The small ozone layer on top of the stratosphere is crucial for human survival. Why ?

Explanation

Ozone found in the upper part of the atmosphere called stratosphere acts as a shield absorbing ultraviolet radiation form the sun.
UV rays are highly injurious to living organisms.
The thickness of the ozone-layer in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU).
Ozone layer absorbs the harmful UV-rays.
It causes aging of skin, damage to skin cells and various types of skin cancers.
In human eye cornea absorbs UV-B radiation and a high dose of UV-B causes inflammation of cornea, called snow-blindness, cataract, etc.
Such exposure may permanently damage the cornea.
Chlorofluro carbons deplete the ozone layer.
The part of atmosphere with lesser concentration of ozone is called ozone hole.

Ozone hole is the area above Antarctica, shown in purple colour, where the ozone layer is the thinnest. Ozone thickness is given in Dobson unit (see carefully the scale shown in colour violet to red). The ozone hole over Antarctica develops each year between late August and early October. Courtesy: NASA
Steps leading to ozone depletion : UV-rays split CFCs and release atomic chlorine (CI)
UV-rays also split ozone into oxygen.
Chlorine atoms trap oxygen atoms and ozone is not formed again from oxygen.
This leads to depletion of ozone in the stratosphere.