Ozone: Shield or Smog?
Good ozone up high, harmful ozone at ground level
What this lesson covers
Why it matters
Ozone (O₃) is the same molecule everywhere — but high in the stratosphere it is a life-saving shield, while down at ground level it is a harmful part of smog. Same molecule, opposite reputations.
Predict first
Stratospheric ozone and ground-level ozone are the identical molecule (O₃). Why is one 'good' and one 'bad'?
'Good' ozone sits in the stratosphere (20–30 km up), absorbing the Sun's harmful UV. 'Bad' ozone forms at ground level from vehicle exhaust and sunlight, irritating lungs and forming smog.
- High up it absorbs harmful UV and shields life; near the ground it is a toxic pollutant — correct
- The good ozone is a different, heavier molecule
- Ground-level ozone is actually oxygen, not ozone
What you do
Sort each statement: does it describe 'good' stratospheric ozone, or 'bad' ground-level ozone?
Check yourself
Why is the stratospheric ozone layer vital for life?
The ozone layer screens out damaging UV; warming is the greenhouse effect, and breathing oxygen is O₂ at ground level.
Ground-level (tropospheric) ozone is considered a pollutant because it…
Down where we breathe, ozone is toxic and a key ingredient of photochemical smog.
- It absorbs most of the Sun's harmful ultraviolet radiation — correct
- It traps heat to keep Earth warm
- It supplies the oxygen we breathe
- irritates the lungs and is part of smog — correct
- blocks UV too well, cooling the Earth
- is a completely harmless gas