1 in 100 people around the world die from secondhand smoke each year, a new study reveals, and nearly two-thirds of the deaths occur in children.

Health officials have known that more than 1 billion people around the world smoke and 5 million people die each year from tobacco-related illness, according to the World Health Organization. That's about one person dying every six seconds.

But just how many people are sickened by secondhand smoke has been less clear, which led researchers to try to investigate how big the problem is. Based on 2004 data gathered from 192 countries, researchers estimate "as many as 40 percent of children, 35 percent of women, and 33 percent of men are regularly exposed to secondhand smoke indoors," according to a WHO study published in the British medical journal The Lancet.

"Tobacco use is one of the biggest public health threats the world has ever faced," says the WHO. More than 80 percent of the more than 1 billion smokers worldwide live in low- and middle-income countries, where the burden of tobacco-related illness and death is heaviest. The study authors estimate that 165,000 children under the age of 5 die each year from lower respiratory infections caused by second-hand smoke – and most of these deaths occur in Africa and south Asia.

Just two months ago, the Centers for Disease Control and Prevention released a report that found more than half of children in the United States, between ages 3 and 11 show signs in their blood of exposure to secondhand smoke. Previous studies have found that even extremely low levels of exposure to cigarette smoke produced detectable abnormal genetic activity in these cells.

The more than 1 billion smokers are exposing billions of non-smokers to one of the top indoor pollutants according to the WHO. Researchers believe more needs to be done to create complete smoke-free indoor environments at work, in public places and on public transportation. Jonathan Samet and Heather Wipfli, two leading public health experts from the University of Southern California say their research from 31 countries found that 88 percent of parents who smoked did so at home and that over 80 percent smoked near their children. In an accompanying commentary, they emphasize the need for smoke-free homes, which can help lower the number of people sickened and dying from someone else's smoke.

Wipfli and Samet say educating and empowering women can make a big difference is protecting children and non-smoking adults from the deadly effects of secondhand smoke. "Few sources of indoor air pollution can be completely eliminated. However, smoking indoors can be eliminated," they say.