Mountaineers at altitudes higher than 12,000 ft (3658 m) are subject to retinal hemorrhages, probably 
secondary to hypoxic vasodilatation combined with sudden rises in intravascular pressures. The 
hemorrhages usually resolve spontaneously with return of normal visual acuity on return of the 
climber to lower altitude. The prevention of snow blindness is essential. Mountaineering sunglasses 
or goggles should filter out at least 90% of wavelengths below 400 nm and be designed to block most 
reflected light coming from the sides and below. Erythropsia (vision that is temporarily tinged red) is 
due to retinal overexposure to ultraviolet light and eliminated by the use of ultraviolet light–absorbing 
glasses.