Which gas is both harmful and beneficial to life on earth?

Ozone (O3) has a dual nature - it is beneficial in the stratosphere where it forms the ozone layer that protects life from harmful UV radiation, but harmful at ground level where it is a toxic pollutant and component of smog. This Jekyll-and-Hyde character makes ozone unique among atmospheric gases. Oxygen is essential for life but not harmful in normal concentrations. Carbon dioxide and methane are primarily known as greenhouse gases without significant protective roles.