This post was reviewed by E. Idil Temel, one of our subject matter experts.
This study was conducted by researchers from Columbia University Irving Medical Center.
The paper we’ll be demystifying can be found here, if you’d like to follow along!
TL;DR: Researchers investigated the effects of UV light on aerosolized human coronaviruses alpha HCoV-229E and beta HCoV-OC43, finding that relatively low doses of UV radiation inactivates 99.9% of viral particles. They also found decreased expression of the viral spike protein needed for infection.
UV light tends to be middle child of the electromagnetic spectrum. While visible light nourishes plants and lets us see, microwaves heat our food, and radio waves play the Top 40, UV light only enters our mind when a CSI scans for clues or when we pass by a tanning salon. Most of the time, we’re actively avoiding it with sunscreen and sunglasses, as excessive amounts damage the skin and eyes. However, the recent crisis has given UV light an important new potential gig.
UV light, like visible light, comes in a range of different wavelengths. While the visible light spectrum is divided into colors, UV light has subtypes such as Ultraviolet A and B, or near and far ultraviolet, based on their wavelength and amount of energy they carry.
“Far-UVC” is the subtype under the spotlight today. One of its main applications is sterilization; in fact, it has been known and used to inactivate airborne viruses for many years. It does so by disrupting its DNA, blasting it on the molecular level with its concentrated payload of energy. This is also the reason that far-UVC can be a hazard to humans in high enough amounts, as our DNA can suffer the same type of damage.
Two years ago, a team of researchers from Columbia University had already found that far-UVC light could inactivate aerosolized H1N1. These were viruses suspended in a fine spray of floating liquid particle, suspended in the air. Drawing on their experience, the team used their aerosol investigation talents to tackle human coronavirus aerosols instead.
Two viral isolates were used to make a viral stock. HCoV-229E was grown in human diploid lung MRC-5 fibroblasts, and HCoV-OC43 was grown in WI-38. The collected viral stock solution was adjusted for TCID50, the concentration in which 50% of cells will be infected. The cells were checked for infection by looking for cell rounding and other telltale effects through a microscope.
To simulate a cough or breath, the researchers loaded 107-108 times the TCID50 the viral solution into a nebulizer used for delivering drugs as an aerosol to respiratory patients. The force behind the aerosol was generated by an air pump running at 11 L/min. The target was a BioSampler, a bottle-shaped apparatus that collects aerosol droplets. The whole assembly was kept in a chamber to maintain humidity, temperature, and particle size, and to avoid contaminants entering or exiting.
Researchers used a 222-nm KrCl excimer lamp, normally made for disinfecting the contents of a microbiology experiment, to generate the ultraviolet light. The far-UVC lamp shone through a plastic window in the chamber and aluminum foil on the opposite side helped reflect some of the radiation back onto the aerosols. Using a silicon detector, it was calculated that particles were exposed to 2 mJ/cm2 of radiation. Researchers reduced this dose to 1 mJ/cm2 and 0.5 mJ/cm2 by placing special windows in front of the lamp.
Before each sampling, the virus aerosol was run through the chamber for 5 minutes. During the sampling period, the aerosols were irradiated for 30 minutes. The windows were used to give certain groups different levels of radiation. Control samples were prepared by leaving the lamp turned off.
The sunburned virus solution was collected from the Biosampler and assess in a couple of ways. First, a diluted sample of the treated virus was applied to human lung cells that were mentioned above. After incubation for 3-4 days, they were checked for plaques: dead zones created by an infectious virus. Each plaque corresponds to a single virus, so the quantity is measured as PFU, for plaque-forming units.
Using some straightforward math, the researchers determined the D90 of the viral isolates, the UV dose required to inactivate 90% of the viruses. The HCoV-229E isolate had a D90 of 0.56 mJ/cm2, while HCoV-OC43 had a D90 of 0.39 mJ/cm2. A 1.2 to 1.7 mJ/cm2 dose of 222-nm light inactivates 99.9% of both viruses.
Next, the researchers assessed the viruses themselves, checking for level of the viral spike glycoprotein, a component of SARS-CoV-2 that facilitates infection. They used immunofluorescence, a method that labels a specific protein with a glowing molecular tag, which in this case was the spike glycoprotein. 150 μL of the treated viral solution was applied to a layer of host cells. Once the immunofluorescence protocol was applied, the infected cells were evaluated under a fluorescence microscope, a specialized type of microscope that makes the molecular tag of the spike protein visibly glow green. The researchers found a decrease in this green fluorescence from UV-treated samples, indication a lower expression of the infectious viral spike protein.
While very exciting, it is worth noting that this study’s aerosol chamber isn’t a perfect recreation of a real-life aerosol event. In reality, particle size is not as standard as those created by the nebulizer, and humidity and temperature may differ from this experiment’s range. That being said, the fact that such low doses of UV radiation can inactivate human coronaviruses is great to hear. In fact, this same group is currently testing their method on SARS-CoV-2 directly, although the results have not yet been published. With the current UV limit for people being 23 mJ/cm2, it’s completely within reason to set up far-UVC lamps in hospitals and other high-traffic areas.
All the more reason to get out there and enjoy some sun! Six feet apart, of course.