Cigarette smoke contains over 4,000 chemicals, including 43 known cancer-causing (carcinogenic) compounds and 400 other toxins. These include nicotine, tar, and carbon monoxide, as well as formaldehyde, hydrogen cyanide, and DDT.
Cigarettes also contain a chemical very similar to rocket fuel. This helps keep the tip of the cigarette burning at an extremely hot temperature, that way the nicotine turns to vapor in your lungs and is absorbs easier.
Radioactive materials are also found in cigarette smoke; polonium is the most common. A smoker of one and a half packs per day may be exposed to radiation equal to 300 chest x-rays a year. Radiation is a strong aging factor. Acetaldehyde, a chemical released during smoking, causes aging, especially of the skin, as it affects the cross-linking bonds that hold our tissues together. Some authorities believe that cigarettes are our greatest source of radiation.
Once inhaled, it only takes six seconds for nicotine to reach your brain. As you smoke, the amount of tar inhaled into your lungs increases, and the last puff of a cigarette contains twice as much tar as the first puff. Tar is a mixture of substances that together form a sticky mass in your lungs. Carbon monoxide makes it harder for red blood cells to carry oxygen throughout your body. Majority of the chemicals you inhale stay in your lungs. The more you inhale, the better it feels, and the more damage you cause to your lungs.
Nicotine in small doses acts as a stimulant to the brain. In large doses, it's a depressant, inhibiting the flow of signals between nerve cells. In even larger doses, it's a lethal poison, affecting the heart, blood vessels, and hormones. Nicotine in the bloodstream acts to make the smoker feel calm.