On the walls inside the lungs there are tiny little hairs called
cilia that keep the lungs clean and protect them from dust, dirt
and germs.

Cigarette smoke kills the cilia so that there’s nothing to
protect the lungs. This means there’s nothing to filter the dust
and dirt and it gets harder to breath.  When this happens,
mucus or phlegm builds up and causes blocked noses or sore throats.
So smokers get sick more often and some of them end up with

Over time, after lots of years of smoking, the lungs get so
damaged they develop a condition called emphysema.

Not all smokers get lung cancer but anyone who smokes long
enough will get emphysema.

It causes chronic shortness of breath and in the worst cases,
means that sufferers have to wheel an oxygen tank around with them
everywhere they go.