Q: My baby is 4 months old and has fair skin. We're planning to take a spring vacation in Hawaii—is it OK to use sunscreen on him?
A: Applying sunscreen to babies younger than 6 months is generally not recommended because it can be absorbed through their thin skin and nobody knows for certain if it's harmless or not. At this age the best sun protection is to keep your baby in the shade and covered in loose clothing and a hat with a brim to shield his eyes and face.
If you do have to take your baby out in strong sunlight, apply a children's sunscreen with an SPF of 15 to 30 a half-hour beforehand and reapply it every few hours. Preparations that combine a chemical sunscreen (which absorbs UV rays) with a physical sunblock in the form of zinc oxide or titanium dioxide (both block UV rays) have a higher SPF and offer longer-lasting protection. Any brand will do; a more expensive product isn't necessarily better.
You should also know that sun is good for young children in reasonable amounts because it gives them a free dose of vitamin D, so don't keep your baby wrapped up like a mummy all the time.