Which air purification technologies can tackle COVID-19?

During the early weeks of the COVID-19 pandemic, it was common to see people on social media sharing strategies for disinfecting groceries to remove any possible trace of SARS-CoV-2. Over the past year, though, research has shown that surface transmission is a relatively unlikely route of infection, while transmission through the air poses a far bigger threat—not only via contaminated droplets sprayed out during a coughing fit but also from the fine mist of virus-bearing aerosols emitted when someone speaks or even breathes.

That shift in focus has spurred a booming market for air purifiers that promise to remove or neutralize SARS-CoV-2 particles. For example, the consumer electronics jamboree CES recently featured a bewildering array of high-tech air-scrubbing products. “There’s a phenomenal amount of hype related to air-cleaning products right now,” says Richard Shaughnessy, director of the Indoor Air Program at the University of Tulsa.

According to the US Environmental Protection Agency, simple preventive measures such as wearing masks, practicing social distancing, and opening windows to let a draft flow through are the best ways to avoid airborne transmission indoors. But the US Centers for Disease Control and Prevention (CDC) adds that “when used properly, air purifiers can help reduce airborne contaminants, including viruses, in a home or confined space.”

The underlying technologies in air purifiers broadly fall into four categories: filtration, ultraviolet disinfection, electrical ionization, and catalytic oxidation. Some of these technologies have been around for decades, but the big question is: Do they work against SARS-CoV-2? So far, some have solid data, while others need more study.

Something in the air

If you want to capture a virus, you first need to know how it gets around. The SARS-CoV-2 virus is about 60–140 nm wide, “but it’s rare that you have naked virus particles in the air,” Shaughnessy says. Instead, the virus hitches a ride on tiny specks of water and biological material.

Public health agencies have tended to base their advice on reducing transmission via virus-loaded droplets, which are typically larger than 100 µm and fall to the ground within about 2 m (Science 2020, DOI: 10.1126/science.abf0521). But a growing body of evidence suggests that much smaller aerosols can also cause infections, a route the World Health Organization classifies as “airborne spread.” This cloud can travel dozens of meters from the source and potentially remain suspended in the air for hours. Although there is still debate among researchers about the relative importance of droplets and aerosols, “we have to pay attention to that airborne fraction,” Shaughnessy says.

Filtration is the most common approach used to capture airborne SARS-CoV-2, and it generally gets the thumbs up from scientists and regulatory agencies. Many devices rely on the high-efficiency particulate air (HEPA) filter, which traces its origins back to the gas masks of World War II. Back then, the US Army Chemical Corps asked Nobel Prize–winning chemist Irving Langmuir to study how the masks’ asbestos filters worked—and how they could be improved to guard against particle inhalation (Appl. Biosaf. 1998, DOI: 10.1177/109135059800300111).

Langmuir and other researchers found that the mesh of fibers in these filters trapped particles through several different mechanisms. Particles smaller than 100 nm are buffeted around by gas molecules until they contact a fiber, where they get trapped by van der Waals forces. Meanwhile, larger particles may be captured by van der Waals or electrostatic forces as air carries them over a fiber, but they can also embed themselves in a fiber, like bullets in a cinder block.

Langmuir found that filters using these three capture mechanisms were least effective on particles about 300 nm wide and were more efficient at removing smaller and larger particles. As a result, manufacturers now assess the effectiveness of a HEPA filter on 300 nm particles—a filter should remove at least 99.97% of them from the air. HEPA filters were commercialized in the 1950s, and today they are typically made from glass or polymer fibers, each roughly 0.5–2 µm wide, which are formed into a pleated sheet and held in a frame.

HEPA filters have long been used in hospitals, and a good track record of peer-reviewed clinical evidence shows that they can reduce viral infections (Environ. Sci. Technol. 2020, DOI: 10.1021/acs.est.0c03247). “The consensus is that HEPA filtration is the best technique” to remove viruses from the air, says John Holecek, a senior scientist at nanoComposix, a nanomaterials consultancy and contract research company based in San Diego. For the past 6 years, the New York Times has contracted Holecek to carry out tests on air purifiers for its Wirecutter product-review website.

Many heating, ventilating, and air-conditioning (HVAC) systems are not designed to push air through the tight weave of a HEPA filter, and instead use filters that are classified by their minimum efficiency reporting value (MERV), a scale from 1 to 16 that rates their ability to capture particles between 300 nm and 10 µm. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) says that filters rated MERV 13 or higher are good enough to capture airborne viruses. This is partly because the accumulation of trapped particles closes pores in the filter and improves its performance. “A slightly dirty filter is actually more efficient than a brand-new, clean filter,” says Keith Watkins, assistant director of facilities for the City School District of New Rochelle in New York and past president of the National School Plant Management Association.

