Around one in a hundred deaths worldwide is due to passive smoking, which kills an estimated 600,000 people a year, World Health Organisation (WHO) researchers said on Friday.

In the first study to assess the global impact of second-hand smoke, WHO experts found that children are more heavily exposed to second-hand smoke than any other age-group, and around 165,000 of them a year die because of it.

"Two-thirds of these deaths occur in Africa and south Asia," the researchers, led by Annette Pruss-Ustun of the WHO in Geneva, wrote in their study.

Children's exposure to second-hand smoke is most likely to happen at home, and the double blow of infectious diseases and tobacco "seems to be a deadly combination for children in these regions", they said.

Commenting on the findings in the Lancet journal, Heather Wipfli and Jonathan Samet from the University of Southern California said policymakers try to motivate families to stop smoking in the home. 

"In some countries, smokefree homes are becoming the norm, but far from universally," they wrote.

The WHO researchers looked at data from 192 countries for their study. To get comprehensive data from all 192, they had to go back to 2004. They used mathematical modelling to estimate deaths and the number of years lost of life in good health.

Worldwide, 40 percent of children, 33 percent of non-smoking men and 35 percent non-smoking women were exposed to second-hand smoke in 2004, they found.

This exposure was estimated to have caused 379,000 deaths from heart disease, 165,000 from lower respiratory infections, 36,900 from asthma and 21,400 from lung cancer.

For the full impact of smoking, these deaths should be added to the estimated 5.1 million deaths a year attributable to active tobacco use, the researchers said.