This is a great question, and thank-you for asking it. The first thing you need to consider is that there are SO MANY SPF’s on the market… how do you even decide what brand to purchase? Let me give you some tips on what to look for in an SPF and this should give you a good idea on how to choose your SPF. Firstly, many of the sun blocks and SPF products on the shelves are NOT Broad Spectrum SPF’s, so they do not fully protect you from the sun. So these products still allow a fair of the sun’s rays to strike and damage your skin. So always look for products that state “Broad Spectrum SPF”. Secondly, how to choose your SPF rating depends on how quickly your skin burns when you go into the sun. So for example if it takes my skin 10mins to start feeling the sensitivity caused by the sun, that is considered to be my “burn time”. I take my burn time and multiply that by the SPF rating, and that gives me a rough estimate of how long that product is able to protect me. So if I apply an SPF 10, and my burn time is 10mins, then that SPF will protect me for approximately 100mins. I personally tell all my patients not to wear anything less than an SPF 30, broad spectrum product. Some good ones include, Skin Medica’s SPF’s and Heliocare.