A bit of the old ultraviolet

Ultraviolet light also crops up in many commercial air purifiers, with good reason. “We’ve known that UV kills microbes since the early 20th century,” says David J. Brenner, director of the Center for Radiological Research at Columbia University Irving Medical Center, who says it’s common for operating rooms in hospitals to be sterilized with 254 nm light, part of the ultraviolet C (UVC) band.

UVC typically deactivates viruses by triggering a chemical reaction that fuses adjacent thymine bases in DNA or uracil bases in RNA. These dimers jam the molecular machinery responsible for replication, effectively killing the microbe, and researchers have shown that UVC is highly effective at killing SARS-CoV-2 (ACS Photonics 2020, DOI: 10.1021/acsphotonics.0c01245).

The downside is that direct exposure to 254 nm light at similar doses can also be damaging to humans because of the same dimerization process. So UVC must be used judiciously. ASHRAE says that the lights can be installed inside HVAC ducts, or mounted in occupied spaces as long as they are at least 2.1 m above the floor and shine upward. Modeling studies have suggested that this could be an effective way to reduce COVID-19 transmission in poorly ventilated rooms (PeerJ 2020, DOI: 10.7717/peerj.10196), and laboratory research found that the light can completely decontaminate SARS-CoV-2 from surfaces in a few seconds (Sci. Rep. 2020, DOI: 10.1038/s41598-020-79600-8).

In July, Big Ass Fans in Lexington, ­Kentucky, started selling a $500 UVC up light add-on for its $1,244 Haiku ceiling fan. In the US, a lot of the guidance on air purification methods in response to COVID-19 has focused on HVAC systems, says Alex Risen, public relations manager for the company. “But a lot of our customers with manufacturing facilities, they don’t have HVAC—what are they supposed to do?”

The system has been tested by Innovative Bioanalysis, a laboratory services company based in Cypress, California, which found that the light could reduce the amount of SARS-CoV-2 in 1 m³ of air by 99.999% in 10–20 min. Risen says that customers include Toyota, Carnegie Mellon University, and the US National Guard.

The CDC supports the use of germicidal UVC against SARS-CoV-2, but industry organizations are concerned that some devices risk exposing people to the light. A joint statement from the National Electrical Manufacturers Association and the American Lighting Association states: “The online retail market is growing rapidly with handheld and portable consumer oriented UVC germicidal devices, many of which do not employ proper containment or other equivalent means of protection.”

One solution could be found further along the spectrum. Far UVC light, typically at a wavelength of 222 nm, is very quickly absorbed by proteins and liquids. “That means far UVC light can’t penetrate the dead cell layer at the surface of the skin or the tear layer on the surface of the eye, so it can’t reach living cells,” Brenner says. A recent clinical trial found that humans exposed to a 2 min burst of 222 nm UVC at 500 mJ/cm²—more than 100 times the current regulatory exposure limit for 1 h—experienced no skin damage (PLOS One 2020, DOI: 10.1371/journal.pone.0235948).

Viruses within tiny aerosol droplets, though, are highly vulnerable to far UVC. Last year, Brenner and his colleagues found that 222 nm UV was just as good as 254 nm UV at killing two common types of coronavirus and calculated that far UVC light at the regulatory exposure limit for humans would kill 95% of exposed viruses in 11 min (Sci. Rep. 2020, DOI: 10.1038/s41598-020-67211-2). Brenner suggests that far UVC lamps could be mounted on the ceiling of occupied rooms and operated continuously to disinfect the air.

Ion questions

Some air purification systems rely on a process called bipolar ionization, in which high-voltage electrodes create a blizzard of positive and negative ions from molecules in air, such as oxygen and water. Some manufacturers claim that these ions attach to viral particles in the air and weigh them down until they fall to a surface; others claim that the ions react with the virus’s surface proteins, preventing it from infecting cells.

Shaughnessy points out that merely dragging SARS-CoV-2 out of the air is perhaps only a temporary solution. “The more we learn about COVID-19, the more we’re looking into the impact of resuspension of settled virus particles back into the air,” he says. Meanwhile, the CDC says that although bipolar ionization technology has been around for decades, it “has a less-documented track record in regards to cleaning/disinfecting large and fast volumes of moving air within HVAC systems.”

