Why is stratospheric ozone considered “good” while tropospheric ozone is considered “bad” ozone?

What “good” vs “bad” ozone is really about

Ozone is the same molecule, $O_3$, in both cases. The difference is where it is in the atmosphere and what effects it has there.

Stratospheric ozone, what it does for you

In the stratosphere (roughly 10–50 km up), ozone forms a layer that absorbs UV-B and UV-C radiation.

• This reduces UV exposure at Earth’s surface.
• Less UV means lower risk of skin cancer and cataracts, and less damage to ecosystems. So in the stratosphere, ozone is protective, which is why it is called “good.”

Tropospheric (ground-level) ozone, how it forms

In the troposphere (the air we breathe), ozone is mainly produced as a secondary pollutant when sunlight drives reactions involving nitrogen oxides and volatile organic compounds:

• Precursors: $NO_x$ (from vehicles, power plants) and VOCs (from fuels, solvents, some natural sources).
• Sunlight helps convert these into $O_3$. This is why ozone pollution is often worse on hot, sunny days.

Why ground-level ozone is harmful

Tropospheric ozone is a strong oxidant.

• Human health: irritates airways, worsens asthma, reduces lung function.
• Environment: damages leaf tissue, reduces crop yields and forest growth.
• Air quality: a major component of photochemical smog. So in the troposphere, ozone acts as a pollutant, which is why it is called “bad.”

Quick summary

Same molecule ($O_3$), different location:

• Stratosphere: blocks harmful UV, beneficial.
• Troposphere: pollutant that harms lungs and plants.