There's light on the horizon when it comes to controlling the airborne spread of infectious diseases like COVID-19...
Over the last year the COVID-19 pandemic has delivered a hefty financial and human cost across the globe. As I type this in the UK (25/01/21), we are currently seeing some of the worst daily statistics so far. Thankfully, there does appear to be light on the horizon. Vaccine delivery programmes have begun with ambitious plans to have the most vulnerable in society protected within the first half of 2021. There is also another tool in our toolbox in the form of a technology that has been around for over 100 years and which we can use in our fight against the spread of SARS-CoV-2 as well as other respiratory infections.
That technology is Ultraviolet C (UVC) radiation. UVC is a subcategory of UV radiation, alongside the more commonly known UVA and UVB. It is a short wavelength form of light in the range 100 – 280 nanometres (nm) and a common source we're all familiar with is the Sun. But because UVC is almost entirely blocked by the planet's atmosphere, cosmic UVC is not regarded as threat to public health. Nevertheless, UVC can also be produced by artificial sources.
The use of UV radiation to treat disease, or kill viruses and bacteria, was pioneered by Niels Ryberg Finsen, who was awarded the Nobel Prize for Medicine for his work in this area. Since then, many researchers have carried on this work and furthered our understanding of the mechanisms involved, and the most effective strategies to use.
"But", I hear you ask, "isn’t UV bad for you?". Well, yes, but as George Orwell might write, "all UV is harmful, but some UV is more harmful than others". In fact, we are all too aware that some UV is actually good for you (it powers the process of vitamin D synthesis in your skin, for example), so the UV good/bad landscape is already rather complicated. What I mean to say here is that UV radiation is not one homogeneous entity: wavelength matters. Wavelength dictates how strongly a packet of UV of light (called a photon) interacts with something. For example, a photon with a wavelength of 300 nm has over 1000 times the destructive effect on DNA than a photon with a wavelength of 350 nm. But, a 350 nm photon will typically travel much farther into the skin than a 300 nm photon. Both photons in this example are "UV photons", but with very different effects. As such, there are published exposure limit values (ELV) which let us know how much of different types of UV radiation (and visible light too) we may be exposed to over an 8-hour "day", before a hazardous threshold is reached.
So, let’s talk a bit about the hazards of UVC specifically. The vast majority of the work on UVC sterilisation, or ultraviolet germicidal irradiation (UVGI), has focused on the wavelength of 254nm. Why 254nm? Quite simply this wavelength has a dominant peak in mercury vapour lamps, which were some of the first lamps used for UVGI.
While effective at killing viruses, including coronaviruses like SARS-CoV2, there are associated hazards. There is evidence that irradiation of the skin with 254 nm UVC can cause markers of DNA damage to appear, called CPDs (cyclobutane pyrimidine dimers). Dr Ewan Eadie, Head of Scientific Services at the Photobiology Unit at NHS Tayside, explains why this is important: "If a UV photon can penetrate far enough into the skin to reach the part where skin cells are made, and then damage the DNA of those cells - and if the cell doesn’t repair the DNA damage properly or destroy itself and copies that mutation when it replicates - then there is a potential that it can lead to skin cancer. However it is important to remember that our bodies are designed to deal with attacks on our cells, it is happening to us constantly. The vast majority of the time the cell repairs or decides to destroy itself and there is no further harm."
An alternative is to use UVGI at 222 nm, so-called "far-UVC". ("far" in this case refers to the wavelength being "further away" from visible light). While UVGI at 222 nm is still effective at killing viruses, the damage caused by this radiation is largely localised to the upper epidermis of the skin, and is far less than 254 nm UVC. Dr Eadie continues, "at a wavelength of 222 nm, the UV gets absorbed quickly in the 'dead’ skin cells at the very top of the skin, it never reaches the part where the skin cells are made. Damage to these ‘dead’ cells can’t spread to other skin cells and so shouldn’t result in skin cancer. In our self-exposure we irradiated our inner forearms to 260 times the ELV [exposure limit values] and couldn’t find any DNA damage where skin cells are made."
Now that we’ve covered broadly the two types of UVC used in UVGI, how do we actually use these in practice? There are a few implementations to consider. "Upper room" UVGI involves shining UVC radiation at the upper portion of a room and circulating the air such that the air is continually disinfected. As the UVC is not incident on surfaces at "person level", it is possible with such a design to have people in the room while this system operates. The downsides are that surfaces in the lower portion of the room won’t be disinfected, and caution must be exercised when using these light sources to avoid unnecessary overexposures due to incorrect use.
Right now, the New York City subway is trialling UVGI by irradiating the insides of the subway carriages with UVC when the subways are not is use, such as during the night. While this method will undoubtedly reduce the risk of transmission of viruses between days, it won’t stop the person-to-person spread of infection during the intervening day.
With new data on the relative safety and comparable virus-killing effects of far-UVC, perhaps we could have far-UVC lights alongside room lighting, constantly sterilising the air and surfaces in a room whilst the room is in use. Importantly, far-UVC could make this a safe proposition. According to UVC pioneer from Columbia University, New York, David Brenner, speaking on the future of far-UVC sterilization, "The goal of being able to rapidly decontaminate the air in occupied public spaces is a really attractive one, a major step back towards normality. This is particularly true in light of the new SARS-CoV-2 variants, in that far-UVC would almost certainly inactive these with equal efficiency."
This brings us to another benefit of UVGI, if you aren’t already convinced. We’ve heard a lot recently about different strains of SARS-CoV-2, often with somewhat unhelpful nicknames like the "UK strain" or the "Brazil strain", much in the same manner as the "Spanish flu" didn’t necessarily originate in Spain. These variants are a test for current vaccines. Will the vaccines still be effective against these new strains? According to Devi Sridhar, Chair of Global Public Health at the University of Edinburgh and member of the Scottish Government COVID-19 Advisory Group, "...as new variants emerge we don’t know whether our vaccines will protect against them or whether having COVID once means you can’t get it again.” But what we do know is that there are no known mutations in a virus that protect it from UVGI. Ever. There is no reason to believe that UVGI would be any less effective at killing new variants of SARS-CoV-2, or any other future viruses that may come our way for that matter.
Imagine if we already had this infrastructure in place? Hospitals, care homes, schools, public transport, supermarkets – these spaces could be "pandemic ready", allowing a good deal of the country to keep functioning while reducing the spread of the virus. Of course, this would require forward planning and likely costly investment. Compare that to the human and financial costs to date however, and it seems like an investment worth making.
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