You can disinfect surfaces by shining ultraviolet (UV) radiation on them. This is a bluer wavelength than what the human eye can detect. What wavelengths work best to kill SARS-CoV-2, which is the virus responsible for COVID-19? How much radiation is sufficient?

Scientists must overcome two major obstacles to answer these questions. They must first separate the virus from any other substances in the environment. They must also illuminate the virus using a single wavelength UV light, and with minimal modifications to the experimental setup.

Recent collaborations between the National Institute of Standards and Technology and the National Biodefense Analysis and Countermeasures Center, a U.S. Department of Homeland Security Science and Technology Directorate lab, have overcome these obstacles and produced what could be the most comprehensive test of how different UV and visible wavelengths impact SARS-CoV-2.

The collaborators present their new system of projecting a single wavelength light onto a COVID-19 virus sample in a secure laboratory in a paper published in Applied Optics this week. The lab has been classified as Biosafety Level 3 or BSL-3 and is intended for the study of microbes that can be potentially fatal if inhaled. The experiment had more UV and visible light wavelengths than any other study on COVID-19.

What is SARS-CoV-2’s kryptonite then? It turns out that the virus is just as susceptible to UV light wavelengths as other viruses, such as the flu. The UVC range of 222 to 280 nanometers (nm) was the most effective. The UVC wavelengths (full range 200 to 280nm) are shorter than those UVB wavelengths (280-315nm), which can cause sunburn.

Researchers found that virus surroundings may have a protective effect. It took less UV radiation to kill viruses in pure water than in simulated saliva. This saliva contains salts, proteins, and other substances that are found in human saliva. Simulated saliva simulates real-world situations such as coughs and sneezes. These findings may be more informative than previous ones.

Michael Schuit, NBACC, stated that "I think one the major contributions of this research is that we were in a position to show that idealized results don't always accurately predict what will happen when there's an actual scenario at play." "UV decontamination methods can be less effective when there is material such as simulated saliva around the virus."

These results can be used by regulators and manufacturers of UV disinfection equipment to determine how long surfaces, in medical settings, planes, or liquids, should be irradiated in order to inactivate the SARS-CoV-2 virus.

Cameron Miller, a NIST researcher, stated that there is a push for UVC disinfection in the commercial environment. "Long-term this study should lead to standards and other methods for measuring the UV dose needed to inactivate SARS CoV-2 and other harmful virus."

This project was a continuation of earlier work that the NIST team and another collaborator did on inactivating microorganisms from water.

A little light is a good thing

The wavelength of UV light can damage pathogens in different ways depending on its nature. Some wavelengths can cause damage to microbes' DNA and RNA, which could lead to them losing their ability to reproduce. Some wavelengths can also destroy viruses by breaking down proteins.

Even though UV light has been known for its disinfection capabilities for over 100 years, the last decade has seen a surge in UV disinfection research. One reason is that UV light sources are often made from toxic materials like mercury. Some of these concerns have been alleviated by the recent introduction of nontoxic LED lamps.

The NIST collaborators collaborated with NBACC biologists to conduct this study. Their research informs biodefense planning for biological threats like Ebola virus and anthrax.

Miller stated that the NBACC was able "to grow the virus, concentrate and then remove all other viruses." We were trying to convey to the public how much light is needed to kill only the SARS-CoV-2 virus.

The team tested the virus in various suspensions during the study. Scientists used the saliva mimic to test the virus in different suspensions. The scientists tested the virus suspensions as liquids as well as dried droplets on stainless steel surfaces. This represented what an infected person might cough up.

NIST's task was to direct UV light from a laser onto samples. They needed to find the right dose to kill 90% of this virus.

This setup allowed the collaboration to measure the virus' response to 16 wavelengths. These wavelengths ranged from the very low UVC (222 nm) to the middle of the visible wavelength spectrum (488 nm). Because some blue light is known to have disinfecting properties, researchers included longer wavelengths.

There is no cake like it

It was difficult to get the laser light onto samples in a secure laboratory. Scientists working in a BSL-3 laboratory wear scrubs and hoods that have respirators. After leaving the lab, you will need to take a long shower and then change into civilian clothes.

Equipment like the laser used by the team would have needed to be sterilized in a more rigorous manner.

Miller stated that it was a one-way street. Anything coming out of the lab must be incinerated, heated-sterilized or chemically disinfected using hydrogen peroxide gas. We didn't want to take our $120,000 laser in.

Instead, NIST researchers devised a system that allowed the laser and some optics to be placed in a hallway outside of the lab. The light was piped through a 4-meter long fiber optic cable, which passed under a lab door. The negative pressure prevented air from leaking out of the lab by keeping it flowing through the hallway.

Researchers could create any wavelength they wanted using the laser's full tunable capabilities. The laser only produced one wavelength at a given time. Because light bends at different angles depending upon its wavelength, researchers had to design a prism system to change the angle at which light enters the fiber. This would ensure that the fiber aligns properly. To change the exit angle, they had to manually turn a knob that was designed to adjust the position and orientation of the prism. The goal was to keep it as simple and straightforward as possible with as few moving parts as possible.

Schuit stated that "the device that NIST developed allowed us to quickly test a wide range of wavelengths at very precise wavebands." We would have to manage a lot of different devices if we tried to test the same wavelengths without this system. Each device would have produced different waveband widths. These would have required different configurations and would have had many additional variables.

Mirrors and lenses were required to manipulate the light. However, the researchers made it so that only a few lenses were needed, as each lens leads to a decrease in UV light intensity.

The team used inexpensive parts for the materials that needed to be brought into the laboratory to project the light from fiber onto COVID virus samples. Steve Grantham, NIST physicist and Thomas Larason were key members of the team. "So, nothing was very expensive, and if it's never used again, it's no big deal."

Communication between the laser area of the lab and the interior was difficult as people couldn't go in and out at will. So, they used a wired intercom system.

Miller stated that despite the difficulties, the system performed remarkably well considering they only had a few months to create it. Miller stated that there were a few areas where they could make improvements, but that their gains would be minimal.

NIST plans to use the system to study other microorganisms and viruses that high-security biologists might be interested in.

Miller stated, "When the next virus or any other pathogen comes along, all that's required is to roll the laser system up, push a fiber underneath there, and then they'll connect it with their projector system." We're now ready for the next.