ScienceCBSE Class 10 · Our Environment

Ozone Layer and Its Depletion

Updated 14 October 2026 · Fact-checked

Ozone (O3) is a molecule of three oxygen atoms. Near the ground it is poisonous, but high in the atmosphere it shields the earth from harmful ultraviolet radiation, which can cause skin cancer. Its sharp fall since the 1980s is linked to CFCs, so CFC production was frozen and CFC-free refrigerators made mandatory.

Understand Ozone Layer Depletion

What ozone is. Ozone, $O_{3}$ , is a molecule made of three atoms of oxygen. Ordinary oxygen, $O_{2}$ , is essential for all aerobic life. Ozone is a deadly poison if breathed, but at the higher levels of the atmosphere it performs an essential job.

Why it matters. The ozone layer shields the surface of the earth from ultraviolet (UV) radiation from the Sun. UV rays seriously harm living things; in people, one known effect is skin cancer. So damage to the ozone layer would let more UV reach the earth and harm living things across ecosystems.

How ozone forms up there. Ozone in the upper atmosphere is produced by UV radiation acting on oxygen:
1. High-energy UV radiation splits some oxygen molecules into free oxygen atoms: $O_{2} UV O + O$ .
2. These atoms combine with oxygen molecules to form ozone: $O + O_{2} \to O_{3}$ .

Depletion. From the 1980s, atmospheric ozone started falling steeply. Scientists connect this fall with man-made chemicals, the chlorofluorocarbons (CFCs), whose uses included refrigerants and fire extinguishers.

Steps taken. In 1987 an agreement brought about by the United Nations Environment Programme (UNEP) held CFC production at its 1986 level (a freeze). Today, refrigerator makers everywhere must produce CFC-free models.

What you can do. Choose CFC-free products, and support safe disposal of old refrigerators and air-conditioners so that their gases are not released carelessly. More broadly, the lesson of the ozone story is that human activities can harm the environment on a global scale, and that agreements between countries can help limit the damage.

Do not confuse. Ozone depletion (a thinning of the protective layer high in the atmosphere, letting in more UV) is a different problem from waste disposal and from the build-up of harmful chemicals in food chains, which are the other environmental issues in this chapter.

Ozone Layer Depletion: formulas and key facts

Splitting of oxygen by UV: $O_{2} UV O + O$; High-energy UV radiation breaks oxygen molecules into atoms.
Formation of ozone: $O + O_{2} \to O_{3}$; Free oxygen atoms join oxygen molecules.
Key facts (NCERT): $drop from 1980s; CFCs; UNEP 1987: freeze at 1986 levels$; CFC-free refrigerators now mandatory.

How to solve Ozone Layer Depletion questions

For ozone questions, cover what, where, why it matters, cause of damage and remedy.

1Define ozone as $O_{3}$ , three oxygen atoms; poisonous near the ground.
2Say where it is useful: higher levels of the atmosphere.
3Give its job: shields the earth from UV radiation, which causes skin cancer.
4Write the two formation equations with UV over the first arrow.
5Name the cause of depletion: CFCs (refrigerants, fire extinguishers), noticed from the 1980s.
6State the steps: UNEP 1987 agreement freezing CFC production at 1986 levels; CFC-free refrigerators mandatory.

Ozone Layer Depletion in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

CFCs and ozone protection

Suppose your family is replacing a very old refrigerator, and the shopkeeper points out that all new models are labelled CFC-free. Your younger brother asks why that matters if the fridge works just as well either way.

Explain to him what CFCs have to do with the ozone layer, why the ozone layer matters to people, and what was done internationally about it.

Try it first, then show the solutionSolution

CFCs (chlorofluorocarbons) were used as refrigerants and in fire extinguishers.
The sharp drop in atmospheric ozone from the 1980s has been linked to these synthetic chemicals.
Ozone high in the atmosphere shields the earth from the Sun's UV radiation, which is highly damaging and can cause skin cancer.
So an agreement under UNEP froze CFC production at 1986 levels, and making CFC-free refrigerators is now mandatory; buying CFC-free models avoids adding to the damage.

Answer: CFCs have been linked to ozone depletion; the ozone layer blocks harmful UV that can cause skin cancer; CFC production was frozen and CFC-free refrigerators made mandatory.

Memory hook: CFC-free fridge, safer sky: less ozone loss, less UV.

Tricks to remember Ozone Layer Depletion

Short memory aids. Say each one aloud once, then use it on a question the same day.

