The more it builds up, the more likely it is that you will develop a malignant tumor. It's important to watch for cancers even if you are avoiding the sun now. Sunscreen doesn't give full protection. It should be the last line of defense, say dermatologists. They should be applied 30 minutes before going out, every two hours thereafter, and always after swimming or sweating heavily.
Avoidance is the best protection.
Use plenty of sunscreen, and don't stay out long. You can get burned through any shirt that you can see through. A wet T-shirt equals an SPF of about 5, or next to nothing. A dry, tightly woven shirt has a rating of about 60. Your head needs protection.