Firstly to ensure your sunscreen protects against both UV rays, you should choose a broad spectrum sunscreen. Secondly, you need to consider the sunscreens SPF. SPF stands for sun protection factor. SPF will determine how long the sunscreen will protect you against UVB rays (not both rays it should be noted). SPF 15 to 50 should be adequate depending on your skin type. Sunscreen can wash off with sweat and while swimming. If you are planning on being wet you may benefit from a water-resistant sunscreen.