1/21 Why would UV light be a really bad method for destroying the SARS-CoV-2 virus causing #COVID19 in your body or on your skin while it is a good method for disinfecting surfaces? TL;DR It destroys the viruses RNA. But also your DNA, which may lead to cancer. 
2/21 I am a Chemistry Professor and I work in nanomedicine @UNSWScience @NanoMed_UNSW @ARCCoEBionano making amongst other things, self-assembled virus-like particles for cancer treatment. In my field, some "smart" particles can be triggered with light. cbns.org.au/photoswitchabl…
3/21 But in designing such nanoparticles one rule prevails - never ever create them to be triggered with UV light. Ideally only use lower energy visible or near-infrared light. The reason we all avoid UV light is that we know it damages DNA. And can lead to cancer.
4/21 So how come UV light damages DNA? Why not also visible (normal) light? And how does this help us in destroying viruses, bacteria and other pathogens? Enter a subset of Organic Chemistry - Photochemistry. Light can trigger various chemical reactions. en.wikipedia.org/wiki/Photochem…
5/21 What is "light" again? It is a stream of photons or what we call also electromagnetic radiation. These photons differ in energy. Low energy photons carry radio, TV and phone (incl. 5G!) signals. Their energy is way too low to cause any chemical, and hence biological effect.
6/21 Slightly higher energy microwave & infrared photon may heat up, say water. With visible light (red to blue) you to start some photochemistry happening. Retinal photoisomerism gives us vision and photoinduced electron transfer underpins photosynthesis. en.wikipedia.org/wiki/Retinal
7/21 Visible light does not though have enough energy to cause a lot of chemical reactions, that is, irreversible actual bond breaking and formation between molecules. And thankfully so! Hence our DNA and most of molecules we are made of are not affected by visible light at all.
8/21 Once we reach the higher energy ultraviolet (UV) part of the spectra, we start to see a lot more photochemistry happening. Even then, the so-called UVA and UVB that we do get a bit from sunlight are nowhere near as powerful as UVC (deeper UV) light. darkniteglow.com/uv-lights-expl…
9/21 UVC (below 280 nm) is the most damaging to DNA. Most disinfectant systems use UV-light sources with a strong output in that range. UVC readily cases the formation of dimers in DNA nucleobases (the pyrimidines, especially thymine). This is bad news. iverson.cm.utexas.edu/courses/310N/M…
10/21 Most cells, including ours can though repair this UV-induced dimer damage. But too much UV light and a some point the DNA is beyond repair. This causes cell death (good if we are killing bacteria!) or worse, mutations and then cancer (bad for us!). ncbi.nlm.nih.gov/pubmed/1574863…
11/21 The UVB which we do get a bit of from sunlight can do the same chemistry. The cells that die cause pain = sunburn! (UVA activates melanin, making the skin darker which protects against UVB). Occasionally, our skin cells can mutate leading to cancer. who.int/uv/faq/uvhealt…
12/21 RNA, e.g. in the SARS-CoV-2 can undergo very similar chemistry. When you shine UV light on a virus the RNA is damaged. The viral particle may look intact, but once the RNA is damaged it can no longer infect you. N.b. viruses can't repair their RNA! ncbi.nlm.nih.gov/pmc/articles/P…
13/21 Longer UV irradiation causes also chemical reactions (crosslinking) between the RNA and proteins in the virus particle. N.b. Studies have not yet been done on this in SARS-CoV-2 but there is no reason to think it is any different from other viruses. ncbi.nlm.nih.gov/pubmed/31076437
14/21 Does sunlight destroy the SARS-CoV-2? Well, probably. But UVB is nowhere near as effective as UVC - and there is no UVC in sunlight. At best, sunlight might have 1% of the efficiency of common UVC light sources (with a maximum output around 254 nm). ncbi.nlm.nih.gov/pmc/articles/P…
15/21 I can't find any good studies yet on sunlight vs SARS-CoV-2 but hopefully we will see these soon. It "ought" to work and how long the virus stays outdoor is therefor likely to be a bit shorter than indoors. But we need better data on this: @BBCNews bbc.com/future/article…
16/21 Can we use UV light to disinfect our hands or even stick it down our lungs? I hope with everything I said here before you can see why that is a very BAD idea! Yes, you might destroy the virus but only after you kill a lot of your own cells. And possibly trigger cancer!
17/21 Like so many thing with #COVID19 this is all about probability. Could you destroy the virus by sticking your hands under a UV light? Probably. Would this cause cancer. Probably. Would you have died from COVID if you didn't? Probably. Which factor prevails? Who knows?
18/21 But you see, that is not the point! There are other better ways to disinfect your hands! E.g. soap & water or alcohol based sanitizer. Both work quicker/better than UV light. And they don't cause cancer! So that is why you should NEVER use UV light!
19/21 Same with UV treatment of, say the lungs. Risk of cancer is quite high. Damage = burn to your lung tissue almost guaranteed. But it may not destroy all the viruses - because it has a lot of hiding places inside your body! Conclusions - UV light is a very bad therapy option.
20/21 Those of us working on cancer therapies know that UV light inside our body is a bad idea. I hope this thread helps you understand why UV light can disinfecting surfaces but very dangerous near our skin or inside our body. Don't look into the sun! nbcnews.com/politics/white…
21/21 We should study the effect of sunlight on the SARS-CoV-2 on surfaces better as it might help with public health strategies if we know how quickly it destroys the virus outdoors. But it would be a waste of resources to try to use UV light to treat the virus. Thank you!
As a side point here - which factor prevails will depend heavily on both the type of light (UVB or UVC) and exposure time. The longer or more powerful (UVC) light - the higher the cancer risk. Key question is what dose is required for the virus vs the cancer risk with that dose?
