Recent comments in /f/askscience

brad_y t1_jcmuzj2 wrote

Reply to If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

I see a lot of correct comments here. I work in the UVC disinfection industry and the best way I describe it to people is basically the Sun produces UV-A, UV-B, and UV-C. Because of C’s wavelength, it never makes it through earths atmosphere, thus nothing on earth has a resistance to it. It’s pretty cool how UVC has been used since the late 1800s to help with outbreaks and now it’s used in almost every operating room to quickly disinfect in between surgeries to prevent hospital acquired infections. Hope that helps!

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Ausoge t1_jcmuezg wrote

Reply to comment by joyamazingpinoy in If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

That will probably be more to do with moisture being able to escape - with open windows, your house will be a lot drier than if you keep them closed for weeks at a time. You wouldn't need to open windows to get the UV effect, just your curtains.

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Neurokeen t1_jcmt6y7 wrote

Reply to comment by -Metacelsus- in Are there any known endogenous retroviruses that can cause active infections, and is this possible in principle? by amlyo

It actually gets to a really interesting question as to "what counts as infected?"

If there's latent proviral inserts that never activate and propagate, to the point that they're ubiquitous in the DNA of the host, then it strains the definition a bit in most contexts. If you're doing genomics then for those purposes it makes sense to call it an infection. If you're doing something at the level of epidemiology, then probably not.

Having a bunch of boundary cases in definitions is pretty much a staple for biology though, so it's not worth losing any sleep over.

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fromagionado t1_jcmszjn wrote

Reply to Energy can not dissappear or be created, only change form, right? Earth is blasted by the sun 24/7, where does the excess energy go, because I guess it doesn't stay here or we'd cook by mr_greenmash

In addition to reflected energy, most of the energy that gets soaked up on the light side of the planet continues to get radiated away during the night. That sort of balances it out. Then, depending on how well the atmosphere is capable of insulating Earth (increasingly well due to build up of aptly named greenhouse gases), the planet settles into an equilibrium state and that's the temperature we experience on average.

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scrapmek t1_jcmsagb wrote

Reply to comment by echawkes in If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

This is the main difference, there was actually an incident where medical grade UVC bulbs made for sterilizing were accidentally used at an event which caused 'sunburn' and eye damage to the attendees.

Many organisms have evolved some resistance to UVA and UVB since it reaches the planets surface, although they can still cause enough damage to have a mild sterilizing effect.

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purpleoctopuppy t1_jcmr0ik wrote

Reply to comment by TinyDemon000 in If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

UV Index of 14 is 350 mW/m², a UV steriliser bulb I found online is 28 μW/cm² which is 280 mW/m² ... huh, those are pretty comparable.

(The bulb is measured at 1m, so in practice it'll probably be 16× stronger or more, but still I wasn't expecting comparable orders of magnitude. Also note different UV wavelengths)

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babar90 t1_jcmqnd7 wrote

Reply to comment by -Metacelsus- in Are there any known endogenous retroviruses that can cause active infections, and is this possible in principle? by amlyo

Can't we argue that by definition a live virus should have a reproduction number strictly greater than 1 (ie. exponential) and be able to spread from cells to cells? For PERV-A seems they obtain exponential replication in human 293T cells https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2950858/figure/pone-0013203-g001/

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nriabko t1_jcmq3fm wrote

Reply to If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

There are more UV photons per unit area in artificial UV versus solar. UV kills living things because it breaks apart DNA molecules (specifically, it creates thymine and cytosine dimers which prevent DNA transcription and replication and create mutations in the sequence), and the cells die if they sustain enough damage, and artificial UV is so concentrated that it does this much more quickly than the sun, since cells do have the ability to repair DNA damage. It’s like the difference between playing catch with your friend or having the whole class throw dodgeballs at you all at once.

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Oudeis16 t1_jcmoiig wrote

Reply to If UV radiation is used to disinfect and sterilise things then why isn't everything the sun touches (your skin, the sidewalk etc) sterile? by Critwhoris

If fire cooks food, why isn't my steak grilled if I wave it over a candle?

If sound can damage eardrums, why am I not deafened by the pitter-patter of rain?

If alcohol kills germs, how can I get sick when I'm drunk?

There's an application of UV that will sterilize. That doesn't mean the simple existence of any kind of UV will fully sterilize all things.

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