Determining which wavelengths of UV light work best for COVID-19 virus disinfection
To disinfect a surface, you can illuminate it with a blast of ultraviolet (UV) light, which is bluer than the human eye can see. But to specifically inactivate SARS-CoV-2, the virus that causes COVID-19, which wavelengths are best? And how much radiation is enough?
To answer those questions, scientists must overcome two major obstacles. First, they must completely separate the virus from foreign substances in the environment. Second, they must illuminate the virus with a single wavelength of UV light at a time, with minimal changes in the experimental setup between tests.
A recent collaboration between the National Institute of Standards and Technology (NIST) and the National Biodefense Analysis and Countermeasures Center (NBACC), a laboratory of the U.S. Department of Homeland Security Science and Technology Directorate, overcame both obstacles and completed what is arguably the most thorough ever test of how different UV and visible wavelengths affect SARS-CoV-2.
In a new article published this week in Applied Optics, the collaborators describe their new system for projecting a single wavelength of light at a time onto a sample of the COVID-19 virus in a secure lab. Classified as Biosafety Level 3 (BSL-3), the lab is designed to study microbes that are potentially lethal if inhaled. Their experiment has tested more wavelengths of UV and visible light so far than any other research involving the virus that causes COVID-19.
So, what is the kryptonite of SARS-CoV-2? It turns out there’s nothing out of the ordinary: The virus is susceptible to the same wavelengths of UV light as other viruses, such as those that cause the flu. The most effective wavelengths were those in the “UVC” range between 222 and 280 nanometers (nm). UVC light (full range from 200 to 280 nm) is shorter than the UVB wavelengths (280 to 315 nm) that cause sunburn.
Researchers also showed that the environment of the virus can have a protective effect on the virus. In the study, a smaller UV dose was needed to inactivate viruses when placed in pure water than when placed in simulated saliva, which contains salts, proteins and other substances found in real human saliva. Suspending the virus in simulated saliva creates a situation similar to real-world scenarios involving sneezing and coughing. This may make the findings more directly informative than those of previous studies.
“I think one of the great contributions of this study is that we’ve been able to show that the kind of idealized results we see in most studies don’t always predict what happens when a more realistic scenario is involved,” says Michael Schuit. from NBACC. “Having material like the simulated saliva around the virus can reduce the effectiveness of UV disinfection methods.”
UV disinfection device manufacturers and regulators can use these results to determine how long to irradiate surfaces in medical environments, aircraft or even liquids to achieve inactivation of the SARS-CoV-2 virus.
“Right now there is a lot of pressure to get UVC disinfection into the commercial atmosphere,” said NIST researcher Cameron Miller. “In the long term, this study will hopefully lead to standards and other methods for measuring the UV dose needed to inactivate SARS-CoV-2 and other harmful viruses.”
This project built on previous work the NIST team did with another collaborator to inactivate microorganisms in water.
Shed a little light
Depending on the wavelength, UV light damages pathogens in different ways. Some wavelengths can damage the RNA or DNA of microbes, rendering them unable to replicate. Other wavelengths can break down proteins and destroy the virus itself.
Although people have known about the disinfection capabilities of UV light for over a hundred years, the past decade has seen an explosion in research into UV disinfection. One reason is that traditional sources of UV light sometimes contain toxic substances such as mercury. Recently, the use of non-toxic LED lamps as a UV light source has alleviated some of these concerns.
For this study, the NIST staff collaborated with biologists from NBACC, whose research informs biodefense planning about biological threats such as anthrax and the Ebola virus.
“What NBACC could do was grow the virus, concentrate it and remove everything else,” Miller said. “We were trying to get a clear message about how much light we need to inactivate just the SARS-CoV-2 virus.”
In the study, the team tested the virus in different suspensions. In addition to using the saliva mimic, scientists also put the virus in water to see what happened in a “pure” environment, with no components to shield it. They tested their virus suspensions both as liquids and as dried droplets on steel surfaces, which represented something that an infected person might sneeze or cough.
NIST’s job was to focus a laser’s UV light on the samples. They were looking for the dose needed to kill 90% of the virus.
With this setup, the collaboration was able to measure how the virus responded at 16 different wavelengths, ranging from the very low end of the UVC, 222 nm, all the way to the middle part of the visible wavelength range, at 488 nm. Researchers have included the longer wavelengths because blue light has been shown to have disinfecting properties.
Not a piece of cake
Getting the laser light on the samples in a secure lab was not trivial. Researchers in a BSL-3 lab wear scrubs and hoods with gas masks. When you leave the lab, you will have to take a long shower before you change into civilian clothes.
Equipment like the team’s expensive laser would have to undergo a significantly more strenuous sterilization procedure.
“It’s kind of a one-way door,” Miller said. “Everything that comes out of that lab has to be either incinerated or autoclaved [heat-sterilized], or chemically disinfected with hydrogen peroxide vapor. So taking our $120,000 laser wasn’t the option we wanted to use.”
Instead, the NIST researchers designed a system where the laser and some of the optics were located in a hallway outside the lab. They routed the light through a 4-meter-long fiber optic cable that passed through a seal under a lab door. Negative pressure allowed air from the hallway to flow into the lab and prevent anything from leaking back out.
The laser produced a single wavelength at a time and was fully tunable, allowing researchers to produce any wavelength they wanted. But because light bends at different angles depending on the wavelength, they had to create a prism system that changed the angle at which the light entered the fiber so that it was properly aligned. Changing the exit angle involved manually turning a knob they made to adjust the position of a prism. They tried to make it all as simple as possible, with a minimum number of moving parts.
“The device the NIST team devised allowed us to quickly test a very wide range of different wavelengths, all on very controlled and precise wavebands,” Schuit said. “If we were trying to do the same number of wavelengths without that system, we would have to juggle a lot of different types of devices, each of which would have produced wavebands of different widths. They would have required different configurations, and there would have been a lot of extra variables in the have been mixed.”
To manipulate the light, mirrors and lenses were needed, but the researchers designed them to use as little as possible, because each of them results in a loss of intensity for UV light.
For the materials that had to go into the lab to project the light from the fiber onto the samples of the COVID virus, the team tried to use inexpensive parts. “We’ve 3D printed a lot of things,” said NIST physicist Steve Grantham, a key member of the team along with NIST’s Thomas Larason. “So nothing was really expensive and if we never use it again it won’t be a big deal.”
Even communication between the laser field and the inside of the lab was difficult because people couldn’t go in and out as they wanted, so they used a wired intercom system.
Despite the challenges, the system worked surprisingly well, Miller said, especially since they only had months to put it together. “There are a few areas where we can probably improve, but I think our gains would be minimal,” Miller said.
The NIST team plans to use this system for future studies of other viruses and microorganisms that biologists would like to conduct in high-security labs.
“When the next virus comes along, or whatever pathogen they’re interested in, we just have to roll the laser system up, push a fiber under it, and they’ll hook it up to their projector system,” Miller said. † “So now we’re ready for next time.”
Thomas Larason et al, Traveling Tunable Laser Projector (TTLP) for dose determination for UV blue disinfection, Applied Optics (2022). DOI: 10.1364/AO.460317
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