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How smoking causes cancer

Smoking body diagram updatedSmoking causes over a quarter of all cancer deaths in the UK and nearly one in five cancer cases.

This page describes how the various poisons in cigarette smoke cause cancer by damaging our DNA, preventing DNA repair and weakening the ability to remove toxins from the body.

What happens in your body?

Chemicals in cigarette smoke enter our blood stream and can then affect the entire body. 

This is why smoking causes so many diseases, including a dozen types of cancer, heart disease and various lung diseases.

The diagram on the right shows just some of the types of cancer that are caused by smoking.

Damaging our DNA

The main way that smoking causes cancer is by damaging our DNA, including key genes that protect us against cancer. Many of the chemicals found in cigarettes have been shown to cause DNA damage, including benzene, polonium-210, benzo(a)pyrene and nitrosamines.

This is already bad news, but it’s made worse by other chemicals in cigarettes. For example chromium makes poisons like benzo(a)pyrene stick more strongly to DNA, increasing the chances of serious damage. And chemicals like arsenic and nickel interfere with pathways for repairing damaged DNA. 

This makes it even more likely that damaged cells will eventually turn cancerous.

Weakening the body’s defences

As well as less effective DNA repair, smokers can’t handle toxic chemicals as well as those with healthy lungs and blood.

We all have special cleaner proteins called ‘detoxification enzymes’ that mop up harmful chemicals and convert them into harmless ones. But the chemicals in smoke, such as cadmium, can overwhelm these cleaners.

Other chemicals like formaldehyde and acrolein kill cilia, the small hairs that clean toxins from your airways.

Cigarette smoke also impacts the immune system – increasing cells which can encourage tumour growth in the lungs and suppressing the ones which kill cancer cells.

Concentration of chemicals

Most of the harmful substances in tobacco smoke are found at low levels in a single cigarette. But smokers are exposed to large amounts overall and these chemicals can build up to high levels in our bodies. 

For example, heavy smokers can be exposed to up to 150 times the background level of radioactive polonium-210.

Cancer is a multi-step process

It usually takes many years, or decades, for smoking to cause cancer. Our bodies are designed to deal with a bit of damage but it’s hard for the body to cope with the number of harmful chemicals in tobacco smoke. Over time DNA damage builds up and makes it more and more likely that our cells will become cancerous.

This is why a smoker’s risk of cancer and other diseases increases the more cigarettes they smoke a day, and the more years they spend smoking. And why the sooner you can quit the better. Check out our reducing the risk  page to find out more about factors affecting the risk and advice for quitting.

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Question about cancer? Contact our information nurse team
Updated: 25 September 2009