Eric,
While I agree that a good quality clear canopy (eg
mecaplex) gives a gratifying amount of UV protection
I think that your sources under-estimate the risks
of UVA and the implications of a tan.
As regards tanning, to quote the current opinion of
the British Association of dermatlogists:
'There is no such thing as a safe or healthy tan. A
tan is a sign that already damaged skin is trying to
protect itself from further damage. The protecting
power of a tan is weaker than that of a mild sunscreen
of SPF 2-4.' See:
http://www.bad.org.uk/public/cancer/
UVA doesn't burn the skin as quickly as UVB but it
is much more deeply penetrating and the trend of medical
opinion in recent years has been towards an increasing
appreciation its cancer causing potential. UVA penetrates
cloud cover and light clothing.
I don't know whether UVA tanning beds are used a lot
in the US but in the UK they are very popular and manufacturers
and tanning studios propogate a great deal of misinformation
about 'bad' UVB , 'harmless' UVA and 'safe' tanning.
The message I take from your very helpful finding that
quite a bit of the longer wavelengths of UVA gets through
canopies is the opposite from your conclusion. To
me it emphasises importance of using UVA protective
sunscreens and clothing when flying.
John Galloway
At 23:30 29 September 2006, Eric Greenwell wrote:
I recently bought an Oregon Scientific UV888 Personal
UV Monitor (~$30).
Among other things, it measures the UV flux and computes
the UV Index
(UVI), then determines a 'safe exposure time' based
on your input of
skin type and the SPF of the sunscreen you are using.
Naturally, I made a measurement in the sun (UVI = 8),
then another one
inside the cockpit (UVI = 0), indicating the canopy
provides quite a bit
of protection. Testing other kinds of plastic, like
baggies, plastic
wrap, Lexan, etc., gave readings from 0 to 7, so not
every kind of
plastic is protective.
These measurements made me curious about what the UVI
index is, so I did
some searching and learned a few things:
* The UVA band (315 nm to 400 nm) is the primary
tanning and
wrinkling band
* The UVB band (280 nm to 315 nm) is the primary
skin damage band
* The UVC band is almost entirely blocked by the
atmosphere, so has
essentially no effect
* The UVI is a weighted value that accounts for
the skin's response
to different irradiation frequencies (almost
none to UVA, quite a
bit to UVB)
The most interesting discovery for me is that tanning
can occur without
harmful skin damage (I know, we hate wrinkles, but
they don't kill!), so
getting a tan, such as the tan described by a letter
writer to a recent
Soaring magazine, does not mean you are also damaging
your skin.
Looking at an 'erythemal dose rate' chart, you can
see almost all (99%)
of the dosage in the direct sun occurs below about
330 nm. Since our
canopies block UV below about 360-370 nm, they provide
essentially
complete protection from damaging rays. The fraction
of the UVA that
comes through will still give you some tanning, and
some fabrics will
fade slowly, but your skin is protected from the UVB.
This is very good
news, that the canopy affords even more protection
than I first thought.
So, cover yourself as much as possible (hat, long sleeves,
etc), put the
sunscreen on when you get to the gliderport, do your
rigging and outside
preparation early in the morning, and take shelter
under a wing, under
an umbrella, or inside a building or vehicle (glass
protects, too) while
waiting to take your tow. Breathe easy while you are
flying the glider,
and apply more sunscreen when you land, unless the
sun is low.
This is the main reference I used for my education
and conclusions:
* http://www.aero.jussieu.fr/~sparc/News21/21_Long.html
Other interesting references a
* A simpler version of the main reference (National
Weather Service):
http://tinyurl.com/evfc7
* UV meter:
http://tinyurl.com/f3uuy
--
Eric Greenwell - Washington State, USA
Change 'netto' to 'net' to email me directly
'Transponders in Sailplanes' on the Soaring Safety
Foundation website
www.soaringsafety.org/prevention/articles.html
'A Guide to Self-launching Sailplane Operation' at
www.motorglider.org