Good up high, bad nearby
Ozone is a shield in the upper atmosphere but a poison at ground level.
Use it when: Explaining ozone's two faces.
Split then stick
UV splits $O_{2}$ into O atoms; O sticks to $O_{2}$ to make $O_{3}$ .
Use it when: Formation equations.
86-87
Agreement in 1987 froze CFCs at 1986 levels.
Use it when: Recalling the UNEP step.

Ozone Layer Depletion: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Ozone and its importance · 2 marks · Very short answerBoard level

What is ozone and how does it affect an ecosystem?

Try it first, then show the solutionSolution

Ozone ( $O_{3}$ ) is a molecule of three oxygen atoms; it is poisonous at ground level (1 mark).
In the upper atmosphere it shields the earth from harmful UV radiation, which can cause skin cancer and damage organisms; so it protects all ecosystems (1 mark).

Answer: O3, three oxygen atoms; shields living things from harmful UV in the upper atmosphere.

Example 2: Formation equations · 3 marks · Short answerHarder board level

Write the equations showing how ozone is formed in the upper atmosphere and name the energy source. Why is ozone formed mainly high up rather than near the ground?

Try it first, then show the solutionSolution

$O_{2} UV O + O$ (1 mark).
$O + O_{2} \to O_{3}$ (1 mark).
The energy comes from high-energy UV radiation from the Sun, which acts on oxygen at higher levels of the atmosphere before it reaches the ground (1 mark).

Answer: UV splits O2 into atoms, which join O2 to form O3; UV acts mainly in the upper atmosphere.

Example 3: Why depletion is a concern · 3 marks · Short answerHarder board level

Give reasons why a thinning ozone layer worries scientists, and describe the measures adopted to slow the damage.

Try it first, then show the solutionSolution

The ozone layer shields the earth from UV radiation; with less ozone, more UV reaches the surface and can cause skin cancer and harm other organisms (1 mark).
Ozone amounts began to fall sharply in the 1980s, linked to CFCs used in refrigerants and fire extinguishers (1 mark).
In 1987 UNEP forged an agreement to freeze CFC production at 1986 levels; making CFC-free refrigerators is now mandatory worldwide (1 mark).

Answer: More UV would harm living things; CFC production frozen (UNEP 1987) and CFC-free refrigerators made mandatory.

Example 4: Two faces of ozone · 3 marks · Short answerHOTS / competency

A student says: 'Ozone is poisonous, so we should try to get rid of it.' Another says: 'Ozone is good, so we should make more of it near the ground.' Evaluate both statements.

Try it first, then show the solutionSolution

First statement: wrong. Though poisonous to breathe, ozone at higher levels of the atmosphere protects life by absorbing harmful UV radiation; losing it would expose organisms to UV (1½ marks).
Second statement: wrong. Near the ground ozone is a deadly poison for living things; its useful role is only high in the atmosphere (1½ marks).

Answer: Both are wrong: ozone is protective high up and harmful near the ground.

Example 5: Comparing environmental problems · 5 marks · Long answerHOTS / competency

Compare ozone depletion and biological magnification under these heads: the substances responsible, where the damage happens, and how humans are affected. Suggest one action for each that individuals or governments can take.

Try it first, then show the solutionSolution

Substances: ozone depletion is linked to CFCs (refrigerants, fire extinguishers); biological magnification involves non-degradable chemicals such as pesticides (1 mark).
Where: ozone depletion thins a protective layer high in the atmosphere; biomagnification happens inside living organisms along food chains (1 mark).
Effect on humans: more UV reaching the earth can cause skin cancer; humans at the top of food chains collect the highest concentration of pesticides (1 mark).
Action for ozone: use CFC-free products; governments freezing CFC production and making CFC-free refrigerators mandatory (1 mark).
Action for biomagnification: reduce the use of non-degradable pesticides; check pesticide levels in food (1 mark).

Answer: CFCs vs pesticides; atmosphere vs food chains; UV and skin cancer vs pesticide residues; CFC-free products vs reduced pesticide use.

Where students lose marks in Ozone Layer Depletion

The slips examiners see most often, and what to write instead.

Mistake: Saying ozone is always harmful or always helpful
Only one role is remembered.
Wrong working
Ozone is a harmless gas that protects us.
Right working
Ozone is poisonous near the ground but protective in the upper atmosphere.
Do this: State both roles with their location.
Usually costs: 1 mark.
Mistake: Wrong cause of depletion
Confusion with global warming.
Wrong working
Carbon dioxide causes ozone depletion.
Right working
Depletion has been linked to CFCs.
Do this: Ozone depletion → CFCs.
Usually costs: 1 mark.
Mistake: Missing UV on the formation equation
Condition left out.
Wrong working
$O_{2} \to O + O$ with nothing over the arrow.
Right working
$O_{2} UV O + O$
Do this: Always write UV over the splitting step.
Usually costs: ½ mark.
Mistake: Wrong international detail
Dates are mixed up.
Wrong working
CFCs were banned completely in 1986.
Right working
In 1987 UNEP forged an agreement to freeze CFC production at 1986 levels.
Do this: Learn the NCERT wording of the step taken.
Usually costs: ½ to 1 mark.

