A new study analyzing data from 2004 in 192 countries says second-hand smoke kills more than 600,000 people every year.
The first ever look at the global impact of passive smoking was published Friday in the British medical journal The Lancet.
Researchers found that 40 percent of children, and more than 30 percent of non-smoking men and women were exposed to second-hand smoke.
Scientists estimate that about 60 percent of the deaths were caused by heart disease and 30 percent by lower respiratory disease, followed by asthma and lung cancer.
Altogether, passive smoking accounted for about one percent of the world's deaths.
Scientists say passing and enforcing smoke-free laws for public spaces could significantly reduce mortality and health care costs from passive smoking.
But researchers also noted that children's exposure to second-hand smoke happens most likely at home. They say that "the mix of infectious diseases and tobacco seems to be a deadly combination."
Some information for this report was provided by AFP and AP.