Bob,

Increased cancer rates in commercial pilots and crew are likely due to
higher exposures to radiation (cosmic rays) at high altitude for long
periods of time rather than UV light. Additionally, when these rays
strike the metal skin of the aircraft, they scatter, changing energy
levels and actually producing more damage to tissue than direct
exposure. In contrast, glider pilots don't usually fly that high, or
for as long, and don't have that metal skin effect to deal with.

As far as the UV exposure goes, it would be helpful to see some
readings from the meter, measured both inside and outside the cockpit.
Typically, clear plexiglass will knock out 50-70% of UV from the sun
depending on thickness, etc. (see http://www.rplastics.com/plexiglass-transmittance.html
for example) Simple plastic sunglasses can be made to absorb harmful
UV rays in the same way. Of course we must still wear protection,
(mostly from our exposure on the ground when putting the bird
together) but we don't want to leave the impression that flying
gliders significantly increases your chances of getting skin cancer.

Matt