As cold weather comes and doing things outside becomes harder, more focus is being paid to how we can make the indoors safer when it comes to preventing the spread of the novel coronavirus SARS-CoV-2 (which causes the disease COVID-19). One method that’s been getting more attention is the use of ultraviolet (UV) light to disinfect surfaces or even the air. But there are different types of UV light, so you should be aware (and don’t go using ones that need these signs saying “beware”!)

In our cell culture room (where we work with insect cells which we get to make proteins for us to purify and study) we have a germicidal UVC light that’s on a timer to go on overnight and purify the surfaces so we keep the cells germ-free. It should only be on overnight, but just in case we have a warning light outside the door that tells us when it’s on so we can turn it off before we go in – and therefore don’t have to worry about frying our skin! 

You see, that’s the problem with conventional UV, you don’t want it to contact people (despite what you might have heard on TV) – but it can be really useful in killing microbes like bacteria, fungi, and – viruses.  

There’s nothing super fancy about UV light. It’s just like “normal” light (little packets of energy called photons traveling in waves) except it’s more energetic. In fact, the only reason we humans consider some light “normal” is that it’s in our visible range – our eyes have receptors capable of detecting it. In addition to the ROYGBIV of the rainbow (which combined gives you white light). There’s actually a whole spectrum of “light” (aka ElectroMagnetic Radiation) stretching out from either side of the visible range (e.g. on one side you have microwaves, etc. which are lower-energy and on the other you have UV, etc. which is higher energy). 

Because UV light is higher energy, it has the potential to damage microbes like bacteria and viruses by messing with its proteins and DNA or RNA (more on the science of how it does so later in the post). But it also has the potential to damage humans and other animals – if it can get through our protective outer layers of dead skin (and eye tears). 

So the key is to use a form of UV light that’s least harmful to humans but still harmful to human-harmers. The UV light in sunlight can be divided into 3 main classes defined by their wavelengths (given in nanometers (nm) – the shorter the wavelength, the higher the frequency and higher the energy): UVA (315-399nm), UVB (280-314), & UVC (100-279nm)

You’ve likely heard about UV A & UV B in commercials for or labels on sunscreens claiming to block them. UVA, the “sluggish-est” only penetrates the outer layers of our skin and are blamed for wrinkles and other long-term damage and UVB is known to be carcinogenic (cancer-causing). 

Sunscreen companies don’t usually talk much about the 3rd form of UV light that’s present in sunlight – UV C – because it’s blocked before sunblock could block it! It’s the most energetic of the bunch and has the best virus-killing-power, but the atmospheric ozone layer blocks most of it. UVA & UVB still get through and cause sunburn, DNA damage, etc. (hence sunscreen-wearing). There’s some evidence that sunlight can slow viral growth but it’s UVC (the blocked-out one) that’s the biggy. 

Just because the ozone blocks it doesn’t mean we can’t generate it ourselves down here on the ground. Most “germicidal UVC” lamps typically have a wavelength of ~260 – 285 nm and are used to disinfect rooms and surfaces when people are not inside. There’s also this method where low levels of UV light are put out only in the upper part of the room, away from people, to kill the upper air, or UV lights are installed in the air ducts. 

But there might be a way to use UV light more safely around people – potentially even disinfecting rooms people are in.  Maybe a wavelength shift could be a paradigm shift when it comes to cleaning?

That’s the hope for some scientists at Columbia University working on “far-UVC light.” I first heard about this a few months ago, and then heard more on This Week in Virology (TWiV) episode 666, 

Basically, Columbia University’s Center for Radiological Research developed a lamp that shines a low level of far-UVC light (207–222 nm). This short-wavelength UVC light is thought to be able to kill viruses and bacteria without harming human tissues because the wavelengths of light they use get absorbed by our dead cell layers before they can reach our live cells. But microbes aren’t so fortunate – they don’t have layers of dead cells protecting them – or even layers of cells at all – so when they get hit by the light, they can get damaged. Their latest paper follows up on this report from 2018 

They say it’s effective against cold-causing coronaviruses as well as some other germs, and now are testing it against SARS-Cov-2 (the virus that causes Covid-19). That’s cool – but an important thing to remember is that that technology is still just being tested in a lab. There are a few companies making far-UVC products now, but they haven’t yet gotten FDA & EPA approval. 

All those products you see on the market are NOT this safer version of UV light, instead they’re the “normal” UVC. Which is definitely NOT safe for skin – or eye – contact. I’ve mostly mentioned skin because that’s the biggest region UV light is likely to hit, but UV light can also cause eye problems. You might be used to the term carcinogenic (cancer-causing) but apparently “cataractogenic” is also a real term – and a real danger. 

