|The study finds that a third of those killed annually by passive smoking are children [EPA]
Second-hand tobacco smoke kills upwards of 600,000 people every year, nearly a third of them children, according to a global assessment in The Lancet, a British medical journal.
The findings, released on Friday in the first ever global study, indicate that unlike "lifestyle" diseases, which stem largely from individual choice, the victims of passive smoking pay the ultimate price for the health-wrecking behaviour of others, especially family members.
Among non-smokers worldwide, 40 per cent of children, 35 per cent of women and 33 percent of men were exposed to second-hand smoke in 2004, the most recent year for which data was available across the 192 countries examined.
In addition to 5.1 deaths caused by active smoking, the final death toll from tobacco for 2004 was more than 5.7 million people, the study concluded.
Nearly half of the passive-smoking deaths occurred in women, with the rest divided almost equally between children and men, according to the study.
Some 60 per cent of the deaths were caused by heart disease and 30 per cent by lower respiratory infections, followed by asthma and lung cancer.
All told, passive smoking accounted for one per cent of worldwide mortality in 2004.
Adult deaths caused by second-hand tobacco were spread evenly across the spectrum of poor-to-rich nations.
But for children, poverty made things much worse, the study found.
The adult-to-child ratio of deaths in high-income Europe, for example, was 35,388 to 71 while the ratio in Africa was nearly reversed: 9,514 to 43,375.
"Children's exposure to second-hand smoke most likely happens at home," the researchers noted. "Infectious diseases and tobacco seems to be a deadly combination."
The tragedy of children impacted by others' smoke is even greater when calculated in years of life lost, rather than lives lost.
One reason twice as many non-smoking women die is simply because they outnumber their male counterparts by 60 percent.
But in the developing world, they are also 50 percent more likely to be exposed to harmful smoke.
Enacting smoke-free laws for public spaces could significantly reduce passive smoking mortality and health care costs, said Annette Pruss-Ustun, the lead researcher.
Currently, only a small fraction - 7.4 per cent - of the world population lives in places with stringent smoke-free laws, and even in these jurisdictions, compliance is spotty.
Earlier research has shown that where laws are enforced, exposure to second-hand smoke in high-risk settings such as bars and restaurants is cut by 90 per cent.
Anti-smoking regulations also lower cigarette consumption, and improve one's chances of kicking the habit.
The researchers recommend fully applying the World Health Organisation's (WHO) Framework Convention on Tobacco Control, which includes high taxes on tobacco products, banning tobacco advertising and the use of nondescript packaging.
"There can be no question that the 1.2 billion smokers in the world are exposing billions of non-smokers to second-hand smoke, a disease-causing indoor pollutant," noted Heather Wipfli and Jonathan Samet, of the University of Southern California in Los Angeles.
"Broad initiatives are needed to motivate families to put their own policies into place to reduce exposure ... at home."