Smoking and Lung Cancer

Tobacco smoke is a complex mixture of more than 3,000 different substances, including the addictive stimulant nicotine, benzene, ammonia, hydrogen cyanide, carbon monoxide, and tar. The burning tar elements in the smoke are known to be strongly cancer-causing (carcinogenic). The risk of developing lung cancer increases with the number of cigarettes smoked per day, thair tar content, the number of years that a person has smoked, and the depth of inhalation into the lungs. Another risk factor is regular exposure to other people's cigarette smoke, whish is known as passive smoking.

Tar is just one of thousands of chemicals in tobacco smoke. In smokers, healthy lung tissue becomes dotted with deposits of tar.

The process:
1. In healthy airway lining, the columnar cells topped by tiny, hairlike cilia line healthy airways (bronchi). Basal cells constantly divide to replace naturally damaged columnar cells.
2. Initial damage: Over time, columnar cells damaged by smoking become squamous cells, which gradually lose their cilia. The mucus-secreting goblet cells die
3 .Cancer begins: to replace the damaged cells, basal cells start to multiply at an increased rate. Some of these new basal cells develop into cancerous cells.
4. Cancer spreads: the cancerous cells replace healthy cells. If these cells break through the basement membrane, they can enter blood vessels to travel elsewhere

Source:
The Human body book, Steve Parker, Robert Winston, DK.p143
http://health.nytimes.com/health/guides/disease