Is Sunscreen SPF Additive Amongst Products

Dr. Neal Schultz

I have a question regarding this video, and I was hoping you can enlighten me with your wise knowledge!

You mentioned that the spf protection is only related to the FIRST product that is put on the skin, and not any other products layered on top. However, I have been advised by others to use first the CHEMICAL suncreen, then the PHYSICAL sunscreen. The notion was that the physical sunscreen can form sort of a barrier akin to a metal shield that can help protect my skin, while the chemical sunscreen can also protect any further damage. In short the combination of the two sunscreens will increase my protection to sun damage.

What are your feelings of layering the two types of sunscreen together?

On another note, will Vitamin C products help reverse signs of aging, and why is it that it must be used at night? Also, I'm thinking of making do-it-yourself vitamin C serum, is ferulic acid powder, and hyaluronic serum two good addition ingredients useful in the vitamin c serum recipe?

@Sandy: These are terrific questions. The first one (re: chemical plus physical sunscreens) is a lot of fun because while academically interesting, from a practical point of view it may not make that much of a difference. Yes, the first product on your skin (in this case the chemical sunscreen) is the one that can deliver the promised sun protection (i.e., rated spf). And, it is true that regardless of where the physical sunscreen is in order of application, it can be of some help (if it's applied first it provides it's full rated protection and if applied on top of other products it provides some protection but not its full rating)...but the spf's still are not additive. Intuitively, I agree that the physical "shield" sunscreen should add some additional protection, but all sunscreens when tested, are tested without other products being applied. The problem with applying both is how do we know that there is not a possible chemical incompatibility between the particular chemical sunscreen you use and the particular physical sunscreen you use that will actually results in less protection? (A great idea for a research project!) I don't know if that can happen but I'll bet no one has ever tested it so who knows?

I believe that Sunscreens work best when applied alone. And from a practical perspective, if you are sufficiently motivated to use two layers of sunscreen, then I'll bet one of them is at least an spf 30 which gives you at least 95% protection. And if you want or feel you need a higher % protection, use a 45 ,50 or 60 (versus pairing a chemical and a physical sunscreen). I professionally don't know what the extra 3 or 4% protection translates to for your skin in real life (i.e., when you go from an spf of 30 to 60) but I do know that the way to really get the rated spf protection is more a function of properly and frequently reapplying the sunscreen (see How to Properly Apply Sunscreen) then combining it with other forms. I think people should pick one sunscreen (with spf of at least 15 to 30) that they like the feeling of when it's applied so there is no disincentive to reapplying it, and stick with that single sunscreen.

Regarding your Vitamin C question, the only signs of aging Vitamin C products help reverse is brown pigmentation from the sun. The real value of Vitamin C is in helping to actually prevent photoaging and skin cancer caused by the sun. It must be applied at night so it can be absorbed into the skin to work before it is inactivated by the sun. If applied during the day, the sun destroys it before it can be absorbed and do it's magic inside the skin cell (see How Antioxidants Protection Your Skin). There's nothing wrong with trying to make your own topical Vitamin C product…the Hyaluronic Serum should be fine but I would leave out the Ferulic Powder as I don't know whether that will adversely affect whatever form of Vitamin C you are using.

Thanks again for your great questions!