Various methods and apparatus for determining an ability of a region of skin to withstand exposure to harmful radiation, as well as a safe exposure time of the skin to harmful radiation, are disclosed. In various embodiments, methods and apparatus according to the invention provide real time information about the cumulative effects of exposure to harmful radiation, such as ultraviolet radiation, based on an individual's skin type, or "natural" protection factor, and the actual effectiveness of any sunscreen applied on the individual's skin, independently of the labeled sun protection factor (SPF) of the sunscreen. In one embodiment, the invention periodically measures the individual's skin darkness (natural protection factor), the dynamic actual effectiveness of sunscreen, if applied, and a total exposure level of the skin to potentially harmful radiation, and determines a safe exposure time of the skin to the radiation. The safe exposure time may be continuously updated based on changes in skin darkness, sunscreen efficiency, and total exposure level over time.