What is ozone?
Big idea: Ozone (O₃) in the stratosphere protects life from harmful UV radiation. This is different from ground-level ozone, which is a pollutant.
Good ozone vs bad ozone
Stratospheric ozone (GOOD)
- Located 15–35 km above Earth
- Forms the ozone layer
- Absorbs harmful UV-B and UV-C radiation
- Protects life on Earth
- We want MORE of this
Tropospheric ozone (BAD)
- At ground level
- Component of smog
- Respiratory irritant
- Damages plants and materials
- We want LESS of this
Remember: Good up high, bad nearby! Stratospheric ozone protects us; ground-level ozone harms us.
How the ozone layer forms
- UV radiation splits O₂ molecules: O₂ → O + O
- Free oxygen atoms combine with O₂: O + O₂ → O₃
- UV breaks ozone: O₃ → O₂ + O
- This creates a dynamic equilibrium — ozone constantly forms and breaks down
- The balance maintains a protective layer
Exam tip: Dont confuse the ozone layer (stratosphere, UV protection) with the greenhouse effect (troposphere, temperature regulation). Different layers, different functions!
Importance of the ozone layer
Big idea: The ozone layer absorbs most UV-B radiation, protecting living organisms from DNA damage, skin cancer, cataracts, and ecosystem harm.
Types of UV radiation
- UV-A (315–400 nm): Mostly reaches Earth; causes skin aging, some cancer risk
- UV-B (280–315 nm): Partially absorbed by ozone; causes sunburn, skin cancer, cataracts
- UV-C (100–280 nm): Completely absorbed by ozone and O₂; would be very harmful
Effects of increased UV-B
- Human health: Skin cancer (melanoma, carcinoma), cataracts, immune suppression
- Terrestrial ecosystems: Reduced plant growth, crop damage, DNA mutations
- Aquatic ecosystems: Kills phytoplankton (base of marine food chains), damages fish larvae
- Materials: Degrades plastics, paints, and building materials
Phytoplankton are especially vulnerable — they cant escape UV and are the base of ocean food webs AND a major carbon sink. Losing them would have cascading effects.
Exam tip: Be ready to explain impacts at different levels — individual organisms, populations, ecosystems, and human society.
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IB-style question — the role of the ozone layer [3]
Sunlight reaching the top of the atmosphere contains visible light and ultraviolet (UV) radiation. The ozone layer sits high up in the stratosphere.
Outline the role of the ozone layer and why it matters for living things. [3]
How to answer it, step by step
- What the ozone layer does
• It absorbs most of the harmful UV radiation from the Sun.
• This stops a lot of that UV from reaching the ground. - Why this matters
• Less UV reaching organisms means less damage to living cells and DNA.
• So it acts like a natural sunscreen that protects life on Earth.
Final answer
Say what it absorbs (UV) AND the benefit (protects living things from cell/DNA damage) — a one-word 'protection' answer won't get all 3 marks.