Ask A Scientist© Chemistry Archive

name         Monzur M.
status       other
age          30s

Question -   Can glass completely prevent ultraviolet ray penetration?
Does UV penetration depend on the thickness of glass? As a lab worker I
need to check my DNA samples using an UV illuminator. Are my regular eye
glasses sufficient to protect my eyes?
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Glass is not a perfect UV blocker, but it is pretty darn good.  The little UV
lamp you use will not be enough to injure your eyes if you wear glasses.

And yes, as you expect, the thicker the glass, the more UV it will block.
Basically, if thickness x transmits 50% of the UV, thickness 2x will
transmit 25%, 3x will transmit 12.5%, and so on.  If you have access to a
spectrometer, you can check the transmittance of your glasses.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Hi, Monzur !!


Just to confirm : as you know, there are three types
of Ultra-violet light :
UV-A : With wavelength 400-320 nm is less intense
            than UV-B, but is more penetrating and can
            reach and damage easily the retina
UV-B : Wavelength 320-286 burns the skin
            and damages the cornea and ocular lenses.
            It is associated with cataracts
UV-C : filtered off by ozone layer

Some sun lenses may not offer adequate UV-protection, just
only by darkening the lens to eliminate glare, because the
pupil dilates and more harmful UV can penetrate the eyes.
Typical sun lenses are simply coated with a bath in material
that protects against UV waves. Better lenses contain
such material integrated in the composition of the lens.
Only with this UV-bath, the protective material can be
removed through abrasion and scratches.

Going back to your questions :

1 - Can glass completely prevent ultraviolet ray penetration ?
      No, not completely but a high percentage can be
      prevented. The percentage transmitted can be measured
      in a lab with the help of an instrument. At least 70% of the
      visible light should be filtered.

2 - Does UV penetration depend on the thickness of glass ??
       Yes, for a given concentration of UV-absorbing material,
       if it makes part of the glass composition. If not, the answer
      still is "yes" when the glass is tinted. A common substance
      used is a Poly-Vinyl Alcohol.

3 - You work as a lab to check DNA  samples and uses UV
      light to illuminate them. Your question is if "are my regular
      glasses sufficient to protect my eyes". Answer : No, they
      are not.



Alcir Grohmann
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Most window glass will absorb a large fraction of the U.V. radiation. You
can check your safety glasses (always wear them in any case, of course) by
shining the U.V. light through the glasses backed by a sheet of high quality
paper (e.g. copier paper). You should not see any fluorescence behind the
glasses. The paper will give a strong blue fluorescence if not filtered.
Also use side shields on your glasses, because the U.V. light can also hit
your eyes indirectly.

Even so, prudence dictates that you not "look into" the U.V. light or shine
it directly on your skin. It can give you a "sun burn".

Depending upon the type of lamp, you may produce ozone from the
photochemical reaction of short wavelength U.V. and atmospheric O2. If you
smell any ozone ( the thundershower odor) use the lamp only in a fume hood.
Ozone is very toxic as well as irritating.

Vince Calder
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