It's important to understand that even though you may be using a sunscreen, some UVB rays will still penetrate through your skin. You cannot filter out 100 per cent of the sunís rays.
For example, SPF2 products protect your skin from 50 per cent of the UVB rays, so you can stay in the sun twice as long as you would be able to without protection. If you burn after five minutes without protection, once this SPF is applied you could stay out for ten minutes without burning.
This is just an example as SPF2 is far too low, but here is further clarification:
SPF 8 blocks 87.5 per cent of UVB rays
SPF 15 blocks 93.33 per cent of UVB and some UVA rays
SPF 30 blocks 96.6 per cent of UVB and some UVA rays
SPF 60 blocks 97.98 per cent of UVB and some UVA rays