Peninsula Dermatologic Surgery Mohs

321 Middlefield Road, Suite 245
Menlo Park, CA 94025
T 650.326.7222 F 650.326.7332


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
Organic UVB
PABA 15.0 283 260-313 UVB
Padimate O 8.0 311 290-315 UVB
(octyl methoxycinnamates)
7.5 311 280-310 UVB
Cinoxate 3.0 290 270-328 UVB
Octisalate 5.0 307 260-310 UVB
Homosalate 15.0 306 290-315 UVB
Trolamine salicylate 12.0 260-355 269-320 UVB
Octylocrylene 10.0 303 287-323 UVB
sulfonic acid)
4.0 310 290-340 UVB
Organic UVA
Oxybenzone 6.0 290,325 270-350 UVB, UVA2
Sulisobenzone 10.0 366 250-380 UVB, UVA2
Dioxybenzone 3.0 353 206-380 UVB, UVA2
(menthyl anthranilate)
5.0 336 200-380 UVA2
Avobenzone 3.0 360 310-400 UVA1, UVA2
(terephthalydene dicamphor sulfonic acid [Mexoryl SX])
10.0 345 295-390 UVA1,  UVA2