Since we're taking a look at all things sun-care this week, lets address - again! - one of the greatest beauty myths there is.
Namely, that you can add up your protection factors to achieve an overall level of SPF.
The myth goes something a little like this. "So, if my foundation has SFP15 and my moisturiser has SPF10 then BINGO! I must be getting a total of SPF25!"
Well, no. It just don't work like that, sister.
Let me draw you a simple, yet very illustrative analogy. If you go to the pub and order two pints of 4% strength beer and pour them into a larger container, what do you have? Unfortunately, by the very power of pouring and mixing divine intervention has not occurred and it has not suddenly transformed into a beer of 8% strength. No, what you have now is merely two pints volume at - you guessed it - 4% strength.
And it is exactly the same with sunscreen - you're only protected to the highest level you applied (i.e. SPF15 in your foundation), not the aggregate of the lot.