Polycarbonate (the likely window material) has good transmission in the visible range but very poor transmission in the ultraviolet. An increased cellular damage rate at altitude, if true, would be due to more exposure to x-rays, gamma rays, and bombardment from energetic particles in the solar wind (hyrdogen and helium nuclei). A countermeasure would need to be focused against those sources. Something like sunscreen designed to block UV would offer no advantage.