The continual reapplication of sunscreen is key to its effectiveness in protecting you from skin cancer, explains Ramsay Farah, MD, the chief of dermatology at Upstate. He says sunscreen is important for everyone, no matter how dark their complexion, and addresses concern about the chemicals that may be contained in sunscreens. He advises his patients to seek products with zinc oxide or titanium dioxide, substances that are inert but effective in blocking the sun's rays.