I believe that the increased skin cancer risk for crew has more to do with lifestyle, ie lots of time off and opportunity to spend time outdoors. I and a close family member have health physics backgrounds, and have done the analysis based on known risk tables.
Some specifics...
1) Cosmic radiation at altitude will cause a very slight but noticeable increase in risk of cancers associated with hard radiation exposure. We did the math based on my exposure as a domestic pilot. It would be a bit higher for long-haul due to:
a) Higher cruise altitudes
b) More time spent there (very high percentage of block time at cruise altitude).
c) Commonly cruising at higher latitudes, cosmic radiation gets funneled towards the poles by the earth's magnetic field (ex northern lights).
2) UVB is known to cause skin cancer, especially melanoma. UVB does not really penetrate modern glass windows in a jet.
3) UVA has less risk of skin cancer, but is known to cause aging of your skin. UVA can penetrate our windows, although layers of glass probably cut it down a bit. UVA also contributes to fatigue.
4) Older planes, and GA planes, with plastic windows, don't block much UV at all... you might as well be at the beach. I have a little more noticeable skin aging on my left side, despite being an FO and having spent most of my career as an FO. But I was a CA on a turboprop with poly side windows for a few years...
I use sunblock on my face when flying (daylight). I used to use it on my neck and arms but it turns shirt sleeves and collars yellow. That's mostly so I don't edge up aged more on one side than the other. Can't stop aging, but I'd prefer it be symmetrical
The radiation exposure calculations we did years ago were based on risk tables from the medical community. Newer studies might change that data.