Question

what would happen if our oxygen was decreased by 50%?

Answer 1

we could die

Answer 2

yep, lot more light headedness etc. We would probably all have to shift to lower altitudes to survive as that's where the majority of the oxygen is. People living in higher altitudes such as the himalayas would have a higher chance of survival, since they are better adapted to living in an environment with lower oxygen.

In essence though, it would make life as we know it unsustainable. And we would all die in most probability if it is a sudden change especially.