Still, the past year has produced an avalanche of white papers in support of bipolar ionization against SARS-CoV-2, often produced for manufacturers by contract research organizations or academic laboratories. The electronics company Sharp, for example, had its Plasmacluster air purification technology tested by researchers at Nagasaki University and Shimane University in Japan. The researchers claim that protons and superoxide anions (O₂^–) generated by the device reduced the concentration of aerosolized SARS-CoV-2 passing through a test chamber by 91% after 30 s of exposure.

For now, ASHRAE says that “convincing scientifically-rigorous, peer-reviewed studies do not currently exist on this emerging technology” and cautions that some systems may generate ozone, an irritant that can aggravate respiratory ailments and reduce pulmonary function. “Don’t let anyone tell you that a small amount of ozone is a good amount of ozone,” Shaughnessy says. “Any increase, even up to 10 ppb, has been shown to have an impact on morbidity.”

Call the doctor

Electricity isn’t the only way to produce sterilizing ions. Some systems use photocatalytic oxidation to produce ions and radicals, the same principle behind self-cleaning glass systems. When UV light shines on materials such as titanium dioxide, it generates negative electrons and positively charged species called holes that can then react with air to generate superoxide anions, along with hydroxyl (OH) and hydroperoxyl (OOH) radicals (Build. and Environ. 2015, DOI: 10.1016/j.buildenv.2015.01.033).

“This creates a highly oxidative localized environment in which the reactive oxygen species react with lipids, proteins, carbohydrates, and nucleic acids of viruses,” says Trupti Kotbagi, vice president of technology at ThruPore Technologies, a University of Alabama spinout based in New Castle, Delaware. “These reactions lead to a cascade of damaging events preventing a virus from functioning and effectively killing it.”

ThruPore was launched to commercialize a porous carbon support for heterogeneous catalysts, but last year it pivoted to focus on COVID-19. In September 2020, ThruPore received a $256,000 grant from the US National Science Foundation to develop an oxidation catalyst containing modified porous carbon seeded with nanoparticles of zinc oxide, a material widely used in sunblock creams. The company says that the catalyst is effective at killing aerosolized MS2, a bacteriophage virus used as a proxy for SARS-CoV-2, although the results from those experiments have not been published.

A host of other companies already sell stand-alone purification devices that use UV-activated titanium dioxide, but ThruPore says that its catalyst can be retrofitted to existing HVAC systems by spraying the catalyst powder onto a conventional non-HEPA filter. These treated filters are already being tested in occupied office buildings in Delaware. “The biggest benefit of our technology is that it lasts for the lifetime of the filter, at least 90 days,” says Franchessa Sayler, CEO of ThruPore, who hopes to win EPA approval for the technology, branded as Dr. Filter, later this year. “And there’s no other modifications that you have to make to the HVAC system itself. You simply spray the filter and install it as you normally would.”

Shaughnessy says that although the principle of the technology is sound, there is still a dearth of independent, peer-reviewed studies on the effectiveness of photooxidation catalysts against SARS-CoV-2 in real-world air purification systems.

All in one

Faced with a choice between these technologies, some companies have simply bundled them all into one device. Aura Air in Tel Aviv, Israel, combines a HEPA filter, an absorbent carbon filter, an antimicrobial copper mesh, a UVC light, and a bipolar ionizer in its air purifier. “They’re taking existing technologies and putting them together into a package,” says John Egan, founder of Red Barn Advisory, a health consultancy that supplies the $500 units in the US. Customers include New England Biolabs in Ipswich, Massachusetts, and the City of Cambridge, Massachusetts, which has recently installed 1,000 Aura Air units in its municipal buildings.

Some air purifiers, including those manufactured by Aura Air, continuously record operational data, providing companies with a vital audit trail of their air quality. That offers a rather different kind of protection to employers, who may fear being sued for not taking due care of employees’ safety during the pandemic.

Those concerns will only be heightened by news that New York State attorney general Letitia James filed a lawsuit against Amazon on Feb. 16, claiming that it showed a “flagrant disregard for health and safety requirements” and that this lapse left its workers exposed to higher risk of COVID-19 infection at two of its New York warehouses. State labor law requires employers to provide “reasonable and adequate protection” for their employees’ health, but the lawsuit alleges numerous infractions—including a lack of proper ventilation. Amazon denies the claim. Air purification industry sources speaking off the record to C&EN say that more lawsuits, targeting other employers, are on the way.

Despite the bold claims and potential benefits of these technologies, Shaughnessy says that an air purifier on the other side of a room will do relatively little to reduce the risk from a maskless, infected person sitting right next to you. His advice is simple: keep your distance, wear a mask, and avoid crowded, stuffy indoor spaces. And if you do need to be inside, Holecek adds, “opening a window is probably the best solution.”

Mark Peplow is a freelance writer based in the UK.