Know Your Numbers

Sun Protection Factor (SPF) is a measure of how much solar energy (UV radiation) is required to produce sunburn on sunscreen-protected skin relative to the amount of solar energy required to produce sunburn on unprotected skin. As the SPF value increases, sunburn protection increases.

Are you at risk? Many people believe that, if they normally get sunburned in an hour, then an SPF 15 sunscreen allows them to stay in the sun for 15 hours (i.e., 15 times longer) without getting sunburned. This is not true, because SPF is not directly related to the amount of time of solar exposure, but to the amount of solar exposure. The intensity of solar impacts the amount of exposure. For example, the following exposures may result in the same amount of solar energy.

  • One hour at 9:00 a.m.
  • 15 minutes at 1:00

In addition to solar intensity, there are a number of other factors that influence the amount of solar energy that a person is exposed to, including:

  • Skin type
  • Amount of sunscreen applied
  • Reapplication frequency (due to time in the sun, perspiration and water exposure)

Source: U.S. Food and Drug Administraition