The far-UVC folks are hoping that it can be used in public spaces to help keep things clean – and that would be great, but I’m a bit skeptical, because UV doesn’t “work” instantaneously – it’s not like if an infected person sneezed the UV would kill the virus before that sneeze droplet reached the person they’re talking to.

The damaging power of UV depends on the intensity of the radiation (does your light source have a “heavy flow” of electrons?), the distance from the object (some of the photons are inevitably going to get “distracted” or absorbed by other things en route), and how long the object is exposed. If you know these things you can calculate the “dose” 

Intensity at the light source & distance from the light source combine to give you the “irradiance,” reported in milliwatts (mW) per square centimeter (cm²). Times that by how long the object is exposed to get the dose. If you times mW/cm² by seconds (s) you get mWs/cm² which is aka 1 mJ/cm².

This all sounds really technical, and you don’t need to know all the fancy science terms – but what you do need to know is that you have to have an adequate dose to kill. not all UV lights are created equally, and the UV lights sold to consumers often don’t tell you their irradiance. And that matters… If you have a really weak light, you’re gonna have to get it really close to the object and/or shine it on the object for a really long time. But if you have a stronger light, you’re more likely to accidentally harm yourself with it in the process.

What dose do you need to kill SARS-Cov-2? It depends on the material of the object. A study used one of those hospital-grade germicidal UV lamps, held 50 cm (a little under 2 feet) away from objects to see what dose of UV light was needed to kill SARS-Cov-2. At this distance, the irradiance was 0.005 mW/cm² (aka 5 microWatts (μW)/cm² and it took ~1hr to completely disinfect an N95 mask (corresponding to a dose of ~18 mJ/cm²) and ~12 min for steel (corresponding to a dose of ~3.6 mJ/cm²). 

Like I mentioned above, most companies don’t tell you their irradiance, but it’s probably a lot less than a hospital-grade lamp. Which means you’re gonna have to shine light on things for a long time in order to disinfect them. 

The Illuminating Engineering Society (an INDUSTRY GROUP) told CNN: “Ultraviolet disinfecting ‘wands’ or other ultraviolet products for residential use — as they are inadequately proven and unregulated — may pose a safety hazard and are unlikely to provide the protection expected” 

And apparently companies make it look like their products have EPA certification, but it’s really just the facility, not the actual product 

And the FDA recently put out a warning and Q & A: 

Now that I’ve gotten all the warnings out there for the people who don’t want the super geeky details, here’s that detail for those like me who’ve just gotta know what’s going on. I already have the catchphrase “push electrons not people” – but if I could have a second one it’d be “What’s the mechanism?” As a passionate molecular mechanic who spends her days working to figure out how a certain protein does what it does, I’m always itching to know how things work!

And I’m also passionate about trying to teach anyone who’s interested, even if they haven’t been super privileged like me to have advanced science training. So, now that I’ve gotten the general stuff you can find in news articles out of the way, let me step back and try to get to more of the stuff you’d find in the pages of “dry” textbooks that expect you to know a lot going in. 

Let’s start at the very beginning. In order to understand what UV light’s doing, you need to know what light is. Light is made up of pockets of energy called photons and different types of light have photons with different amounts of energy. These photons travel as waves – the more energetic the photon, the closer together the wave peaks (higher frequency) and, since all light travels at the same linear speed, the shorter the wavelength (like a little energetic kid running zig-zaggedly so he keeps pace with his grandpa on their walk).

When you shine light on something it’s like streaming pennies and nickels and dimes of free energy money for molecules in its path, but the molecules have to have the right “slot size” in order for them to accept these coins. This “slot size” corresponds to differences in energy levels between subatomic particles called electrons, which are what atoms share to form bonds to form molecules). The “slot size” is the difference in energy between the places in that molecule where electrons are allowed to live in and this depends on what the molecule is and what’s around it.

If a photon (coin) comes along & the molecule has the right slot size, the molecule will absorb it. If the light was part of the visible light spectrum (the part of the electromagnetic radiation (EMR) spectrum our eyes are specialized for detecting and interpreting) you’ll see a color change because the molecule basically stole a slice of the rainbow – white light is made up of light of all colors, each corresponding to a different wavelength. ROYGBIV = white, but RYGBIV or ROYBIV doesn’t, and this is the basis of how dyes work.

But molecules can also absorb light that is outside of our visible range. This includes less energetic (longer wavelength, lower frequency) light, like infrared or microwaves, as well as more energetic (shorter wavelength, higher frequency) light, like ultraviolet (UV) or x-rays. 

We can’t see when this happens (at least with our naked eye), but it has the potential to alter molecules. When a molecule absorbs light, what happens is that the electrons in the molecule get temporarily excited and they “jump up” to a higher energy level. But it’s hard to stay excited, so they often just give the energy back – usually as heat, etc, but sometimes as light (which is the basis of fluorescence). Sometimes, however, the absorbed energy from the light can be used to break, form, or alter chemical bonds – including in DNA & RNA. 

