Why is tobacco dangerous?

Tobacco contains thousands of chemicals and byproducts that can make it harmful. The three most dangerous byproducts of tobacco are nicotine, tar and carbon monoxide. 
 
Nicotine is the pharmacologically active agent in tobacco that acts on the brain primarily as a stimulant, but which also has sedative effects. Nicotine is largely concentrated at the base of the tobacco leaf stem. In this form, it is a deadly poison that has been used for centuries as a lethal pesticide. 

The nicotine "kick" that most smokers get causes a rush of adrenaline that stimulates increased blood pressure, respiration and heart rate. It directly causes a release of brain chemical called dopamine in the region of the brain that controls pleasure and motivation. Nicotine's effect on the brain's "pleasure center" is what creates a craving and reaction similar to that seen with other drugs of abuse, such as cocaine and heroin. 
 
Tar is the gummy substance that is left behind when tobacco is smoked or chewed. It is the primary carcinogenic (cancer-causing) agent in tobacco. Over time, tar builds up inside the cells of the lungs and causes severe damage. 
 
Carbon monoxide, a deadly, poisonous gas, is readily released with each puff of smoke. The most toxic agent found in tobacco smoke, carbon monoxide lowers or displaces the level of oxygen in the bloodstream, thereby increasing heart rate, blood pressure and respiration. 

While nicotine, tar and carbon monoxide are clearly the three most dangerous byproducts of tobacco, approximately 4,000 other known chemicals are released as byproducts of both cigarette smoke and smokeless tobacco. Forty-three of these chemicals increase the risk of cancer, while hundreds more are toxic and lethal. Some of the more common chemicals found in tobacco smoke include: acetone (solvent thinner); ammonia (household cleaner); formaldehyde (embalming fluid and preservative); hydrogen cyanide (poison); methane (flammable gas and fuel); naphthalene (dry-cleaning fluid); nickel and cadmium (metals); and vinyl chloride (plastic). The average pack-a-day smoker inhales about 150,000 doses of these chemicals in one year, with up to 90 percent remaining trapped in the lungs.