Secondhand smoke causes cancer – and the evidence is so compelling the International Agency for Research on Cancer (IARC) says it ‘puts a final stop to all controversies fuelled at various degrees by the tobacco industry.’ IARC’s Monograph on tobacco smoke and involuntary smoking was prepared by a scientific working group of 29 experts from 12 countries, which reviewed all published evidence related to tobacco smoking and cancer, and which concluded both smoking and exposure to secondhand smoke should be classified as ‘Group 1’ carcinogens, definitely causing cancer in humans.

IARC Monographs on the evaluation of carcinogenic risks to humans, volume 83, Tobacco smoke and involuntary smoking, 2004. Risks 161.