Sunscreen can protect you from damage to your skin. Although you do need some sunlight for overall health, too much can cause problems. The sun’s ultraviolet rays darken the melanin in your skin to produce a tan, but excessive sun exposure can result in sunburn and skin damage. The sun protection factor, or SPF, tells you how much protection a particular sunscreen provides.
Melanin and Sun Exposure
People respond differently to sun exposure. Some may be able to stay out in the sun for an hour or so without burning, while others will develop a burn within as little as 15 minutes. The amount of melanin in your skin is a good indication of how long you can be exposed without burning -- if you are very fair-skinned you need more protection. Dark-skinned people are more likely to tan instead of burn and may be able to use a lower SPF.
The Minimum SPF
A sunscreen’s SPF rating provides an indicator of the level of protection you will have while wearing it. If you can stay in the sun for 15 minutes without burning, an SPF of four should increase your time to one hour. An SPF of eight would allow you two hours of sun time. The minimum SPF anyone should use, however, is an SPF of 15, according to both the Virginia Department of Game and Inland Fisheries and the American Council on Exercise.
Other Factors
Other factors that can affect the SPF you need include how long you expect to be in the sun, the time of day, the altitude and any medications you may be taking. If you burn easily or expect to be in the sun for more than an hour, you need a higher SPF. The sun is stronger between 10 a.m. and 4 p.m. Daylight Savings Time or 9 a.m. to 3 p.m. Standard Time, and at higher altitudes. Some medications, such as antihistamines, estrogens, anti-nausea drugs, tranquilizers and nonsteroidal anti-inflammatory drugs can make you more sensitive to the sun.
Extended Periods Outside
If you are going to be outside for extended periods, Recreational Equipment Incorporated recommends you use a broad-spectrum sunscreen with an SPF of at least 30. REI says a rating of 30 means the sunscreen was able to absorb 97 percent of ultra-violet B rays in laboratory testing. UVB rays are the kind that cause sunburns and can contribute to skin cancer. An SPF of 50 blocks 98 percent of UVB rays, according to REI.
Calculating the SPF
To calculate the SPF you need, multiply the SPF by the number of minutes you expect to be in the sun and divide by 60 to determine hours of protection. The minimum SPF is 15, so if you expect to be in the sun for five hours, an SPF of 15 will protect you for that length of time. If you are very fair and burn easily, choose a higher SPF. REI recommends you apply sunscreen at least 15 minutes before you go outside. Reapply sunscreen every two hours, after swimming or if you perspire heavily.
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