Sun Protection Factor (SPF) is a measure of how effective a sunscreen product is at blocking one particular type of ultraviolet (UV) light from the sun. The numbering system of SPF is a multiple of the amount of time it normally takes you to develop sunburn. SPF 15, therefore, is a cream that can allow you to stay outside 15 times longer than usual before sunburn starts. Although a cream with an SPF is partially protective, it offers no advantages in protection from certain other forms of ultraviolet light.
Generally, a person with pale skin may begin to burn after about 15 minutes in strong sunlight. Those with darker skin are more resistant to sunburn, and usually take longer to burn. Damaged skin, such as skin cells exposed to sunburn, can potentially lead to skin cancers later in life; most doctors strongly recommend that people avoid getting sunburns. Sunscreen is one option that allows people to go out in the sun without having to cover up, although doctors also recommend that everyone covers up as well, with clothing such as hats and long-sleeved tops.
Two different types of ultraviolet light can cause damaging changes to skin cells. The type that SPF blocks is UVB light, which is implicated in the development of sunburn. This is because the UVB wavelength is short and dissipates its energy in the top layers of skin. Although a type called UVC exists in nature, it does not pass through the earth's atmosphere to reach the surface.
Ultraviolet A (UVA) is another form of light that can damage skin. It has a longer wavelength than UVB, and can get into the lower layers of the skin. Skin aging is associated with this type of ultraviolet, and it is not blocked by an SPF cream. Instead, creams with components that can block this variety of UV may be represented by another symbol, such as a star rating out of four.
Each person differs in the amount of time it takes them to become sunburned under the midday sun. With a sunscreen of SPF 15, however, each person can stay out 15 times longer than usual before their skin becomes damaged through sunburn. If a person normally gets burnt after 15 minutes, then putting on this cream can allow him or her to stay outside for 15 x 15 minutes, which is 3.75 hours.
Of course, each cream only lasts for a specific period of time. Even with an SPF 15 applied, a sunbather has to reapply the cream every two hours, or he or she may burn. In addition, skimping on the cream means that the skin is insufficiently protected, which can also lead to inadvertent burns. Swimming and showering can also remove an SPF 15 cream, so it needs to be reapplied after.