There is no question most skin cancers are related to sun exposure, yet even with sunscreen sales approaching $1 billion a year, skin cancer rates continue to climb. Melanoma diagnoses have risen nearly 2 percent a year since 2000 and are increasing even more among young white women. Some experts blame inappropriate use of sunscreen, saying that people do not apply enough lotion (a golfball-size dollop) or do not reapply it every two hours as instructed. But there’s another major concern: Until recently, many sunscreens with a high sun protection factor, or SPF, were designed primarily to protect people from ultraviolet B rays, the main cause of sunburn. These sunscreens may have enabled users to stay out longer but did not necessarily protect them from ultraviolet A rays. These are associated with aging and skin damage, but some experts believe they may also be implicated in skin cancer. Experts are urging people to limit their time in the sun, especially at midday, and protect their skin with hats, shirts and cover-ups instead of relying exclusively on sunscreen.