Question: If I put on sunscreen in the morning, am I still covered for my walk to lunch from work four hours later?
Answer: I talk about sunscreen all day with my patients. It's important to apply sunscreen on a daily basis; probably the best time is when you wake up in the morning right after your shower. I tell my patients they should apply sunscreen to the face, the neck, the upper chest and the arms, because those are the most frequently exposed areas to the sun.
I also tell my patients if they're going to exercise at lunch or walk to lunch with friends and they're outside they need to reapply sunscreen. Sunscreen only lasts about two hours after application and needs to be reapplied in sufficient quantities, probably about the size of a quarter onto the hand and apply that dollop or quarter onto the face for adequate protection.
An SPF of 30 is probably the magic number you need to know and also know that you can protect your skin with clothing: hats, umbrellas and tightly woven clothing.