Many people believe that sunscreen will prevent skin cancer because it can prevent sunburns. In fact, studies have never been carried out to compare people's risk of skin cancer with their use of sunscreens.
We do know that SPF sunscreens are effective at preventing sunburn and these chemicals block UV-B light (but not UV-A light) from the sun. We also know that most commercially available sunscreens do NOT contain full spectrum UV-A blocking chemicals. In addition, UV-A and UV-B both cause skin cancer, so by using a sunscreen, not all the cancer causing radiation will be blocked.
It has also been known for about 40 years that topical Vitamin C and Vitamin E block both UV-A and UV-B radiation. As with all topical products, perspiring and swimming will remove the active product, so they cannot be solely relied upon for adequate UV protection.
Use of a hat and protective clothing will block most of the UV radiation coming from above. But for incidental radiation reflected from water or sand, the topical vitamins are more effective than commercial sunscreens at full spectrum UV protection. Remember that on cloudy days UV radiation continues to reach the earth's surface, so sun protection should be practiced. In addition, UV radiation is transmitted though glass, so be sure to protect your skin while in the car. Affordable UV-blocking window films are commercially available, which effectively shield passengers from the burning and tanning rays of the sun.