I used an SPF 50…,by Rutledge Forney, MD

“I read that an SPF 15 is no different than an SPF 30?” “Anything more than an SPF 30 is a waste?” “How often should I reapply sunscreen?”
 
These and other comments greet me everyday. Are they true? Why am I so adamant about using at least SPF 3O everyday and much higher when you are doing outside activities?
It is true that if you are a lab animal and the lab techs are applying your spf product according to standard instructions, using 2 mg per cm² of body surface, then a SPF 15 is very close in protection to a SPF 30. After all, the techs are paid to make sure the lab animals don’t get tan and the lab techs aren’t paying for the sunscreen.
 
But humans apply their own sunscreen. And most of us are, shall we say, thrifty. Very few people apply it generously. In fact, most people apply sunscreen sparingly and put on ¼ to ½ the amount needed to reach the advertised SPF level. So, for most of us, an SPF 15 is actually an SPF 4 to 8 and an SPF 30 is an SPF 8 to 15
An 8 ounce tube of sunscreen should cover your exposed body 8 times, so if you go to the beach for a week the tube should be finished at the end. If you spend more than 80 minutes in the sun, you should reapply so two full days on the beach should finish that 8 oz tube.
 
That is why you burn and tan even if you bring a tube of SPF 50 to the beach. Too little for too long will result in burning and tanning.
The chart below, published by EltaMD©, shows how the different ingredients in their sunscreens absorb different types of harmful sun waves.