Dr Siegfried believes the hormone may exacerbate the effects of exposure to tobacco smoke - and to other substances known to be linked to lung cancer such as the gas radon.

BBC: Lung cancer risk greater for women

For these reasons, they are unable to demonstrate whether an observed reduction in hospital admissions was due to measurable reductions in exposure to tobacco smoke or to other factors, such as the long-term continuing decline in heart disease rates, other factors that may affect smoking prevalence, changes in other risk factors for heart disease, or mere chance fluctuations.

FORBES: Do Smoking Bans Really Save Lives? If So, Whose?

Most studies are based on imprecise recall and anecdotal evidence concerning the exact amount of exposure to environmental tobacco smoke.

BBC: NEWS | Health | Cancer alert for smoking parents

He said it was possible that people had modified their lifestyles to reduce exposure to risk factors such as tobacco smoke.

BBC: Childhood asthma starting to fall

Smoking bans could have an effect on reducing heart attacks or heart disease generally by one of two major routes: either by inducing current smokers to quit or by reducing the exposure of non-smokers to environmental tobacco smoke (ETS).

FORBES: Do Smoking Bans Really Save Lives? If So, Whose?