Our cells use DNA to hold our genetic blueprint (genome) – containing instructions for making (and regulating) all the proteins, etc. we need to live. You don’t want to mess with it because if you do, things like cancer can occur.  SARS-Cov-2 (the virus that causes Covid-19) is an RNA virus – it has an RNA genome instead of a DNA one – and it’s single-stranded, unlike our genome, which is double-stranded. 

DNA & RNA letters (nucleotides) have light “coin slot sizes” that correspond to light in the UV range. When DNA or RNA absorbs UV light it can “spend” the energy money to form improper bonds, like strong bonds called pyrimidine dimers between neighboring bases in the same strand. If these don’t get fixed, or if they get incorrectly “fixed” before the genome gets copied, there will be permanent mutations in all future daughter cells. 

Our cells have complex machinery on guard to try to correct errors before the DNA gets copied and the error gets passed on, but if they get too much UV exposure they get overwhelmed and errors slip through the cracks. And if those mutations do things like mess up a regulatory molecule, cells can start growing uncontrolledly to form a cancerous tumor. 

So it might sound like hitting a virus with this light isn’t a good idea – we want to STOP them from growing right? Key thing is, when it comes to viruses, they have much smaller genomes with much less room for error and much less proofreading & fixing power, so their RNA gets *fatally* flawed. In addition to causing dimers, UV light can make viral RNA strands actually break, and therefore it can kill viruses instead of giving them superpowers.  

The virus has no hope of fixing this broken RNA and, without functioning RNA it can’t survive, so UVC light *can* be used as a disinfectant – BUT for SURFACES (and maybe air) – NOT for people. If the virus is on someone’s skin and you shine them with UV light you *might* kill the virus on the skin, but you’re also putting that person at risk of skin cancer. And if the virus is inside someone (they’re already infected) – shining UV light on them will not cure them – the UV light would get absorbed by their skin (potentially messing up their DNA) before it had any chance of even getting to the virus to kill it.


Some final details at the even geekier level…

One way UV can damage nucleic acids is an electron jumps up and, before it has a chance to fall back down, it gets “caught” by another molecule. It’s kinda like a coordinated trapeze act. It can only happen if the molecular players are exactly in the right place at the right time. It may sound really unlikely, but this can happen when you have 2 pyrimidine residues (Ts or Cs) next to each other on the same strand (so they’re stacked on top of each other in the helix) and you hit it with UV light. These photons are like the right coins for T & C, so they get absorbed – an electron jumps up and – thanks to that base stacking, there’s another electron’s home right above it. So it moves in. This leads to new COVALENT bonds being formed between these bases.⠀

Unlike the weak bonds between the DNA strands that can melt apart, these covalent bonds are “stuck” and they can cause problems during copying. These dimers most frequently form between 2 thymines -> thymine dimers and our cells can *usually* detect the bulge and fix it, typically through something called nucleotide excision repair. This involves cellular quality control recognizing that the DNA has an awkward bulge due to the dimer, cutting out a short stretch of DNA encompassing the dimer, and then filling back in the removed letters using the second strand as a template. ⠀

If the problem doesn’t get fixed before it’s DNA copying time, the copiers (DNA Polymerases) just do the best they can to “guess” what the stuck letters are – for T-T dimers, they usually get it right, writing the complementary letters A-A. But, for dimers of cytosine (C-C), they’re more likely to mess up, and write A-A instead of G-G. Now, when this new strand gets used as a template for making more copies of the strand with the dimer, instead of C-C, that strand will read T-T. And this mutation will get passed on to all the daughter cells made from this cell. If the mutation is in a key regulatory molecule, it can lead to uncontrolled cell growth (cancer). ⠀

So it’s really important that we use caution if we have to work with UV light. Take, for example, when we’re exposing our EtBr (or another other fluorescent dye, like DAPI, the dye in the EZ vision stain our lab uses) -stained DNA gels on a UV tray. We *want* the dye to absorb the UV light and give it back to us as light we can see. We DON’T want the DNA bases absorbing that light and forming dimers – in our gel or in our cells. So we scan our gels in a shielded box and, if we want to recover the DNA, we limit its exposure.more here: 

In addition to the whole-room UV lights in our cell culture room, we have UVC lamps inside the culture hoods that you can turn on AFTER you’re done working in them to disinfect before the next person uses it. And remember – UV light is “invisible” to us so that blue or purple you see is “artificially added” so you know it’s on (like how they add a nasty scent to gas). 

Finally, I’ll just end with a plea – if you choose to use UV, please, pretty pretty please, keep the light away from people!

more on topics mentioned (& others) #365DaysOfScience All (with topics listed) 👉

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