Why do you not need sun protection whe the UV index is 3?

In Australia, they recommend sun protection when the UV index is above 3.
Why is this? Surely whenever it's above 0 there are still UV rays hitting your skin?
Would 10 minutes in the sun with a UV index of 6 be the same as 20 minutes in the sun with a UV index of 3? If so, surely 10 minutes in a UV 6 index would be bad, and therefore 20 minutes in UV index 3 would also be bad?


