This UV Light Wavelength is Harmless to Humans, But Inactivates SARS-CoV-2

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Karl Linden recently said something about SARS-CoV-2 that we haven’t heard since the pandemic started almost two years ago.

“Of almost every pathogen we have ever studied, this virus is one of the easiest, by far, to kill,” said the CU Boulder professor of environmental engineering, “with UV light.”

The UV light caveat is an important distinction here, of course. Still, Linden’s sentiment, which follows his new study published in Applied and Environmental Microbiology, could very well open doors to virus mitigation that the public has yet to experience.

For the study, Linden and his team compared 5 different UV wavelengths from emitted from a variety of sources. Specifically, the researchers completed a side-by-side analysis of an:

  • unfiltered krypton chloride excimer lamp emitting primarily at 222 nm with a small peak at 258 nm
  • a filtered krypton chloride excimer lamp with a 220 nm bandpass filter pre-installed
  • a conventional low-pressure mercury lamp emitting at 254 nm
  • one benchtop UV LED system with a peak emission wavelength of 270 nm
  • one benchtop UV LED system with a peak emission wavelength of 282 nm

“We thought, let's come together and make a definitive statement on what UV exposure is required to kill off SARS-CoV-2,” said Linden. “We wanted to make sure that if UV light is being used to control disease, you're delivering the right dose that's protective of human health and human skin, but also going to be killing off these pathogens.”

The researchers found that SARS-CoV-2 is actually remarkably susceptible to all UV light. While that was good news in and of itself, the team still wanted to find that “sweet spot”—the wavelength at which UV light can kill SARS-CoV-2 but remain practically harmless to humans.

They found it at 222 nm irradiation, which has been found in previous literature to be safe for human exposure beyond that needed to inactivate SARS-CoV-2.

“Created by what’s known as a krypton chloride excimer lamp, fueled by molecules moving between different states of energy, this wavelength is very high energy,” the researchers explain. “Therefore, it’s able to inflict greater viral protein and nucleic acid damage to the virus compared to other UVC devices, as well as be blocked by the very top layers of human skin and eyes—meaning it has limited to no detrimental health effects at doses that are capable of killing off viruses.”

Additionally, Linden and his team discovered that the inactivation range of SARS-CoV-2 across wavelengths is similar to those demonstrated for murine hepatitis virus (MHV), suggesting MHV can serve as a reliable surrogate for SARS-CoV-2. This is particularly important given that MHV has a lower biosafety level requirement (BSL-2) than SARS-CoV-2 (BSL-3), allowing more scientists the opportunity to research the inner workings of the deadly virus.

UV light is already used in hospitals, bathrooms and airplanes as a disinfectant; however, this is done when no humans are occupying the public spaces.

If this research continues to be successful, an effective and safe wavelength of UV light would enable a system for reducing viral spread in crowded public places, like concert venues.

Linden said he imagines systems that could either cycle on and off in indoor spaces to routinely clean the air and surfaces, or create an ongoing, invisible barrier between people in spaces where social distancing is not possible.

UV light disinfection can even rival the positive effects of improved indoor ventilation by providing the equivalent protection of increased air changes per hour within a room. It’s also much less expensive to install UV lights than to upgrade an entire HVAC system.

“There is an opportunity here to save money and energy while protecting public health in the same way,” said Linden. “It’s really exciting.”

Photo: UV light being emitted by a krypton chloride excimer lamp, fueled by molecules moving between different states of energy. Credit: Linden Research Group/CU Boulder

 

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