While all glasses are susceptible to heat damage, plastic frames are more vulnerable than metal frames. Heat is what softens the frames, allowing us to adjust them to fit properly. When left in a hot car, especially if they are squeezed into a tight fitting case, the heat can warp the temples (earpieces), flatten the frame front, loosen the embedded hinges and/or expand the frame making the lenses loose.
Lenses are damaged by the heat too. The Polaroid film used in polarized sunglasses can be damaged by excessive heat. Expansion and contraction of lenses experiencing temperature extremes can cause warping of the lenses resulting in poor optical quality. Anti-reflective and scratch resistant lens coatings can also expand at a different rate than the base lens material causing the coatings to crack or ‘craze’. While newer premium coatings are more tolerant of heat, even those can fail when exposed to the extreme heat.