Passive smoking claims more than 600,000 lives each year around the world - an estimated 1% of all deaths, a major study has found.
Children are the group most heavily exposed to second-hand tobacco smoke, and around 165,000 of them die as a result, said researchers.
In the UK, an estimated 23 children and 4,000 adults die each year as a result of passive smoking, according to the study authors.
A quarter of adult men and just over a fifth of women were exposed to second-hand smoke.
Based on smoking rates among adults, around 55% of all British children were thought to have at least one smoking parent.
The World Health Organisation (WHO) study is the first to assess the global impact of inhaling other people’s smoke.
Based on 2004 data, the figures show smoking in that year killed almost six million people, including non-smokers exposed to secondhand smoke.
Passive smoking was believed to have caused 379,000 deaths from heart disease, 165,000 from respiratory infections, 36,900 from asthma and 21,400 from lung cancer.
In addition 10.9 million years of disability-free life were lost globally because of passive smoking.
The findings are published today in an early on-line edition of The Lancet medical journal.
Dr Annette Pruss-Ustun, from the WHO in Geneva, Switzerland, and her co-authors wrote: “Exposure to second-hand smoke is still one of the most common indoor pollutants worldwide.
“On the basis of the proportions of second-hand smoke exposure, as many as 40% of children, 35% of women and 33% of men are regularly exposed to second-hand smoke indoors.
“We have estimated that second-hand smoke caused 603,000 deaths… worldwide in 2004, corresponding to 1% of all deaths.”
Dame Helena Shovelton, chief executive of the British Lung Foundation, said: “We welcome this new evidence which shows the devastating effects of passive smoking on a global scale, including the deaths of 165,000 children. All of these deaths are preventable.
“This is why the British Lung Foundation is urging the government to prevent children from exposure to second-hand smoke by introducing a ban on smoking in cars when minors are present. Our research has shown that 86% of parents in UK would support a ban in cars.
“This overwhelming evidence and public support can no longer be ignored and as the only UK charity supporting everyone affected by lung disease, we are calling for this legislation sooner rather than later.”