SPF (Sun Protection Factor) – tells you how LONG a sunscreen is effective against UVB rays on the skin. For example, if you went outside without sunscreen and it took 10 minutes for you to develop a sunburn, then a sunscreen with an SPF of 15 will protect you for 150 minutes (10 minutes multiplied by the SPF of 15). Currently there is not an “SPF” for UVA rays. The FDA is developing rating system for UVA blockage and we should be seeing it on sunscreen labels soon.
The American Association of Dermatology recommends a "broad spectrum" sunscreen with an SPF of at least 30.
BROAD SPECTRUM means that it blocks both UVA and UVB rays. Scientists have long thought that UVB radiation causes skin cancer, but recent studies suggest that UVA rays can also induce cancer and cause photoaging (brown spots and wrinkles). UVB rays are more likely than UVA to cause sunburn, but UVA rays penetrate deeper into the skin. UVA radiation also comes from tanning beds and sun lamps. Also important, unlike UVB rays, UVA radiation is not blocked by ordinary window glass.
CHEMICAL SUNSCREENS (for example Avobenzone)- work by absorbing the energy of UV radiation before it affects your skin. Unlike physical sunscreens, no single chemical ingredient blocks the entire UV spectrum, thus multiple chemicals are combined to produce “broad spectrum” protection.
PHYSICAL SUNSCREENS (for example zinc oxide, titanium dioxide) -reflect or scatter UV radiation before it reaches your skin. They provide broad spectrum UVA and UVB protection. Because these are not chemicals, they rarely cause skin irritation and are good for people with “sensitive skin.”
WATER-RESISTANT means the sunscreen maintains the SPF level after 40 minutes of water exposure.
VERY WATER-RESISTANT means it maintains the SPF level after 80 minutes of water exposure.
Table II from Sambandan D and Ranter D. Sunscreens: An Overview and Update J Am Acad Dermatol 64(4): 748-58. FDA-approved (*as of Dec 7, 2009) Active Sunscreen Ingredients.
|FDA-approved active sunscreen ingredient||Maximum FDA-approved concentration (%)||Peak absorp. wavelength (nm)||Range of protection (nm)||Protection provided (UVB/UVA)|
|Titanium dioxide||25.0||Varies||290-350||UVB, UVA2|
|Zinc oxide||25.0||Varies||290-400||UVB, UVA1|
(terephthalydene dicamphor sulfonic acid [Mexoryl SX])