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SPF numbers reflect how well a product screens out ultraviolet B (UVB) rays from the sun. UVB rays are what cause sunburn and contribute to skin aging and skin cancer. An SPF of 15 means that unprotected skin would burn about 15 times faster than would skin with a thick coating of the product. (It does not mean you can safely stay in the sun 15 times longer.)
However, the SPF rating says nothing about UVA rays, which don't get as much attention, but which can penetrate even deeper than UVB rays and also lead to skin aging and cancer. Some sunscreens do not offer any protection against UVA rays. Look for products with avobenzone, oxybenzone, titanium dioxide, zinc oxide or encampsule on the label, which help block UVA.
SPF numbers can also be highly misleading, since they are compounded number ratings, which means that as you go higher, they sound more impressive than they really are. In other words, since lower SPFs block out most of the sun’s UVB rays anyway, those higher numbers deal only with the increased protection as it relates to that small percentage that is left over. As Dr. Aaron E. Carroll and Dr. Rachel C. Vreeman explain, "there is a 1.6 percent or less difference between sunscreens with an SPF 30, 45, 50, and 60. Sunscreen with SPF 15 blocks approximately 94 percent of all incoming UVB rays. Sunscreens with SPF 30 block 96 percent of the UVB rays. Sunscreens with SPF 40 block 97 percent of the rays. The higher SPFs do block more UV rays, but it is not clear whether they are increasingly effective (in actual use) over the SPF 50 mark." (Carroll & Vreeman, 2009, p. 54) When you buy SPF 100 over SPF 30, you're not actually getting something that's 70 times better - you're getting something that might offer 1% more protection, usually at a much higher price.
Applying sunscreen to yourself and your child
Most people do not use nearly enough sunscreen. As Carroll & Vreeman point out, "Sunscreen SPF is tested using 2 mg. of sunscreen per centimeter squared of your body, which equates to two fingers length of product applied to each of the eleven areas of the body. If .you stay at the beach all day, you'd have to use up almost an entire six-ounce bottle of sunscreen in order to follow this recommendation. Since most people don't even come close to following the official recommendation, the level of protection they receive from their sunscreen is probably half, or less than half, of what is described on the bottle." (ibid, pp. 54-55)
Here are the recommended guidelines for applying sunscreen:
1) You should apply sunscreen to children about a half an hour before your outdoor activities. This gives the lotion ample time to bond with their skin. Ideally, you should then reapply about 20 minutes into your outdoor time - one study found this was more effective than waiting two hours (which is the standard recommendation) and then reapplying every two hours thereafter. If you do
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