India, Oct. 2 -- Walk into any pharmacy in India and most sunscreens on the shelf carry one of two numbers: 30 or 50. The jump looks big, so many people pick up a sunscreen SPF 50 believing it protects almost twice as well. The science tells a more measured story. Knowing the real difference between SPF 30 and SPF 50 helps you choose wisely, especially in a country where the UV index stays in the very high range for much of the year.

The number is only half the decision. Skin type shapes what you will actually wear every day. Someone with an oily T-zone usually does with a gel or fluid sunscreen for oily skin, while a person with tight, flaky cheeks needs a sunscreen for dry skin that adds moisture. If shine by afternoon bothers you, a m...