Vitamin C may help to reduce the risks associated with being exposed to second-hand smoke, a study suggests.
Scientists in the United States have found non-smokers who took daily doses of vitamin C protected themselves against the cell damage that can cause cancer.
Their findings follow a study by researchers at the University of Cambridge three years ago, which found the vitamin can also reduce the risks for smokers.
Scientists believe the vitamin's anti-oxidant properties are responsible.
Non-smokers tested
Dr Marion Dietrich and colleagues at the University of California, Berkeley, based their findings on a study of 67 people.
The volunteers, all non-smokers, were divided into three groups.
They were told not to take any other vitamins for several weeks before the study and were not big eaters of fruit and vegetables - natural sources of vitamin C.
One group took 500mg of vitamin C a day for two months. A second group took vitamin C, vitamin E and the anti-oxidant alpha-lipoic acid, while the those in the third group took a dummy pill.
The researchers carried out blood tests on all the volunteers.
They tested for levels of F2-isoprostanes, a sign of oxidative stress or cell damage.
They found levels of F2-isprostanes dropped by 11% in the vitamin C group compared with those taking the dummy pill.
People who had taken vitamin C in combination with the other vitamins saw their F2-isoprostane levels fall by 12%.
"They show that vitamin C may help protect non-smokers from the oxidative damage caused by second-hand smoke," said Dr Dietrich.
Professor Gladys Black of Berkeley's School of Public Health who also worked on the study, said the findings could help people who are exposed to second-hand smoke regularly.
An estimated three million people in the UK are exposed to second-hand smoke while at work.
She said: "The message of the study is clearly not that taking vitamin C makes smoking or exposing others to smoke OK.
"But if you are in a situation where you cannot escape frequent exposure to second-hand smoke, it may be worthwhile to take vitamin C supplements as a precautionary measure and, as always, eat a diet rich in fruit and vegetables."
She added: "The evidence is overwhelmingly consistent that people who eat a lot of fruit and vegetables have a lower risk of cancer."
The Cambridge University study reported that vitamin C could reduce the risks of smokers developing respiratory disease.
But in the long term smoking inevitablity eliminates any protective effect and increases a persons chances of a variety of respiratory disorders, including cancer and obstruction of the airways.
The findings are published in the journal Nutrition and Cancer.
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