These UV devices could keep indoor air free of viruses

Far UV is a new type of germicidal UV (GUV) irradiation – a well-established disinfection technology – utilized to combat the transmission of the SARS-CoV-2 virus and other pathogens that can spread effortlessly in enclosed spaces.

The piano bar in Boston where Edward Nardell performs cabaret songs would usually be an ideal environment for the transmission of airborne diseases. However, Nardell and his audience are shielded from the COVID-19 pandemic by far-ultraviolet (UV) lights that he had mounted on the ceiling.

Nardell is a physician and a researcher specializing in airborne infections at the Harvard T.H. Chan School of Public Health in Boston, Massachusetts. He claims that ensuring indoor air safety commences with ventilation, but it cannot be concluded there. Although ventilation systems that replace the air in a room are in place, they are often inadequate to provide full protection against easily transmitted diseases like coronaviruses.

Room systems that make a concerted effort to purify indoor air, like the ones that utilize high-efficiency particulate air (HEPA) filters, are more effective in removing dangerous particles. Still, they present some drawbacks:

  • Expensive to install and run
  • Frequently emit noise
  • Have restricted coverage – multiple units may be required to provide comprehensive room coverage.

“This is where air disinfection using UV technology becomes relevant,” explains Donald Milton, an environmental health researcher at the University of Maryland School of Public Health in College Park.

According to Milton, with GUV light, one can attain remarkably high levels of air disinfection with little air circulation. He goes on to explain that, with cutting-edge technology, it is possible to use a GUV in the entire room without worrying about air movement since safer wavelengths are now available. GUV can operate covertly in packed areas where diseases can easily spread, such as schools, hospitals and restaurants.

Gunning for germs

Conventional GUV systems use mercury vapor lamps to produce light by passing an electrical current through vaporized mercury (similar to conventional fluorescent bulbs). The lamps emit radiation in the UVC band, with a wavelength of around 254 nanometres. The Earth’s atmosphere filters out UV radiation; thus, life on Earth has not evolved to endure it. The radiation causes photochemical damage by disrupting the nucleic acids, rendering pathogenic viruses and bacteria inert, although not necessarily eliminating them.

The lamps are widely used to:

  • Disinfect water
  • Clean fruits and vegetables
  • Sanitize surfaces in spaces – such as operating rooms

However, due to the harmful effects of this wavelength on human eyes and skin, the light emitted from these systems is kept away from people. Nonetheless, this does not preclude its use in public spaces. Upper-room GUV, a clever strategy devised decades ago, positions the lamps at high levels in a room, harnessing upward air currents to effectively inactivate pathogens far from people.

According to William Bahnfleth, an architectural engineer at Pennsylvania State University in University Park who specializes in indoor air quality, the approach is effective. In a room, air naturally moves upwards from individuals, equipment, and existing ventilation, flows through the radiation zone of the lamps, and subsequently recirculates back down into the occupied area.

While there are no universally accepted and enforced standards for indoor air quality, the typical targets are expressed in terms of how often the amount of air in a room is exchanged per hour. For instance, examination rooms in US hospitals are recommended to have six air changes per hour. This is challenging for ventilation systems and usually requires a lot of energy. On the other hand, upper-room GUV systems can easily reach the equivalent of two or three times those levels of air exchange for disinfection purposes while consuming much less energy than a ventilation system. “It’s mostly impossible for anything other than a hospital or special facility to have six air changes,” says Nardell. “GUV is the only method that gives you this incredibly high number of equivalent air changes because you can disinfect such a large volume of air at once.”

Shelly Miller, a mechanical engineer, and indoor air quality specialist at the University of Colorado Boulder conducted an unpublished study on various combinations of ventilation, filtration, UV, and mask-wearing in different types of buildings. According to Miller, UV was the only technology that routinely achieved acceptable risk levels, including in settings such as offices, hotels, and schools. Miller considers UV to be a highly effective but underutilized tool for air purification.

Riding shorter waves

Following studies led by William Wells, a biologist at the University of Pennsylvania in Philadelphia, upper-room GUV gained widespread adoption in schools and hospitals in the late 1930s and 1940s. Wells and his colleagues demonstrated that the technique significantly decreased the transmission of measles in schools located in suburban Philadelphia. While upper-room GUV is still used in many tuberculosis wards, its use has decreased due to the availability of more effective interventions such as vaccines.

While upper-room GUV using conventional UVC light is effective, it has a fundamental limitation of needing to be kept away from people. The air is only cleaned when it rises to the top of the room and passes by the GUV light, leaving a chance for pathogens to move to a new host. This limitation can potentially be overcome by using shorter wavelengths.

