Introduction: Sun lotions have not been effective in preventing the alarming increase in the incidence of melanoma, possibly, because of incorrect use, lack of protection from UVA and increased exposure times caused by reduced penetration of erythemogenic UVB. Seeking shade prevents deleterious effects from the sun's ultraviolet rays.
Material and methods: We examined the association between UV intensity and the ratio of shadow length to object height. A linear model fitted the data (R = 0.95). With an astronomical navigation equation for the calculation of altitude of the sun and substituting altitude with UV intensity we developed a model predicting the UV load from the point of observation, the time of the year (declination) and the time of day (hour angle). Accumulated UV doses can be calculated by integration.
Results: When the shadow to object ratio is 0.5, skin types I and II will experience erythema after a few minutes, when the ratio is 1 erythema appears after 20-30 minutes, and when the ratio is 2 erythema appears after about one hour. In the Mediterranean, accumulated UV exposure from sunrise to sunset in late summer will be 30-50 times the minimal erythema dose (skin types I + II) of which more than 60% is in the interval where the shadow is shorter than its object.
Discussion: The shadow rule (short shadows--seek shade) is simple and universal and children can be taught it. A rule of thumb is: when shadows are shorter than objects throwing them, avoid direct sunlight, when shadow and object are of equal length restrict sun exposure to half an hour, and when shadows are twice the length of objects an hour in the sun is permissible.