What is the ozone layer and how is it formed?
Ozone is a molecule made of three oxygen atoms (O₃). The oxygen we breathe has only two (O₂). Near the ground ozone is a harmful gas, but high up in the stratosphere (roughly 15–35 km up) it forms a thin ozone layer that works like a sunshade for the whole planet.
It forms in two steps:
- High-energy UV light splits an O₂ molecule into two free oxygen atoms: O₂ → O + O.
- A free atom joins another O₂ molecule: O + O₂ → O₃ (ozone).
The ozone layer absorbs most of the Sun's ultraviolet (UV) radiation. Without it, UV rays would reach living things in much larger amounts.
Ozone depletion: CFCs and why UV rays are harmful
From the 1980s, scientists found that the amount of ozone was falling, especially over the South Pole region each spring (the 'ozone hole'). The main cause was a group of man-made gases called chlorofluorocarbons (CFCs), used in old refrigerators, air conditioners, foam and aerosol sprays.
CFCs are very stable, so they slowly drift up to the stratosphere. There, UV light breaks them and releases chlorine atoms. One chlorine atom can break thousands of ozone molecules, one after another.
Harm caused by extra UV rays
- Skin cancer and sunburn in humans
- Cataract (cloudy lens) in the eyes
- Weaker immune system
- Damage to crops and to tiny plankton at the start of ocean food chains
The Montreal Protocol (1987)
Nations agreed in 1987, at Montreal, to cut down and phase out CFCs and similar gases. It came into force in 1989. Newer fridges use safer gases. Scientists now see the ozone layer slowly recovering, which shows that working together works.
Our activities and garbage: biodegradable vs non-biodegradable
Every home throws away vegetable peels, paper, plastic wrappers, bottles and more. What happens next depends on the material.
- Biodegradable substances can be broken down by bacteria and fungi (decomposers) into simple, harmless substances. Examples: fruit and vegetable peels, leftover food, paper, dry leaves, cotton cloth, wood, cow dung.
- Non-biodegradable substances are not broken down by decomposers. They stay for a very long time or go through only slow physical change. Examples: plastic, polythene, glass, metal cans, many pesticides like DDT.
The reason is simple: decomposers use enzymes that fit only certain natural substances. Man-made plastics do not match those enzymes, so microbes cannot 'eat' them.
Non-biodegradable waste fills landfills, blocks drains, harms animals that swallow plastic, and some chemicals can pass up food chains and build up in top consumers (biological magnification).
Managing waste: disposable cups, 3 Rs, segregation and composting
Disposable cups: kulhad, paper or plastic?
Plastic cups last for hundreds of years. Paper cups break down faster, but need trees and often carry a thin plastic lining. Clay kulhads go back to the soil, yet making them on a huge scale uses a lot of fertile topsoil and fuel for baking. So no disposable cup is perfect; a cup you wash and reuse is the best choice.
Reduce, Reuse, Recycle
- Reduce: use less. Carry a cloth bag, refuse extra packing.
- Reuse: use again. Glass jars for storage, envelopes for notes.
- Recycle: send plastic, paper, glass and metal to be made into new things.
Segregation at source
Keep wet (biodegradable) waste in the green bin and dry (non-biodegradable, recyclable) waste in the blue bin. Mixed garbage is hard to recycle and smells.
Composting
Kitchen and garden waste in a pit or pot, kept moist and turned sometimes, is broken down by microbes and earthworms into dark, rich compost (manure) for plants. Using earthworms is called vermicomposting.
Key formulas and definitions
- O₂ + UV → O + O (UV splits oxygen)
- O + O₂ → O₃ (ozone forms)
- Ozone layer: in the stratosphere, absorbs most UV rays
- CFCs (chlorofluorocarbons): main ozone-depleting gases; Montreal Protocol, 1987
- Biodegradable: broken down by decomposers (peels, paper, cotton)
- Non-biodegradable: not broken down by decomposers (plastic, glass, metal, DDT)
- 3 Rs: Reduce, Reuse, Recycle; green bin = wet, blue bin = dry
Worked examples
1. Sort these into biodegradable and non-biodegradable: tea leaves, polythene bag, newspaper, aluminium foil, egg shells, glass bangle.
Biodegradable: tea leaves, newspaper, egg shells (all natural, microbes can break them). Non-biodegradable: polythene bag, aluminium foil, glass bangle (man-made or mineral, microbes cannot break them).
2. Ozone near the ground is harmful, yet the ozone layer is useful. Explain.
Step 1: Ozone is a poisonous gas to breathe, so at ground level it harms lungs. Step 2: High in the stratosphere we never breathe it. Step 3: There it absorbs UV rays before they reach us. So the same gas is a pollutant low down and a shield high up.
3. Why did the ozone layer thin even though CFCs are released on the ground, far away from it?
Step 1: CFCs are very stable and do not break down in the lower air. Step 2: Over years they drift up to the stratosphere. Step 3: Strong UV there breaks them and frees chlorine atoms. Step 4: Each chlorine atom breaks many ozone molecules, so the layer thins.
4. A school canteen gives tea in plastic cups. Suggest a better plan and give reasons.
Best: steel or glass cups that are washed and reused (Reduce + Reuse), so no waste is made. If disposables must be used, choose paper cups sent for composting or kulhads in small numbers, and put any plastic in the blue bin for recycling. Reason: plastic cups are non-biodegradable and stay for hundreds of years.
Common mistakes
- Thinking ozone is always good. Ozone in the air we breathe is a pollutant; only the high stratospheric layer protects us.
- Saying the ozone hole causes global warming. They are different problems: ozone loss lets in more UV; global warming is due to greenhouse gases trapping heat.
- Calling kulhads the perfect answer. They are biodegradable-friendly, but making crores of them wastes fertile topsoil.
- Writing that plastic 'rots slowly like food'. Plastic is not broken down by decomposers at all; it only breaks into tiny pieces (microplastics).