1. Broad Spectrum
According to the American Academy of Dermatology, a broad spectrum sunscreen is best. That means, the sunscreen protects against both UVA and UVB rays, both of which can cause skin cancer.
2. SPF 30-50
SPF 30 protects you from 97% of UVB rays. SPF 50 protects you from 98% of UVB rays. Anything higher than SPF 50 is misleading.
3. (Very) Water Resistant
In the US, sunscreen can be water resistant (good for 40 minutes) or very water resistant (good for 80 minutes), as tested by the FDA.