Questions about SPF, please answer!?

Do higher SPf ratings really matter? I mean,from the same brand, and you put SPf 30 on one arm and SPf 45 or 50 on the other, does the arm applied with SPF 45 or 50 get less sunburned?





Or from different brands and you apply the same SPF rating. Would the effects differ?





Also, if they say that SPF 15 blocks 93 percent of the sun's rays and SPF 30 blocks 97 percent...does that mean SPF 35 might block 98-99%??? Of course, there is no 100% efficient sun blocks due to factors like activity in the water, sweating and such.





Do these percentages apply only to fair-skinned people and not to dark skinned-people?





So, what percentage does SPF 45 and higher block? It's way above SPF 35 which might be 98-99% efficient. Is it just a marketing ploy? Please cite evidences, proof or data that it is or it isn't.





Lastly, how does the FDA test the efficiency of sunblock? On what do they test it?





THANKS IN ADVANCE!!!
Questions about SPF, please answer!?
Sunblock protects your skin by absorbing and/or reflecting UVA and UVB radiation. All sunblocks have a Sun Protection Factor (SPF) rating. The SPF rating indicates HOW LONG a sunscreen remains effective on the skin. A user can determine how long their sunblock will be effective by multiplying the SPF factor by the length of time it takes for him or her to suffer a burn without sunscreen.





For instance, if you normally develop a sunburn in 10 minutes without wearing a sunscreen, a sunscreen with an SPF of 15 will protect you for 150 minutes (10 minutes multiplied by the SPF of 15). Although sunscreen use helps minimize sun damage, no sunscreen completely blocks all wavelengths of UV light.





The American Association of Dermatology (AAD) recommends that a "broad spectrum" sunblock with an SPF of at least 15 that is applied daily to all sun exposed areas, then reapplied every two hours. However, in some recent clinical trials, sunblocks with SPF 30 provided significantly better protection than sunblocks with SPF15. Therefore at UCSF, they recommend sunblocks with SPF of at least 30 with frequent reapplication.
Questions about SPF, please answer!?
there are many different types of rays that those sun blockers block . . Just keep in mind that applying it once does no good . the higher the SPF the better but see if you can find some that protect from the different kinds .And follow the directions ..there is an explanation of the two main types of rays below.. follow the link
Reply:No sunblock will block 100% of the UV rays. I only use an SPF 30 and like you say, an SPF 30 blockes out about 97% of the sun rays and the most you can block out is 99% so it's plenty of protection. But you must apply regularly, every 2 hours to maintain that level of protection.