Sunlight includes two types of harmful rays – UVA and UVB. Exposure to either type can cause skin cancer. UVA rays are the ones primarily responsible for wrinkles and dark spots while UVB rays primarily cause sunburn. It is notable that UVA rays can pass through windows, while UVB rays are blocked by glass. This is why dermatologists often see more sun damage on the left side of drivers. Both a sunburn and a sun tan are an indication of sun damage.
The SPF value indicates the level of protection from UVB rays only. Not all sunscreen ingredients are created equal. Many of the ingredients used to block the UVA rays actually break down in sunlight. Therefore, you have to read the active ingredient list to find a sunscreen with good UVA coverage. Physical blockers like titanium dioxide and zinc oxide are great at protecting you from both UVA and UVB rays. Additional ingredients that have more stable UVA coverage include Mexoryl or ecamsule (trademarked by La Roche-Posay) and Helioplex (trademarked by Neutrogena). All sunscreens should be reapplied every two hours.