It is like anything including water. In excess it can be dangerous. What a lot of people don't realize is that they are breathing ozone every time they walk out in to the sun. The UV light of the sun reacts with the oxygen in the air forming ozone. It is also the sweet smell you smell after a thunder storm. Ozone is Nature's disinfectant. It also reacts with rainwater to form hydrogen peroxide. Plants pick up this peroxide to help keep themselves healthy.