wrote:
In September, Eric Greenwell started a thread on UV exposure and how
much canopies absorbed. I bought one of the Oregon Scientific units he
recommended (Amazon has a good price, roughly $20, esp if you buy
something else to get over $25 for free shipping.

I have not yet had a chance to try it out under my canopy, but I just
did a test under a white cotton T shirt. The SPF factor seems to be
almost infinite! Pointed at the sun, the meter said 28 minutes was the
safe exposure time for the settings I have on it (SPF=1, moderate
skin). Put under my T shirt, pointed in the same direction, and pulling
the shirt tight to make it as transluscent as possible (as in moving
around) gave a reading of 3959 minutes which is the maximum it will
display. The SPF based on these readings seems to be well over 100, not
the 10 that someone else suggested on that thread. The shirt is
laundered normally, without the special product that poster mentioned
to increase SPF.

I realize this is just one reading, and there may be other factors
(comments please), but the reading is consistent with my experience.
When flying, I use SPF 30 sun screen with titanium or magnesium dioxide
(the only kind my dermatologist says really works) on all exposed
areas, but do not apply it under my T shirt except around the edges,
where it will move and provide partial exposure. I never get even tan
where the shirt covers in spite of no sun screen.

I noticed the same things Martin discusses, and also the much reduced
exposure under almost any tree. The limited research I did on the
sensors used in the UV monitors suggested they do measure according to
the UV Index definition, but my knowledge of the sensors is small. I
suspect the discrepancy between the meter reading and the warnings one
usually sees are due to (perhaps excess) caution on the part of public
officials and others who prefer to err on the safe side.

--
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