Is SPF 15 enough?
In the lab, definitely, but in real life, maybe not. Here's why: In order to get the SPF rating, each sunscreen is tested under a variety of conditions. But the amount of sunscreen applied is always the same, and it tends to be much more than most of us actually use.
According to the preferred testing amounts, you should be using a four-ounce bottle of sunscreen during a single eight-hour day at the beach. Most of us don't use nearly that much. So instead of applying enough to actually reach an SPF 15 rating, we use less and diminish the SPF value. Further, when you perspire or wipe your skin with a towel, you lessen some of the sunscreen's effectiveness.
For this reason, many dermatologists advocate using sunscreen with a higher SPF (30 or above), especially if you are at high risk of skin cancer. Even if you use an SPF 30 sunscreen, it might only carry an SPF of 15, depending on how much is applied.