What Makes Smoking Harmful?

Tobacco smoke contains over 4000 different chemicals, many of which are harmful such as:


This a powerful, fast acting and addictive drug. Most people who smoke are dependent on the nicotine in cigarettes. When a smoker inhales, nicotine is absorbed into their bloodstream and the effects are felt on their brain seven to eight seconds later. Nicotine also has many complex effects on the rest of the body. In small amounts nicotine stimulates nerve impulses in the central and the autonomic nervous system but, in large amounts nicotine inhibits these nerve impulses.

The immediate effects of nicotine are:

The effects on mood and behaviour are complex and depend on the individual smoker and:

Carbon monoxide

This is a poisonous gas found in relatively high concentrations in cigarette smoke. It combines readily with haemoglobin - the oxygen-carrying substance in blood - to form carboxyhaemoglobin. In fact it combines more readily with haemoglobin than oxygen does, so up to 15% of a smoker's blood may be carrying carbon monoxide round the body instead of oxygen.


Oxygen is essential for body tissues and cells to function efficiently. If the supply of oxygen is reduced for long periods, this can cause problems with growth, repair and absorption of essential nutrients. Carbon monoxide is therefore particularly harmful during pregnancy as it reduces the amount of oxygen carried to the uterus and fetus.

Carbon monoxide can also affect the 'electrical' activity of the heart and, combined with other changes in the blood associated with smoking and diet, may encourage fatty deposits to form on the walls of the arteries. This process can eventually lead to the arteries becoming blocked, causing heart disease and other major circulation problems.


When a smoker inhales, the cigarette smoke condenses and about 70% of the tar contained in the smoke is deposited in the lungs. Many of the substances in tar are already known to cause cancer in animals.

Irritants in tar can also damage the lungs by causing narrowing of the bronchioles, coughing, an increase in bronchiole mucus and damage (ciliostasis) to the small hairs which help protect the lungs from dirt and infection.

By law, since 1992, nicotine and tar yields have been displayed on cigarette packets and since the beginning of 1998, all cigarettes sold in the UK have a 12mg upper limit for tar yield. The amount of tar, carbon monoxide, nicotine and other substances that is absorbed into the body from a cigarette varies greatly, and depends on how much the smoker inhales.

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