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Author Topic: Can the sun kill germs?  (Read 23632 times)

Amanda Horton

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Can the sun kill germs?
« on: 21/11/2011 02:01:04 »
Amanda Horton  asked the Naked Scientists:
   
Hi Chris,
  
Recently I was informed that if I wanted to clean my clothes, I should leave them out in the sun for a couple of hours to kill all the germs on them.  Now, being a scientist myself, I queried their evidence for this seemingly unbelievable fact. They told me that of course I was skeptical, since I was British and our sun is strong enough to have this effect on clothing, but it was scientifically proven (not that they could give me any actual evidence). They also quoted UV sanitisers as proof to this "fact".
  
Now, I studied Bioveterinary Science at the University of Liverpool and I lean towards the genetics and infectious disease area of science, so I like to think I know a thing or two about bacteria and viruses. More, at least, than the historian I was arguing with. I thought UV sanitisers worked by producing a wavelength of UV that is blocked by ozone, and actual contributes to the creation of ozone, and as such, it is rarely found on Earth, which is a good thing because it effectively mutants cells which weaken bacteria and viruses, leading to their ultimate demise. Of course, this means it is also dangerous for humans.
  
I said that surely the idea of sun killing germs comes from this use of UV light in industry, and is actually an old wives tale with a foundation in misunderstood scientific fact.  My friend, however, was adament that is was proven and that I was just being closed minded, she even laughed at how silly I sounded.  All I wanted was some hard scientific evidence to back this idea.
  
Curious, I searched the internet, but google only managed to give me some alternative medicine nonsense and lots of conflicting forum posts.  
  
So, as a fan of The Naked Scientist, I thought I would ask you. Can putting clothes in the sun kill germs, and if so, how?
  
Thank you for reading this.
  
Amanda

What do you think?
« Last Edit: 21/11/2011 02:01:04 by _system »

Nizzle

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Can the sun kill germs?
« Reply #1 on: 21/11/2011 06:49:41 »
No, putting clothes in the sun will not kill germs.
If that were true, not only the germs on your clothes would die, but all germs that come into contact with sunlight would.
Obviously this is not the case, since the world is full of germs.

With UV sanitizers, it's all about 'dose'. The dose of UV coming from the sun is, thanks to our ozone layer, far from high enough to sterilize clothing.

Next time you have the flu, sneeze on your historian friend and tell her to stand in the sun for a while if she doesn't want to get sick :p
« Last Edit: 21/11/2011 06:51:23 by Nizzle »

Bored chemist

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Can the sun kill germs?
« Reply #2 on: 21/11/2011 07:16:14 »
Actually, it will kill some of the bugs. It won';t end up sterile, but it's likely to have a significantly lower count.

CliffordK

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Can the sun kill germs?
« Reply #3 on: 21/11/2011 08:28:55 »
I'd imagine that you'd get most of your bug killing from drying and heat provided from the sun.  A clothes dryer might do the same.

CZARCAR

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Can the sun kill germs?
« Reply #4 on: 21/11/2011 14:45:09 »
UV is used in water & air gizmos

Phil1907

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Can the sun kill germs?
« Reply #5 on: 23/11/2011 12:43:24 »
Like to see Bored Chemist's data.  Nizzle is correct and diminished microbial survival would be more  a function of drying and some degree of heat han "sunlight".  Light will not penetrate clothing and UV dosage even at the surface would be minimal - with photoreactivation repair mitigating it's effect.  UV treament used commercially for water and air is much more intense and focused.

Bored chemist

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Can the sun kill germs?
« Reply #6 on: 23/11/2011 19:37:40 »
Like to see Bored Chemist's data.   UV dosage even at the surface would be minimal -

Happy to oblige.
This story
http://www.rsc.org/Publishing/ChemTech/Volume/2009/03/solar_power_kills_bacteria.asp
is probably the best known example.

I have also done some work looking at hydroxyl radicals as a fumigant for reducing general bacterial counts. (I was doing the chemistry- my colleagues were doing the microbiology).
We were using a system that relies of UV light to produce the radicals.
However it's well known that sunlight in air will do this.

Some of the cell culture work done in the lab takes place in clean air cabinets. They are treated with UV after use to degrade any DNA that's present. The UV dose is small enough that it doesn't figure in the risk assessment (and believe me, someone checked). Since it's well documented that sunlight damages skin it's fair to say that the UV levels in the cabinet are lower than those outside on a sunny day.
Also, I have measured the UV output from similar UV tubes- they don't produce as much UV as the sun (at least as far as my monitor was concerned - indicator only: not calibrated).

On top of that there's the fact that plenty of fungi are coloured and the widely held hypothesis is that the colour is there to reduce the damage by UV.



Oh, and since sunlight has enough UV in it to damage us, why would it not damage bacteria?

There's also stuff like this
"The ultraviolet component of sunlight is the main reason microbes die in the outdoor air. The die-off rate in the outdoors varies from one pathogen to another, but can be anywhere from a few seconds to a few minutes for a 90-99% kill of viruses or contagious bacteria. Spores, and some environmental bacteria, tend to be resistant and can survive much longer exposures."
from here
http://www.engr.psu.edu/iec/abe/control/ultraviolet.asp



Now do you have any data to support your assertion that "UV dosage even at the surface would be minimal"?


