Protection from the sun's harmful rays is an essential consideration for a person who enjoys taking part in outdoor activities. Sunscreen and sunblock are the most common ways to obtain this protection, and we often choose the type of sunscreen we will use based on the sun protection factor (SPF) number on the label. The amount of SPF protection a person needs depends greatly on the nature of the activity he will be engaged in, as well as the weather, the time of day, time of year, and the person's location.
In general, the amount of time it takes for a person's skin to be sunburned is multiplied by the SPF number of the sunscreen he uses. For example, if your skin typically starts to turn red after ten minutes of exposure to direct sunlight, a sunscreen with an SPF number of 15 would ideally increase that time to 150 minutes. Numerous factors can change this number, however. Sunscreen wears off when a person sweats, or comes into contact with water, such as when swimming, for example. Also, if the sunscreen is applied at a time of day when the sun's rays are not yet at their peak intensity, the the sunscreen's full effect will be shortened compared to the time of day when it was applied.
The exact SPF protection a person needs perhaps depends most on the color of the person's skin. A very fair-skinned individual will need a lot of SPF protection to avoid what would otherwise be a very quick and painful sunburn. On the other hand, a person with very dark skin may not need any SPF protection at all to prevent sunburn, because the dark skin pigment constitutes a built-in protection from the sun's ultraviolet rays. Most dermatologists recommend that a person who is susceptible to sunburn should use a product with an SPF number of 15 or higher. They also recommend re-applying the sunscreen every hour, to compensate for environmental effects that can remove it from the skin.
A sunscreen's SPF protection is determined in controlled laboratory conditions, which means that results will vary widely, and this is an important concept to remember. It is best to err on the side of caution when choosing SPF protection -- in other words, going with a higher number as opposed to lower, and regularly reapplying it to ensure complete coverage. Even sunscreens with a high level of SPF protection will not block all ultraviolet rays from the sun, but they will greatly reduce the likelihood of sunburn and will mitigate the risk of diseases such as skin cancer, which result from overexposure to the sun.