The sun protection factor is the ratio of the time required to produce redness through a sunscreen product to the time required to produce the same degree of redness without the sunscreen. All sunscreens should be reapplied after swimming or sweating. Patients should be warned, however, that the application will not extend the period of protection. Many chemical sunscreens can themselves cause reactions with sunlight and the skin. Occasionally some patients are allergic to the chemicals within the sunscreen.
Fresh snow is a very effective reflector as are white painted surfaces or sheets of aluminum.
Reflection from water is variable but unless the sun is directly overhead, the reflection from water is only a small percentage of ultraviolet radiation. Reflected light can be especially damaging because it may strike the skin in previously unexposed areas. On a bright, clear day, redness of the skin can develop due to reflected and scattered sunlight, even in the shade or under an umbrella or a hat, particularly near snow or white sand. However, since the skin may be cooler on a cloudy day, some people may not feel hot and therefore may over-extend their exposure to the sun. This means that you can get sunburned while in a pool.