce airborne transmission.

CORONAVIRUS AND THE SCIENCE BEHIND AIR FILTERS

Vaccinations and wearing a mask are the surest ways to prevent the spread of coronavirus, but the Centers for Disease Control and Prevention also recommends portable air filters as a method to remove infectious airborne particles from indoor spaces. This is especially important for higher risk settings such as health clinics, vaccination and medical testing locations, and schools, where virtually half the population cannot yet receive vaccinations.

The particle size of SARS-CoV-2, the virus that causes COVID-19, is about 0.1 micrometer, or one-tenth of one micrometer. To get an idea how small this is, one micrometer is 100 times smaller than the thickness of a sheet of paper. However, when viral particles are exhaled during talking, singing, breathing and coughing, they are trapped in larger particles, some that quickly drop out of the air and others that remain suspended in room air for minutes to hours.

According to the CDC, high efficiency particulate air (HEPA) filters are 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2. Portable filtration units that combine a HEPA filter with a powered fan system do not bring in outdoor air for dilution, but they can clean air within spaces to reduce the concentration of airborne particulates.

Commercial HEPA-based fans cost about $250. Churchill said their purifier would cost less than $100 for a box fan, four filters and duct tape. A single-filter design suitable for smaller rooms can also be made for half the price. He emphasized that recently the U.S. Department of Education clarified that costs for air-quality improvements for elementary, secondary and higher education classrooms can be covered by the American Rescue Plan.

With 99.97% efficiency at capturing human-generated viral particles associated with SARS-CoV-2, efficiency particulate air filters can help keep classrooms safe.

“The purifier tested by Hugh provides a cost-effective way, supported by data, to immediately address one of the most pressing air quality needs schools face, that of addressing COVID-19,” said Stan Green, owner of Clear Energy, an energy performance contractor that helps schools with HVAC and energy efficiency projects. “Funding is always a factor in addressing the many aspects of indoor air quality, but this innovative approach lets schools generate immediate reductions in the presence of SARS-CoV-2 particles at a very low cost. Poorly ventilated spaces will benefit tremendously, but even well-ventilated spaces will see an improvement.”

COLLABORATION WITH ARKANSAS NON-PROFITS

Churchill is an Arkansas Research Alliance Academy Fellow. The ARA Academy represents a community of 32 strategic research leaders across five universities and the state’s only national lab. Douglas Hutchings, director of the ARA Academy, is working with Churchill to develop web-based resources that explain the science behind the purifier and provide instructions for making them.

These resources are available at CleanARAir, where Churchill and Hutchings also provide links to shopping carts on Walmart.com and Amazon, so teachers and parents can purchase parts and kits. The site provides data on filter efficiency.

Churchill and Hutchings have developed educational resources related to the design and function of the purifier. Also available at CleanARAir, these resources include lesson plans in math, science and English/language arts, ranging from basic literacy to advanced calculus. For example, for the math and science components of the project, there are several opportunities to measure flow rates, particle counts and proportional relationships. Students can also experiment with a wide range of designs. Curriculum kits enable students to develop hypotheses and rapidly test them to compare modeled data versus measurement data.

“The DIY approach creates inquiry-based learning opportunities and enables young minds to engage with their environment,” said Hutchings.

Innovate Arkansas, a state-sponsored initiative to help technology entrepreneurs create viable companies, has provided support in developing the supply chain necessary to deploy thousands of kits. The Northwest Arkansas Council is facilitating discussions with local business and philanthropic leaders to provide wholesale prices and get kits to teachers quickly.” Finally, Startup Junkie, a non-profit organization that offers consulting services to entrepreneurs, has helped the researchers with a system to accept tax-deductible donations to help get purifiers in the hands of teachers and schools.