Passive smoking kills more than 600,000 people annually, and close to one-third of them are children, reveals the global assessment report.
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 percent of children, 35 percent 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.
When added to the 5.1 million fatalities attributable to active smoking, the final death toll from tobacco for 2004 was more than 5.7 million people, the study concluded.
Nearly half the passive-smoking deaths occurred in women, with the rest divided almost equally between children and men, said the study, published in the British medical journal The Lancet.
Some 60 percent were caused by heart disease and 30 percent by lower respiratory infections, followed by asthma and lung cancer.
All told, passive smoking accounted for fully one percent 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 fatalities in high-income Europe, for example, was 35,388 to 71.
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 felled 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 they are also, in the developing world, 50 percent more likely to be exposed to harmful smoke.
Passing and enforcing smoke-free laws for public spaces could significantly reduce passive smoking mortality and health care costs, said lead researcher Annette Pruss-Ustun.
Currently, only a small fraction -- 7.4 percent -- of the world population lives in areas with serious smoke-free laws, and even in these jurisdictions compliance is spotty.
Where laws are enforced, exposure to second-hand smoke in high-risk settings such as bars and restaurants is cut by 90 percent, earlier research has shown.
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 tobacco tax hikes, advertising bans, 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," note 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," they say in a commentary, also in The Lancet.