Exam tips

Write both formation equations and mention UV.
Mention skin cancer as the NCERT example of UV harm.
Name CFCs and their uses (refrigerants, fire extinguishers).
Quote the UNEP agreement and the CFC-free refrigerator rule as steps taken.
Keep ozone depletion separate from waste disposal and biomagnification in answers.

Practice questions on Ozone Layer Depletion

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The chemicals mainly linked to depletion of the ozone layer are:
(a) pesticides (b) chlorofluorocarbons (c) carbon dioxide (d) nitrogen
Hint
Used as refrigerants.
Answer
(b) chlorofluorocarbons (CFCs).
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): Ozone at higher levels of the atmosphere is beneficial to life on earth.
Reason (R): Ozone shields the earth's surface from ultraviolet radiation.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
What would UV do without it?
Answer
(a).
Q3 · 2 marks · Very short answerHarder board level
Name two uses of CFCs mentioned in NCERT and the international step taken in 1987 to control them.
Hint
UNEP.
Answer
Refrigerants and fire extinguishers. In 1987 UNEP forged an agreement to freeze CFC production at 1986 levels.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a science club makes a poster titled 'Our invisible sunscreen'. It shows the Sun's rays striking the upper atmosphere, some oxygen molecules splitting, and a layer of a gas that blocks harmful rays.
(a) Name the gas and its formula. (1 mark)
(b) Write the two equations for its formation. (1 mark)
(c) Explain why the poster calls it a 'sunscreen' and what would happen to living organisms if the layer became much thinner. (2 marks)
Hint
The harmful rays are UV.
Answer
(a) Ozone, $O_{3}$ . (b) $O_{2} \to O + O$ (UV); $O + O_{2} \to O_{3}$ . (c) It absorbs harmful UV radiation before it reaches the surface, like sunscreen protecting skin. If it thinned, more UV would reach the earth, causing damage such as skin cancer in humans and harm to other organisms.
Q5 · 3 marks · Short answerHOTS / competency
Explain why making CFC-free refrigerators mandatory everywhere, and not just in a few countries, is necessary to protect the ozone layer.
Hint
The atmosphere is shared by all.
Answer
The atmosphere is shared by all countries, and CFCs released anywhere can reach the upper atmosphere and damage the ozone layer that shields the whole earth. If only a few countries stopped, releases elsewhere would continue the damage. So a worldwide rule and international agreements, such as the one forged by UNEP, are needed.
Q6 · 2 marks · Very short answerBoard level
Distinguish between ozone and oxygen in terms of structure and their effects on living organisms.
Hint
Number of atoms; essential vs poisonous.
Answer
Oxygen ( $O_{2}$ ) has two atoms and is essential for aerobic life. Ozone ( $O_{3}$ ) has three atoms and is a deadly poison to breathe, though in the upper atmosphere it protects life by blocking UV radiation.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Formula of ozone?Show answer
$O_{3}$ .
2.What does the ozone layer protect us from?Show answer
Ultraviolet radiation from the Sun.
3.One disease UV can cause in humans?Show answer
Skin cancer.
4.Chemicals linked to ozone depletion?Show answer
Chlorofluorocarbons (CFCs).
5.When did ozone amounts begin to drop sharply?Show answer
In the 1980s.
6.What did the 1987 UNEP agreement do?Show answer
Froze CFC production at 1986 levels.

Ozone Layer Depletion: frequently asked questions

What is the ozone layer?

A region of ozone (O_3) in the upper atmosphere that shields the earth's surface from harmful ultraviolet radiation.

How is ozone formed?

UV radiation splits oxygen molecules into oxygen atoms, which then combine with other oxygen molecules to form ozone.

What causes ozone depletion?

The sharp drop in ozone since the 1980s has been linked to CFCs, used as refrigerants and in fire extinguishers.

Why is ozone depletion dangerous?

More UV radiation reaches the earth, which is highly damaging to organisms and can cause skin cancer in humans.

What steps have been taken to protect the ozone layer?

In 1987 UNEP forged an agreement to freeze CFC production at 1986 levels, and CFC-free refrigerators are now mandatory worldwide.