Question: How much SPF do I need to use in the summer? My favorite foundation has a 15SPF which I know is pretty low. If I also use a moisturizer with a 15SPF will that bring my protection to a 30 SPF?
Answer: You’re righ,t a 15 SPF is pretty low, but adding another product with a 15 SPF will not give you a 30SPF. Its not fair but the math on sunscren products are not cumulative. Your final SPF will only be the highest SPF you are using. In this case, the final SPF will still be 15. If you use a moisturizer with a 30SPF you final SPF will be able to face the sun with a very respectable 30 SPF.
Please keep in mind that the SPF numbers are just an indication of how long you can stay in the sun without burning. A 30 SPF means that you can stay in the sun for 30 minutes without sundamage. However these calculations were done in the laboratory and made on the assumption that at least two teaspoons of sunblock is used on the face and 1-2 tablespoons are used on the body. I decided to put these standards to the test and the results were both funny and troubling. Clearly few if anyone actually use the prescribed amount of sunblock and we often fail to reapply during the day. This means that the SPF which we have to face the sun is much lower than we think. For that reason I recommend using at least an SF30 and in the summer aim for a 50 SPF. During the day I dust on a mineral sunscreen powder like Brush On Block by Susan Posnick. I can apply it over foundation to boost sun protection.
One final thought. If you have sensitive skin, rosacea, psoriasis you might find chemical sunscreens too irritating. Look for broad spectrum physical sunscreens that get their sun block powers from minerals like zinc oxide or titanium dioxide.