SPF

— For the rest of us, each day’s variable solar conditions require a judgment call. And probably sunscreen, whose SPF (sun protection factor) numbers reveal how effective the product is. A lotion with an SPF of 10, for example, tells us that if current conditions would give you a burn in one hour, the sunscreen will forestall the burn for ten hours. Most sunscreens use either titanium dioxide or zinc oxide, chemicals that reflect, scatter, or absorb ultraviolet light and dissipate it as heat. In practice, creams labeled SPF 30 provide 96 percent of the protection of an SPF 90 product, which suggests that you’ll do fine with a 30 as long as you keep reapplying it as needed during the day.