Sunscreen (also known as sunblock) is a lotion, spray or other topical product that helps protect the skin from the sun's UV radiation, and which reduces sunburn and other skin damage, ultimately leading to a lower risk of skin cancer. The best sunscreens protect against both UV-B rays which can cause sunburn, and UV-A rays which damage the skin with more long-term effects such as premature skin aging.
The SPF (sun protection factor) numbers on a sunscreen package can range from as low as 2 to as high as 60. These numbers refer to the product's ability to screen or block out the sun's burning rays. The higher the SPF, the more protection a sunscreen offers. For example, a fair-skinned person who would normally turn red after 10 minutes in the sun has 10 minutes as their "initial burning time." If that person uses a sunscreen with SPF 2, it takes 20 minutes in the sun for that person's skin to turn red. Now, if that person uses a sunscreen with SPF 15, it multiplies the initial burning time by 15, so it takes 150 minutes, or 2 1/2 hours, for that person's skin to turn red.
Sunscreens with SPF of 15 or higher are generally thought to provide useful protection from the sun's harmful rays. Using sunscreen every day is important, and it should be reapplied often. For maximum effect, generously apply sunscreen 20 to 30 minutes before going outside. If swimming or perspiring, you will need to reapply sunscreen more often. And remember that sunscreen usually rubs off when you towel yourself dry.
Products labeled "waterproof" provide protection for at least 80 minutes even when swimming or sweating. Products that are "water resistant" may provide protection for only 40 minutes. Most sunscreen products expire within two-to-three years, but you should check the expiration date on the container for the date it becomes ineffective.
The use of sunscreen on a daily basis is a good practice for reducing skin cancer risk. The problem is that it can sometimes give a false sense of security. Sunscreen needs to be reapplied often, and even then it only offers a certain amount of protection.
Mistakes in sunscreen use are common and may indicate that people don't understand the importance of protecting themselves from skin cancer. Visible symptoms of skin cancer don't show up for many years. A sunburn will fade in a few days - out of sight, out of mind - but what you no longer see can hurt you. Sun damage remains in the deep layers of the skin. It builds up over time and can eventually cause cancer.
Sunscreen should not be used as a means of staying out of the sun longer, but as a way to reduce the risk of sun damage to the skin when exposure to the summer sun is unavoidable.