Why is O3 beneficial in the upper atmosphere but not in the lower atmosphere?

Answer 1

It provides a barrier to the radiation from space (i.e., the ozone layer).

However it is poisonous to humans in large quantities.

Answer 2

Ground-level ozone is an air pollutant.It is the key component to smog.It can interfere with the immune system and it can change cholestrol to plague(which causes hardening and narrowing of arteries) in the blood stream.It is also considered as cancer causing agent.
But ozone in the upper atmosphere filters potentially damaging ultra-violet light(which can cuse skin cancer) from reaching the earth's surface.