Scientists have discovered another reason why smoking is bad for health.
It seems that smokers have a much lower level of an important chemical that protects against heart disease in their blood. The discovery could help explain why smokers have more than twice the risk of heart attacks than non-smokers.
Paraoxonase is a enzyme which helps to prevent fatty deposits building up and blocking the blood vessels. A team of Swiss researchers examined 367 people with heart disease.
Blood tests revealed significantly reduced levels of paraoxonase activity among current smokers and ex-smokers who had stopped for less than three months. However, the researchers also found that once smokers had given up the habit, their paraoxonase levels began to return to normal.
Head researcher Dr Richard Gestner, from Munich University Hospital, said: "Within two years of smoking cessation, paraoxonase concentration and activity in former smokers is comparable to individuals who have never smoked."
Cigarette smoke is known to contain molecules known as free radicals.
These damage the tissues of the body, and are thought to stimulate the build up of fatty deposits in the arteries. The new research suggests that smoking has a double whammy effect - it stimulates clogging of the arteries, and sabotages the body's attempts to tackle the problem.
Amanda Sandford, of the anti-smoking charity Action on Smoking and Health (ASH), said: "This study emphasises the health benefits of giving up smoking no matter what your age.
"Giving up smoking has got to be the best thing you can do for your health."
"Carbon monoxide and nicotine are well proven to be damaging to the heart and circulation. Perhaps more studies into paraoxonase will reveal further evidence that cigarettes are a danger that sends our hearts' health up in smoke." Diabetes and old age are also associated with lower levels of paraoxonase.
The research is published in the American Cardiac Association journal Circulation.