London: Scientists have discovered that exposure to second-hand tobacco smoke as a child or adult increases a woman`s risk of breast cancer.
Their study of nearly 80,000 women found breast cancer risk was a third higher among those who had clocked up decades of passive smoking, reports the BBC.
But the researchers say their "suggestive" findings in the British Medical Journal need confirming with more studies.
The latest study, led by Juhua Luo from West Virginia University, does not provide a definitive answer on how much exposure is too much or at what age it could be most damaging.
However, it does suggest that extensive exposure to second-hand smoke poses a risk.
Among the 79,990 women aged between 50 and 79 studied, the researchers identified 3,250 cases of invasive breast cancer during 10 years of follow-up.
When the women were asked about their history of smoking and exposure to tobacco smoke a pattern emerged.
Smokers had a 16 percent increased risk of developing breast cancer after the menopause.
For ex-smokers the risk was 9 percent and continued for up to 20 years after an individual had stopped smoking.
The highest breast cancer risk was found among women who had smoked for over 50 years or more.
But women who had never smoked but had lived or worked with smokers for prolonged times also appeared to be at increased risk for breast cancer.
For example, over 10 years` exposure in childhood, over 20 years` exposure as an adult at home and over 10 years` exposure as an adult at work, increased the risk by 32 percent.