« Last Edit: 23/11/2011 19:41:54 by Bored chemist »

CliffordK

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Can the sun kill germs?
« Reply #7 on: 24/11/2011 05:22:30 »
Nice explanation BC.

I guess if you think of human skin, one has multiple layers of dead cells covering the living tissue.

With a bacterium, there is only a single layer cell wall.  So, it is a much more direct route of the UV to the cell nucleus.  Of course, that depends on actual exposure to the light.

Amanda_UK

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Can the sun kill germs?
« Reply #8 on: 24/11/2011 11:12:11 »
Wah! The typos in my original post! Sorry, obviously it should be "out sun isn't strong enough" and "it effectively mutates cells".

Thanks for your answers so far. I really appreciate it as I was going crazy trying to find a reliable answer. Thanks for the useful links Bored Chemist, I like evidence.However, I think it's safe to say that UV light does kill microbes in water as this has been proven several times. I want to know it if would have the same effect on fabric. Isn't it possible that the water magnifies the effects of the UV light and the heat to produce strong enough UV exposure to kill the microbes. Obviously fabric couldn't emulate the same properties as water, so would UV light have the same effect?

Of course the fact the sun can damage our skin cells occured to me, but, as Nizzle said, there are microbe everywhere. How do they survive sunny days?

Also, if we accept that UV exposure from the sun can kill microbes, it's still not an effective way of cleaning clothes, since there would always be area of the fabric not exposed, and how far into the fabric can the sunlight permeate?

How about grass or soil or animals that live outdoors? Some microbes can survive for months outside of a host, does this only apply if they are out of direct sunlight, or are some microbes resistant to the effects of UV light? The questions are endless!!!
« Last Edit: 24/11/2011 11:21:35 by Amanda_UK »

Bored chemist

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Can the sun kill germs?
« Reply #9 on: 24/11/2011 20:31:44 »
"Isn't it possible that the water magnifies the effects of the UV light and the heat to produce strong enough UV exposure to kill the microbes"
Not really.
It's fair to say that the drying and heating contribute to the overall death toll. Having water present is better for the bugs (most don't like drought) and also keeps the temperature down.

Also,, of course, the UV has only a rather limited effect on things that are not in full sunlight.
Most things don't reflect UV very well so the shady side of anything will not be  nearly as well cleaned.
The inside of anything isn't likely to get much of a kill rate either. Perhaps one of the more obvious issues is that bacteria inside dirt will be hidden from the UV.

As I said, some bugs are coloured (in effect- they have a built -in sunshade like our tanned skin). Those are going to be more resistant to UV (though the hydroxyl radicals produced by UV will still damage them).

The simple answer to "Of course the fact the sun can damage our skin cells occured to me, but, as Nizzle said, there are microbe everywhere. How do they survive sunny days? " is that zillions of them don't.
However, the few that survive can reproduce themselves very fast.
There's a lot of approximation to the idea that bacteria can double their numbers every half hour or so, but it's a reasonable place to start.
If the sun wipes out 99.9% of them then, clearly, about 1 in 1000 survives.
Half an hour later that divides and becomes two, after an hour you have 4,
It carries on and after 10 hours or so you have just over 1000 again.



Phil1907

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Can the sun kill germs?
« Reply #10 on: 26/11/2011 12:52:25 »
Please try to remember the original question was offered in context of clothing.  Bugs in that context would be staph, corynebacteria, Malassezia fungi and a variety of other transient and less well studied normal flora as well as dust and soil drtived bacteria.  These are not present as isolated cells but associated with soils such as dust, dried body secretions, skin cells etc.  These associations with the very small number actually be exposed to incident sunlight and with microbial repair of UV damage - would bring very little practical reduction in viability.   
Catalase, superoxide dismutase and other specific and non specific capabilittes (bacterial spores)address some of the other (irrelevant) factors offered.  Yes, pigmentation - melanin, carotenoids etc indeed do mitigate UV and oxidant based killing but are doubtfully involved in context of flora on clothing. 

Bored chemist

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Can the sun kill germs?
« Reply #11 on: 26/11/2011 15:48:42 »
Please try to remember that the title of the thread is can the sun kill germs?
It can.
Also, Phil, rather than wasting time sending me insulting personal messages, you might want to check on what superoxide dismutase and catalase actually do, and why they have nothing to do with the destruction of hydroxyl radicals.
Alternatively, perhaps you might want to answer the question I asked earlier.
Here's a copy.
Now do you have any data to support your assertion that "UV dosage even at the surface would be minimal"?


Mind you , it's not clear that you read what I wrote or you would have noticed that I said "The inside of anything isn't likely to get much of a kill rate either. Perhaps one of the more obvious issues is that bacteria inside dirt will be hidden from the UV."
so saying "Bugs in that context would be staph, corynebacteria, Malassezia fungi and a variety of other transient and less well studied normal flora as well as dust and soil drtived bacteria.  These are not present as isolated cells but associated with soils such as dust, dried body secretions, skin cells etc.  These associations with the very small number actually be exposed to incident sunlight and with microbial repair of UV damage - would bring very little practical reduction in viability.    "
was pretty much redundant.



 

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