Which UV wavelengths best disinfect Covid-19?

13 Jun 2022

US labs NIST and DHS conduct thorough tests to show how different wavelengths interact with virus.

To disinfect a surface, you can illuminate it with a blast of ultraviolet light. But to specifically inactivate SARS-CoV-2, the virus that causes Covid-19, which wavelengths are best? And how much radiation is enough? To answering those questions requires scientists to overcome two main obstacles: first, they need to separate the virus completely from extraneous substances in the environment; and second, they need to illuminate the virus with a single wavelength of UV light at a time.

A recent collaboration between the National Institute of Standards and Technology (NIST) and the National Biodefense Analysis and Countermeasures Center (NBACC), overcame both these obstacles and completed what may be the most thorough test ever conducted of how several different UV and visible wavelengths affect SARS-CoV-2.

The laser-to-fiber-optic coupling systems (left), the laser source (center), and chamber holding samples of SARS-CoV-2. Click for info.

In a new paper published this week in Applied Optics, the collaborators describe their novel system for projecting a single wavelength of light at a time onto a sample of Covid-19 virus in a secure laboratory.

The researchers report that the Covid-19 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 ones in the “UVC” range between 222 nm and 280 nm.

They also showed that the virus’s surroundings can have a “protective effect” on the virus. In the study, it took a smaller UV dose to inactivate viruses when they were placed in pure water than when they were placed in simulated saliva, which contains salts, proteins and other substances found in actual human saliva.

Suspending the virus in simulated saliva creates a situation similar to real-world scenarios involving sneezes and coughs. This may make the findings more directly informative than those of previous studies.

“One of the big contributions of this study is that we were able to show that the kind of idealized results we see in most studies don’t always predict what happens when there’s a more realistic scenario at play,” said Michael Schuit of NBACC. “When you have material like the simulated saliva around the virus, that can reduce the efficacy of UV decontamination approaches.”

Manufacturers of UV disinfection devices and regulators can use these results to help inform how long surfaces in medical settings, airplanes, or even liquids should be irradiated to achieve inactivation of the SARS-CoV-2 virus.

“Right now, there’s a big push to get UVC disinfection into the commercial atmosphere,” said NIST researcher Cameron Miller. “Long-term, hopefully this study will lead to standards and other methodologies for measuring UV dose required to inactivate SARS-CoV-2 and other harmful viruses.”

Overview of the researchers’ setup. Click for info.

In the latest 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, without components that could 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 out. NIST’s job was to direct the UV light from a laser onto the samples. They were looking for the dose required to kill 90% of the virus.

With this setup, the collaboration was able to measure how the virus responded to 16 different wavelengths spanning from the very low end of the UVC, 222 nm, all the way up into the middle part of the visible wavelength range, at 488 nm. Researchers included the longer wavelengths because some blue light has been shown to have disinfecting properties.

The NIST researchers designed a system where the laser and some of the optics stood in a hallway outside the lab. They piped the light through a 4-meter-long fiber optic cable that passed through a seal under a lab door. Negative pressure kept air flowing from the hallway into the lab and prevented anything from leaking back out.

This project built upon earlier work the NIST team did with another collaborator on inactivating microorganisms in water.