What exactly is SPF? The letters stand for Sun Protection Factor, and tell you how long you can stay in the sun before burning. Sunscreens with SPF will delay the onset of burns, but not protect completely. Some sunblocks are listed as "broad spectrum" which indicates that they protect from both UVA and UVB rays.
It is important to know the UV index number for the day. If the number is low or moderate (from 2-5) you should wear sunscreen with at least an SPF of 15. An SPF of 15 will filter out around 90% of the UVB (Ultra Violet Burning) rays. If the number is high (6 or higher), you should wear a minimum of SPF 30. You should also avoid direct sun exposure between the hours of 10 am and 2 pm, when the sun is the strongest.
UVB rays only have the strength to penetrate the top layer of your skin, called the epidermis. UVA (Ultra Violet Aging) rays have the strength to penetrate to the second layer of your skin (the dermis). UVA rays accelerate the aging process.
Prolonged sun exposure can have serious effects on your skin. Some include: 1) changes in pigmentation (brown spots or white spots). Brown spots are a sign of too much melanin in your skin. White spots show no color or pigmentation, which produces irreversible white spots.
2) premature wrinkles called photo aging. UVA rays crack the collagen and cause loss of volume in the dermal layer of your skin, causing the epidermal layer to hang off your body.
3) leather looking skin. The epidermis thickens over time to protect itself from the sun's radiation. The texture of your skin can be rough to the touch.
4) skin cancer. There are several different types of skin cancer, and the fastest growing type is skin cancer.
5) cataracts. The clear lens in your eye can turn cloudy to try and protect itself from repeated exposure to sunlight. It is important to remember to wear UV filtered sun glasses.
Most lotions are a combination of both sunblock and sunscreen. So what is the difference?
Sunblock contains ingredients that reflect and scatter the UVB light and acts as a wall between your skin and the sun. They sit on top of your skin and do not protect against UVA rays.
Sunscreens protect your skin by absorbing and reflecting UV rays and allowing a certain range of UV light to be absorbed into the skin. Look for sunscreens with "broad spectrum" listed on the bottle. They will help protect against both UVA and UVB rays, so you get a little more protection.
Here is the formula used to determine the "time to burn": SPF Number x Time to Burn Without Protection = Time to Burn with Protection
For example, if it takes 10 minutes for your skin to burn without sunscreen, an SPF 15 product will give you 150 minutes in the sun until you will burn. However, this is a very rough guide because the sun's intensity changes throughout the day. Therefore, to be conservative, your 'time to burn' estimate should be calculated when the sun is most extreme.
Important items to remember: Sunscreen CANNOT be reapplied on top of old sunscreen - sunscreen is absorbed into the skin, so applying new sunscreen on top of old sunscreen will not be absorbed properly.
Sunblock CAN be reapplied on top of old sunblock, since it sits on top of your skin and acts as a barrier.
SPF/UPF 15 provides protection against 92% of UV rays SPF/UPF 30 provides protection against 97% of UV rays SPF/UPF 40 provides protection against 97.5% of UV rays
Experts recommend anything above SPF/UPF 15. After that, choosing a rating should be based on how long you expect to stay in the sun and how quickly you get burned. If you're planning to go in the pool for an hour or two, an SPF 15 will do, but if you're out in the field working for long hours or on an outdoor expedition, SPF 40 or 50+ would be the best.