Earth's stratosphere is a lot like Las Vegas, in that what happens up there, stays up there. For one, molecules of ozone, formed naturally by interactions between oxygen and the sun's rays, form a naturally protective layer that shields us from harmful ultraviolet rays. It is only when ozone is produced closer to the ground, such as through industrial pollution, that it is harmful to life.
Though only one extra oxygen atom separates ozone (O3) from the oxygen humans need to breathe (O2), ozone is not breathable, but rather a smog-forming pollutant that is known to cause asthma. That makes sense, given its interaction with the lungs. An unexpected and newly discovered outcome of ozone pollution is depression in teenagers.
