Walt Goettman (wgoettman@mm.wyeth.com)
Fri, 29 Jan 1999 08:33:01 +1008
>
> UVA has one nasty effect on eyes, known as "nuclear cataracts". This is a permanent "tanning" of
> the lens of the eye. "Nuclear" refers to this occuring mainly in the center of the lens of the eye. You
> need lots of heavy exposure to get this. Nuclear cataracts occur mainly among people who work
> outdoors.
> Although incandescent blacklights emit wavelengths that cause nuclear cataracts, they are largely
> safe since you typically get less UV exposure from these than from sunlight. Even considering the
> fact that your pupils are wider under blacklight than under sunlight, there is so much less UV that it
> is safe to stare into the bulb from a few feet away for large amounts of time. (No warranty, so I
> don't recommend this.)
> Fluorescent blacklights are worse, but are usually comparable to sunlight when you are a couple
> feet away. Although I have yet to hear of eye damage blamed on reasonable use of blacklights,
> those who are around them a lot may want UV blocking eyewear.
>
> Persons who have had cataract surgery need extra protection, since the lens of the eye largely
> absorbs UV. The retina does not like strong exposure to UV.
>
> It should be noted that UVA-related eye damage involves a process with at least two steps. The first
> step (excitation of tryptophan or related molecules) normally reverses itself. Damage requires
> excited molecules to receive additional UV before they de-excite. This means that UVA eye damage
> is not linearly proportional to the intensity of exposure, although it may be proportional to the
> duration of exposure of a given intensity.
>
> I have yet to see data indicating clearly what is and is not safe. However, nuclear cataracts are most
> common in people who work outdoors in sunny areas. In case of doubt, use UV blocking eyewear.
> Cheaper amber and yellow sunglasses do actually work - I have tested them. Sunglass lenses made
> of real glass and green in color attenuate UV a fair amount. Clear, blue, purple, or pink glasses may
> not work, even if they are claimed to work. I saw one pair of clear glasses with a sticker claiming
> "100 percent UV blocking" which let through more UV than window glass did.
>
> UVB is worse on eyes than UVA, and can burn the cornea and the conjunctiva. UVC is especially
> burning to the conjunctiva. Please beware that symptoms are delayed considerably from exposure.
> Symptoms typically begin a few minutes after exposure and gradually increase for at least a half
> hour, often an hour to a few hours after exposure. If you have been exposed and any symptoms
> begin, promptly seek emergency medical care to minimize damage.
>
> SOME OTHER HAZARDS
> ------------------
>
> (Not complete)
>
> Prolonged, heavy UV exposure causes some plastics to turn a brownish yellow color. A few of
> these plastics are also affected (to a lesser extent) by shorter visible violet wavelengths such as
> mercury's 404.7 nM deep violet line.
>
> SOME WAYS TO REDUCE UV HAZARDS
> ------------------------------
>
> UVB and UVC, as well as the shortest UVA wavelengths, do not go through ordinary glass. Even
> Pyrex and other more ordinary forms of heat-resistant glass can be used as shields to block UVB
> and UVC.
>
> Another option is to contain all light from the suspect source. For example, EPROM erasers often
> have an interlock switch to prevent operation with the lamp exposed.
>
> In the event you need something transparant to visible light (and most infrared) but which blocks all
> UVC, UVB, and well over 95 percent of UVA, use a GAM ("Great American Market") number
> 1510 (UV shield) filter gel. This is available from some theatrical supply shops. If very severe
> exposure is anticipated, the filter gel should be preceded by glass.
>
Check out http://www.misty.com/~don/light.html for this and lots of
other light source info.
Walt
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