The tobacco plant is a cash crop grown 
in around 125 countries throughout the world. Brazil China India Turkey & USA together produce two-thirds of the total world tonnage.
China is the worlds largest manufacturer of cigarettes, followed by the USA.
The dried leaves of the tobacco plant look pretty innocuous and will seduce you with their aromatic aroma, but the smoke produced from burning tobacco contains around 4000 chemical compounds, both gaseous & particulate, some are poisonous and many are known carcinogens. The leaves contain between 0.5% – 5% of nicotine.
Over the years the industry has learned how to maximise profits by using as little raw material as possible. The result is that reconstituted tobacco leaves including stems & dust are used in cigarette manufacture.
To improve smoking enjoyment chemicals are added to enhance flavour, to numb and desensitise the surface of the mouth & throat & upper respiratory tract.
Ammonia or other alkalis are added to maximise the amount of free base nicotine liberated from the leaves, ready for rapid absorption into the body and brain.