According to David Brenner, a physicist specializing in radiological research at Columbia University in New York City, wavelengths below 254 nm are not as effective at penetrating tissue (this is why far-UV light, which has a wavelength of 222 nm, is attractive). It cannot penetrate beyond the layer of dead cells on the skin or the film of tears on the eye’s surface. However, because bacteria and viruses are much smaller than those layers, far-UV radiation could potentially destroy the pathogens without causing harm to the skin and eyes. Brenner and his colleagues tested this hypothesis using lamps containing krypton chloride gas, which releases UVC radiation primarily in the 222 nm range under electrical excitation.

Initially designed to enhance disinfection in operating rooms, the researchers at Columbia University realized that far-UV radiation could lower the transmission of airborne viruses. In a study conducted in 2018, they demonstrated that over 95% of influenza viruses present in the air were neutralized when they passed through a low-power far-UV lamp. The group had previously demonstrated that exposure to low doses of 222 nm radiation had minimal effects on cells in a 3D human skin model and mice. Furthermore, other researchers found no indications of eye damage in rats exposed to 222 nm radiation.

When COVID-19 emerged, the Columbia scientists conducted similar experiments on strains of coronavirus similar to SARS-CoV-2, again with good results. To expand their investigations, they teamed up with scientists in the United Kingdom, including a team at Leeds University that had access to a large test chamber specifically designed to confine pathogens.

The room-size experiments used Staphylococcus aureus bacteria suspended in the air. According to Ewan Eadie – a medical physicist at the University of Dundee, UK, and the lead author of a paper that outlines the team’s findings – this microorganism is relatively easy to analyse and is expected to be more robust against UV radiation than coronaviruses. He claims that they had no idea of what was going to come out at the end of the experiment.

According to Brenner, the results were impressive, with a rapid reduction in the level of pathogens in the room. He adds that their equivalent air changes per hour were well over 100, which is a significant improvement.

Brenner and his team have already confirmed that exposure to far-UV radiation did not cause skin cancer in hairless mice during a 66-week-long experiment, according to a report they published in May 2022. The goal of their future research is to assess the risks to the eyes and understand the mechanisms by which 222 nm radiation destroys pathogens.

Despite the positive results of laboratory experiments on far-UV disinfection, there are concerns about the technology’s effectiveness in busy indoor spaces. Eadie notes that the laboratories where the experiments are conducted have very clean conditions, and he believes that real-world data is needed to support the efficacy of far-UV in such settings.

A clinical trial is currently being conducted in nursing homes in Nova Scotia, Canada, to investigate the use of far-UV light in reducing the spread of airborne diseases. The study aims to track the incidence of COVID-19 and other respiratory viral infections among 200 residents. Half of them are using common areas equipped with far-UV lamps, while the other half have placebo lights that look identical but do not emit far-UV light. The study started in October 2021, and the results are expected by the end of 2023.

Nardell has repurposed an airborne-infection research facility in Emalahleni, South Africa, to study COVID-19. The facility was initially designed to investigate tuberculosis infection and features a three-bed ward, from which the air is transferred to exposure rooms housing animals that are susceptible to the disease being studied – in this case, hamsters. Nardell explains that hamsters are the preferred experimental animal for COVID. The facility will assess the effectiveness of far-UV radiation against upper-room GUV systems by observing the hamsters for signs of illness.

Far-UV lamp fixtures have already been launched in the market and are being installed globally, not only in buildings but also in infection-prone areas such as buses. However, companies have not waited for peer-reviewed research to release these products. Although some devices are marketed for home use, Brenner warns consumers to be cautious, as using the wrong wavelengths can cause damage.

The cost of the fixtures varies widely, but a ballpark retail price for a lamp installed by specialists is around US$2,000, with an expected lifetime of about 15 months if run continuously. While there is hope that LED-based far-UV lamps will eventually provide cheaper and longer-lived alternatives to gas lamps, current prototype LED far-UV lamps to have impractically low power levels, says Eadie.

For now, Nardell mentions that the far-UV lamps installed in the piano bar where he performs provide approximately 35 air exchanges per hour, making it one of the safest places on the planet for singing. When Brenner and his team were invited to the bar, they attended a cabaret evening without wearing masks, relying on the protection provided by the invisible light emitted by the lamps. Brenner admits that he was anxious about the experience and took several COVID tests during the following week, but ultimately he remained unaffected.


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