The excess risk is almost half that of smoking 20 cigarettes a day, even though the exposure is only 1 per cent of that with smoking. Lifelong non-smokers who live with a smoker, and those who live with other non-smokers, were analysed by the Department of Environmental and Preventive Medicine at St Bartholomew's and the Royal London School of Medicine (JAMA, 1997; 278: 2192).