Ozone layer

Ozone, O₃, is oxygen with three atoms in each molecule instead of two. Near the ground it is a deadly poison. Far above us it helps instead. It soaks up much of the Sun's UV radiation before that can reach the ground. This matters, since UV harms living things and can cause skin cancer in people.

Trouble showed up in the 1980s, when ozone levels high above the Earth fell fast. Scientists tied the fall to CFCs. These are man-made chemicals that cool fridges and fill some fire extinguishers. Countries then acted together. In 1987, through the United Nations Environment Programme, they agreed to make no more CFCs each year than they had in 1986.

A small example

Ozone high up is made by UV radiation acting on oxygen, in two steps:

• O₂ → O + O, with UV written over the arrow. UV splits some oxygen molecules into free atoms.
• O + O₂ → O₃. A free atom joins an oxygen molecule to make ozone.

Count the atoms: 2 become 1 + 1, and then 1 + 2 make 3.

Where marks go

Ozone depletion is in CBSE's Science syllabus for 2026-27. The 2026-27 sample paper has no question on it. A written answer on it can draw on three linked facts: what ozone does, what is harming it, and what was done about it. Each is its own point. Read about how step marking works.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

Deepti Sardana on LinkedIn