Carbon monoxide Carbon monoxide is a poisonous gas that you can't see or smell. When a person smokes, carbon monoxide enters the bloodstream more easily than the oxygen that we need to breathe. The lungs then take in less oxygen, and this can lead to hardening of the arteries and heart disease. There are many other chemicals found in tobacco smoke. Some of these are poisonous, and others can cause cancer.
Many people can develop a 'tolerance' to the nicotine in tobacco.
Tolerance means that you must smoke more to feel the same effects you used to have with fewer cigarettes. Dependence' on tobacco means that smoking takes up much of your thoughts, emotions and activities. Not all people who smoke are dependent. Some benefits of giving up smoking happen as soon as you stop. After 6 hours the heart rate slows down and blood pressure drops slightly.