Professor of Environmental Engineering and Mortenson Professor of Sustainability, University of Colorado Boulder

Karl Linden suggested that many companies promote the use of ultraviolet light for disinfection. As a professor at the University of Colorado at Boulder, he received funding from federal agencies and industry to conduct research. He is affiliated with the International Ultraviolet Association.

The University of Colorado Boulder provides funding as a member of the American Dialogue.

Scientists have long known that ultraviolet rays can kill pathogens on surfaces, air and water. Ultraviolet robots are used to disinfect empty wards, buses and trains; ultraviolet light bulbs in HVAC systems can eliminate pathogens in the air of buildings; ultraviolet lights can kill bugs in drinking water.

Perhaps you have seen UV sticks, UV LEDs and UV air purifiers being promoted as silver bullets to prevent coronavirus. Although decades of research have focused on the ability of ultraviolet rays to kill many pathogens, there is no standard for ultraviolet disinfection products for coronaviruses. These products may kill SARS-CoV-2, the virus that causes COVID-19, but they may not.

I am an environmental engineer and UV disinfection expert. In May 2021, my colleagues and I began to accurately test various UV systems to see which system is the most effective at killing or inactivating SARS-CoV-2.

Light is classified by wavelength (the distance between the peaks of light) and is measured in nanometers. Ultraviolet has a wavelength range of 100 to 400 nanometers—wavelengths are shorter than the violet color in visible light—and are invisible to the human eye. As the wavelength decreases, photons contain more energy.

Different wavelengths of ultraviolet light are more effective than other wavelengths in inactivating viruses, depending on the degree to which the wavelength is absorbed by the virus's DNA or RNA. When ultraviolet light is absorbed, photons transfer their energy to the chemical bonds of genetic material and destroy it. Then the virus cannot replicate or cause infection. The researchers also showed that the proteins that viruses use to attach to host cells and cause infections—such as the spike protein on coronaviruses—are also susceptible to ultraviolet light.

The dose of light is also important. The intensity of light may vary-bright light is more intense and contains more energy than dim light. Exposure to strong light for a short period of time will produce the same UV dose as exposure to dark light for a long time. You need to know the correct dose that can kill coronavirus particles at each ultraviolet wavelength.

Traditional UV systems use wavelengths of 254 nanometers or approximately 254 nanometers. At these wavelengths, even at low doses, light is dangerous to human skin and eyes. Sunlight includes ultraviolet rays near these wavelengths; anyone who has been severely sunburned knows how dangerous ultraviolet rays are.

However, recent studies have shown that at certain ultraviolet wavelengths (especially below 230 nanometers), high-energy photons are absorbed by the top layer of dead skin cells, instead of penetrating into active skin layers where damage may occur. Similarly, the tear layer around the eyes also blocks these bactericidal ultraviolet rays.

This means that people can move around more freely under ultraviolet light with a wavelength of less than 230 nanometers, while disinfecting the surrounding air in real time.

My colleagues and I tested five commonly used ultraviolet wavelengths to see which wavelength is the most effective at inactivating SARS-CoV-2. Specifically, we tested the dose required to kill 90% to 99.9% of the virus particles present.

We conducted these tests at the University of Arizona's Biosafety Level 3 facility, which is designed to deal with deadly pathogens. There, we tested a variety of UV spectrum lamps, including UV LEDs emitting light at 270 and 282 nm, traditional UV tube lamps at 254 nm, and a new technology called excitation dimer or excimer, 222 nm UV light source .

To test each device, we added millions of SARS-CoV-2 viruses to the water samples and coated a thin layer of this mixture on a petri dish. Then we irradiate ultraviolet rays on the petri dish until a specific dose is reached. Finally, we checked the virus particles to see if they can still infect cultured human cells. If the virus can infect cells, the dose is not high enough. If the virus does not cause infection, then the dose of UV source has successfully killed the pathogen. We carefully repeated this process for a series of UV doses using five different UV equipment.

Although all the wavelengths we tested can inactivate SARS-CoV-2 at very low doses, the system that emits ultraviolet light at a wavelength of 222 nanometers requires the lowest dose. In our experiments, less than 2 millijoules of energy per square centimeter are required to kill 99.9% of virus particles. This means that it takes about 20 seconds to disinfect the space that receives low-intensity short-wavelength ultraviolet light, which is similar to the one used in our test.

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These 222 nm systems are almost twice as efficient as traditional UV tube lamps, which are commonly used in UV disinfection systems. But the important thing is that the winning lamp also happens to be the safest for humans. At the same UV intensity required to kill 99.9% of SARS-CoV-2 in 20 seconds, a person can safely be exposed to 222 nanometer light for up to 1 hour and 20 minutes.

This means that widely used UV lamps can be used to safely reduce the level of coronavirus in the presence of people.

Many places or organizations—from the U.S. Air Force to the Space Needle in Seattle to Boeing—are already using or studying ways to use ultraviolet light in the 222 nanometer range to protect public health.

I believe our findings are important because they quantify the exact dose required to achieve different levels of SARS-CoV-2 control, whether it is to kill 90% or 99.9% of the virus particles.

Imagine that coffee shops, grocery stores, school classrooms, restaurants, and concert venues are now safe with this technology. This is not just a solution for SARS-CoV-2. These technologies can help protect human health in public places in future crises and relatively normal periods by reducing the exposure to daily virus and bacterial threats.

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