Household cooking may be exposing thousands of people in developing countries to an increased of lung cancer.

Across the world many people in developing countries use unvented stoves for their domestic cooking that burn smoky coal or wood and produce high levels of indoor pollution.

Research suggests that stoves of this sort may increase the risk of lung cancer.

Scientists found that a switch to vented stoves among farmers in rural China significantly cut the risk of developing the disease.

The researchers compared lung cancer rates among 17,184 farmers from the Xuanwei region who switched from unvented firepits or stoves to stoves with chimneys to those among 4,048 farmers who continued to use unvented firepits or stoves.

Tobacco smoking and frequency of cooking were similar in both groups.

Biggest effect on women

The researchers found levels of indoor air pollution created by vented burning were less than 35% of levels created by unvented burning.

They calculated that changing from unvented to vented stoves was associated with a 41% reduction in lung cancer risk in men, and a 46% reduction in lung cancer risk in women.

The study was carried out by a team of scientists, including Dr Qing Lan, of the Chinese Academy of Preventive Medicine, and Dr Robert Chapman, of the US Environmental Protection Agency.

Writing in the Journal of the National Cancer Institute, they say: "We hope that the documentation presented in this article will stimulate further development and implementation of appropriate stove improvements and of other strategics to reduce indoor pollution in developing countries.

"That many millions of people cannot yet change household fuels renders these efforts urgent."