UV exposure

R.G. Walters mbrgw at s-crim1.dl.ac.uk
Thu Jun 17 06:59:32 EST 1993


In article <C8q6L2.L5w at ncifcrf.gov> pnh at fcs260c2.ncifcrf.gov (Paul N Hengen) writes:
>>The levels of 254nm (the most damaging) UV at ground level (even
>>with ozone depletion) are virtually undetectable.  Transilluminators are
>>VERY strong sources, and even at a distance of several feet are not something
>>I would want to be in the way of, let alone be looking at.  
>
>Robin: Since you know quite a bit about this, I'll ask you my questions...
>
>1. Is 254nm UV detectable by the same procedure at say 10-15 feet from
>   a shortwave transilluminator?

You need a special detector to measure 254nm radiation (and a different one
for 300nm...
>
>2. How much exposure would you expect for say 3 seconds?

One tube emits at something like 1 J/m2/s at a distance of one metre. Standard
transilluminators have around 4 tubes. Applying the inverse square law, at
(say) 4 metres you will suffer around 0.2 J/m2/s.  That's a sufficient dose to
give several pyrimidine crosslinks in an E.coli genome (i.e. several thousand
sites of damage per exposed human cell.

Incidentally, turn this around and you'll see that the dose for your DNA-on-a-
gel is several hundred J/m2/s. It'll be stacked full of damage by the time
you've cut that vital band out - ever wondered why you couldn't clone it?
>
>3. Is the same level detectable from the mid-range (300nm) or longwave (366nm)
>   transilluminator at this distance?

The levels of these "on the beach" increase as the wavelength does, but the
nature of the damage is different. 300nm damages primarily protein (although
protein-DNA crosslinks can be induced), and 366nm is less damaging than both
these. Furthermore, melanin is "designed" to absorb and protect from these
wavelengths, but not UV-C, which isn't seen in the natural environment.
>
>4. What damage is done to the cornea at this level?

I'm unsure of the precise nature of this, but I recall something about 
the sites of damage nucleating sclerosis of the cornea, and the more damage,
the more opaque it becomes (someone else will know more about this than me).
>
>5. What is the source of the "sandy-eye" feeling from mild UV exposure?

Sorry, dunno - you've probably burned yourself, and the eye is slightly
inflamed.

The big thing about transilluminators is that the damage they induce is of
a VERY different nature to that you get on the beach.  The UV heads straight
for the DNA, and if it misses that gets protein instead. Virtually all its
energy goes into damaging cells, whereas for the longer wavelengths a signifi-
cant proportion of the energy gets lost as heat (hence the burning).

Robin Walters.                      Robert Hill Institute, Sheffield UK.

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