I've been in the skincare industry for close to 30 years and I've never been able to get my arms around the sunscreen SPF (sun protection factor) theory relating to UVB. Obviously I know the importance of sunscreen and definitely use it and encourage and promote it. But in my opinion, from an expert point of view the two widely accepted theories contradict one another.

I decided to visit skincancer.org and get some definitive answers once and for all. The website is so comprehensive, informational and the best resource that I've found for information on sun damage and skin cancer and how it relates to over-exposure to UV light. However, when it came to my question regarding the exact meaning of SPF it still made me feel unsure about which definition is right or at least the best way to look at it. I'm not questioning them at all (I know they are correct in both instances, it just simply doesn't make complete sense to me). Without a doubt their team of dermatologists and experts are probably the best in their field but still to me (or the way my brain works) I still don't see how the two theories completely support one another. I don't know why I can't just let it be. I guess because when I give one definition to a reader and then they question it because their understanding was the other; I feel both inept and I would never want to give out such crucial information when I may be wrong.

Here are the two definitions from skincancer.org:

What Is SPF?

Most sunscreens with an SPF of 15 or higher do an excellent job of protecting against UVB. SPF — or Sun Protection Factor — is a measure of a sunscreen's ability to prevent UVB from damaging the skin. Here's how it works:  If it takes 20 minutes for your unprotected skin to start turning red, using an SPF 15 sunscreen theoretically prevents reddening 15 times longer — about five hours.

Another way to look at it is in terms of percentages:  SPF 15 blocks approximately 93 percent of all incoming UVB rays.  SPF 30 blocks 97 percent; and SPF 50 blocks 99 percent.  They may seem like negligible differences, but if you are light-sensitive, or have a history of skin cancer, those extra percentages will make a difference. And as you can see, no sunscreen can block all UV rays.

The former description is the way I've always been taught and the way I've explained it to both my clients and my readers. In other words, by using a factor (which to me and my dense brain denotes a mathmatic formula) you can determine the amount of sun protection by simply using the number on the sunscreen and multiply it by the time it normally takes you to burn. I've always considered it in increments of ten minutes which makes it easy to understand. If you were to be in the sun for ten minutes without any sunscreen at all obviously you have zero protection (except for of course our body's own protection such as melanin) and possibly burn, but if you were to apply sunscreen with an SPF of 15 before you go into the sun you would get 150 minutes (10minutes x 15 SPF) of protection. Of course this is if absolutely none of it was washed or sweated off, etc. I'm talking about an ideal situation just to keep this explanation simple. To me that explanation of what SPF means, makes sense.  However, when you skip to scenario two, they give you percentages of protection by still yet using the SPF number. Here's where it stumps my brain and perhaps one of you can help me out with this.

Ok; here we go. Please don't shoot me!  Now, if we all can agree that the SPF factor is by the amount of time spent in the sun how can they all of a sudden use percentages of blockage? If an SPF is15 and you can stay in the sun 15 times longer and you were indeed in the sun less time than the150 minutes allowed by the factor that seems to me that the efficacy of block is already established. Why then can you say SPF 15 blocks 93%,  SPF 30 blocks 97%, and so on if all things were equal in terms of time? In other words, if you were in the sun the amount of time that the factor recommends why wouldn't they all be considered equal in terms of percentages?

Perhaps it would make sense to me if they used another term for coverage (percentage wise) instead of SPF.  Whew! Am I making any sense? Please help! I'm sure I'm way overthinking this but I'd sure love any input you folks could give me.

Having completely confused you; please always use sunscreen no matter how you interpret the sun protection factor and enjoy the great outdoors! Also, don't forget to apply in the winter and cloudy days as well. The damaging ultraviolet rays don't care if it's clear and 85 degrees or cool and cloudy. The one thing we can all agree on in this skincare industry is, those rays will get ya either way.

The second issue I think we can safely agree on is to use sunscreen that has broad-spectrum protection and has the seal of recommendation by the Skin Cancer Foundation such as DDF Matte Finish Photo-Age Protection.