The dangers of three of these chemicals are particularly important.

•  Nicotine is the substance that causes addiction to tobacco.
•  Carbon monoxide passes from the lungs into the bloodstream leading to hardening of the arteries, which increases the risk of coronary thrombosis.
•  Tar is the major cause of lung cancer

Nicotine acts as a tranquillizer.
Nicotine is the substance that causes addiction to tobacco. It acts as a tranquillizer, but also stimulates the release of epinephrine into the smoker's bloodstream that elevates his/her blood pressure.

When a person inhales cigarette smoke, the nicotine it carries reaches the brain, via the bloodstream, in about 8 seconds, which is less time than it would take to get there had the person injected it intravenously. By the time 15 to 20 seconds have elapsed, the nicotine has spread throughout the whole body, and has even made it to the body's last output, the big toe.

More than 90 percent of the nicotine in inhaled cigarette smoke is absorbed by the lungs. Nicotine enters the body in the form of thousands of little droplets, each suspended in a solid particle of partially burned tobacco (tar). These nicotine droplets are so small that they can penetrate into the tiniest branches of the lungs, where they are picked up by blood that has been sent to the lungs to take up oxygen. From the lungs, it moves quickly to the left side of the heart, where nicotine is pumped out to every corner of the body.

With the first cigarette of the day, heart rate will increase by 10 to 20 beats per minute. Blood pressure will go up 5 to 10 points. Body temperature will drop in the fingertips as the blood vessels there are constricted. The level of nicotine in the bloodstream increases throughout the day with each cigarette.

The overall level of nicotine climbs gradually through the day and then falls during sleep. The level is high enough and its fall gradual enough, however, that for most smokers, nicotine is active in them 24 hours a day and they are never off the drug.

Carbon monoxide is a poisonous gas that is produced when tobacco burns.

Heart Disease

Tobacco enters the body in three ways: by smoking cigarettes, a cigar, or a pipe; by inhaling a packet of snuff through the nose or by chewing tobacco.

Carbon monoxide (CO) passes from the lungs into the bloodstream, where in competition with oxygen, is easily combines with haemoglobin and thus interferes with the oxygenation of tissues. In the long term, persistently high levels of carbon monoxide in the blood of the smokers will lead to hardening of the arteries, which increases the risk of coronary thrombosis.
Carbon monoxide is produced and accumulates as the result of incomplete combustion, from burning tobacco, vehicle exhaust, and other heat producing systems. CO is particularly dangerous in closed and poorly ventilated environments. Inhaled CO can bind to haemoglobin in red blood cells and decreases their oxygen carrying capacity. Carbon monoxide (CO) may contribute to heart disease and also plays a role in the way that smokers process the air they breathe.

Oxygen is carried to the various parts of the body by haemoglobin, a protein in red blood cells. As oxygen enters the bloodstream from the lungs, it binds with haemoglobin When the haemoglobin reaches its destination, the oxygen splits off to be used within cells, and the haemoglobin returns to the lungs to pick up another partner. That partner, however, can just as easily be carbon monoxide as oxygen. Haemoglobin actually binds much more readily with carbon monoxide than with oxygen.

The haemoglobin that binds with CO is unavailable to carry oxygen, and there are only so many haemoglobin molecules in the body at any given time, thus CO interferes with oxygenation of the tissues. Therefore, a fair number of haemoglobin molecules can be engaged in carrying poison rather than life-sustaining oxygen, preventing smokers from participating effectively in high-oxygen-requiring activities such as moving quickly for more than 30 minutes. The body tries to respond to this chronic condition by creating more red blood cells, but the compensation is incomplete.

Emphysema

Mouth Cancer

Tar in tobacco produces chronic irritation of the respiratory system and is the major cause of lung cancer

Smoke particles irritate the lung airways, causing excessive mucus production. They also indirectly destroy the walls of the lungs' alveoli, which coalesce. Both factors reduce lung efficiency, and tar in tobacco smoke has a direct cancer-causing action. The respiratory and the circulatory system are most affected by tobacco use. The tar and other chemicals in smoke damage the bronchi, the passages through which air enters the lungs. The bronchi can become clogged, then infected, or sore and swollen; the infections may then lead to cancer. Smoke coats the bronchi and alveoli in the lungs, making the lungs work less efficiently and causing disease like cancer and emphysema.

Lung cancer is probably the best known harmful effect of smoking. Since pipe and cigar smokers tend not to inhale tobacco smoke, they have a slightly lower risk of lung cancer, although the risk is still significantly greater than that of nonsmokers.

Pipe and cigar smokers have a higher risk of cancer of the oral cavity and upper respiratory tract while tobacco chewers and those who use snuff risk cancer of the oral cavity.

The other important respiratory diseases associated with smoking are chronic bronchitis and emphysema and combinations of the two. These diseases, features of which include increasing breathlessness and coughing up sputum, account for tens of thousands of deaths annually from respiratory failure. In addition, smoking also increases the risk of mouth cancer, lip cancer, and throat cancer.

Smoking is extremely harmful during pregnancy. The babies of pregnant women who smoke might be killed by the effects of smoking. Those who live are more likely to be smaller, have a lower birth weight, and become less likely to survive than those of nonsmoking mothers. Even after birth, there are hazards for the children of parents who smoke. These children are more likely to suffer from asthma or other respiratory diseases.

Smoke also harms the health of non-smokers by way of second-hand smoke. These "passive smokers" suffer considerable immediate discomfort in the form of coughing, wheezing, and watering eyes. The smoke coming from the burning tip of another person's cigarette is called sidestream smoke. Sidestream smoke contains twice as much tar and nicotine as does the smoke that the smoker breathes in because it does not pass through the length of the cigarette and cigarette filter.

We spend more time in our homes than anywhere else. So the thought of cancer-causing chemicals circulating throughout our homes can be quite unsettling. Yet, according to the Environmental Protection Agency, that is exactly what happens when someone lights a cigarette in your home. Those most affected by sidestream smoke are children. Because their bodies are still developing, exposure to the poisons in sidestream smoke puts children in danger of severe respiratory diseases such as asthma, and can hinder the growth of their lungs. On top of that, the effects can last a lifetime. These children are also more likely to become smokers themselves.