Secondhand smoke exposure has been linked conclusively to breast cancer, with half of all cases linked to workplace exposures. The Californian study found exposure to secondhand smoke increased the risk of breast cancer by 70 per cent. The data indicated exposure to secondhand smoke caused up to a fifth of new cases of breast cancer in women under 50. Almost half the risk came from exposure at work.

Kenneth C Johnson. Accumulating evidence on passive and active smoking and breast cancer risk, International Journal of Cancer, published online 31 May 2005.