Smoking is bad for your health. It's harmful because there are so many things in tobacco smoke that can harm your body. The most common diseases caused by smoking are lung cancer, heart disease, and stroke.

How smoking damages your body

There are more than 4,000 chemicals in tobacco smoke. Many of these are poisons. At least 43 of these chemicals cause cancer. Every time you breathe in tobacco smoke, you breathe in poisons that harm your body.^[19] Smoke also contains tar and harmful gases, such as carbon monoxide.

• The tar and chemicals in tobacco smoke cause your lungs to make a thick fluid called mucus. It clogs your lungs and can cause a 'smoker's cough'.

• Your lungs are lined with tiny hairs that help 'sweep' germs and other things out of your lungs. Those hairs can't move easily when you have mucus and tar in your lungs. So you're more likely to get an infection in your lungs.

• The chemicals in smoke make the walls of your airways swell and get thicker. This makes the airways narrower, so you get less air when you take a breath.

• The walls of your lungs have many tiny bags (or sacs) that hold air. Cigarette smoke damages those sacs. When that happens, less oxygen gets into your blood, and you get out of breath and get tired more easily. Eventually, you can get a lung disease called chronic obstructive pulmonary disease (COPD). If you have this, it gets harder and harder for you to breathe.

• The chemicals in smoke can lead to lung cancer. The chemicals damage cells in your lungs, and the damaged cells can then become cancer cells. The more you smoke, the greater your chance of getting lung cancer. If you smoke cigars or a pipe, you also have a higher chance of getting lung cancer than people who don't smoke.^[8] To learn more, see our articles on lung cancer.

• Carbon monoxide from cigarette smoke gets into your blood. It reduces the amount of oxygen your blood can carry. This means your heart has to work harder to get enough oxygen to all the cells in your body. The extra work puts a strain on your heart, which over many years can lead to heart problems.

• Some particles in smoke seem to help 'bad' cholesterol in your blood stick to your blood vessels. When cholesterol builds up in the blood vessels, it makes them narrower and increases the chances that you'll have a heart attack or stroke.

If you keep smoking

We can't say for certain what will happen to you as an individual if you keep smoking. Everyone is different. This is what we know from studies of people who smoked.

• About one-half of all smokers die of a disease that's caused by smoking. The big killers are lung cancer, heart disease, and stroke.^[18]

• On average, the life of someone who smokes is 16 years shorter than the life of someone who doesn't smoke.^[18]

• If you smoke, you're more likely to get cancer in your lungs, intestine, throat, pancreas, kidneys, bladder, or cervix.^[18]

• Smoking is the most important cause of chronic obstructive pulmonary disease (COPD).^[18]

• Smoking causes more than one-quarter of all deaths from cancer in the UK.^[5]

One long-term study looked at more than 100,000 women, and followed them for 24 years. It showed that smokers were almost three times more likely to die during the 24-year study, compared to non-smokers. The study also showed that, 20 years after stopping, the chances of death decreased to be the same as someone who'd never smoked.^[20]