"The combination of infectious diseases and tobacco seems to be a deadly combination for children in these regions and might hamper the efforts to reduce the mortality rate for those aged younger than five years, as sought by Millennium Development Goal Four."
The study authors noted that smoke-free laws appear to have a positive and rapid effect on people's health.
"Policymakers should bear in mind that enforcing complete smoke-free laws will probably substantially reduce the number of deaths attributable to exposure to secondhand smoke within the first year of its implementation, with accompanying reduction in costs of illness in social and health systems," they wrote.
When the figures for deaths attributable to passive smoking are added to those caused by active smoking in 2004 (5.1 million), the study indicates that smoking was responsible for more than 5.7 million deaths that year.
The study authors concluded that countries should enforce WHO's Framework Convention on Tobacco Control without delay by introducing higher tobacco taxes, plain cigarette packaging and advertising bans.
In addition, they called for educational strategies to encourage people to stop smoking in their homes and urged developing countries to pay as much attention to tobacco-related diseases as infectious ones.
They observed: "Prompt attention is needed to dispel the myth that developing countries can wait to deal with tobacco-related diseases until they have dealt with infectious diseases. Together, tobacco smoke and infections lead to substantial, avoidable mortality and loss of active life-years of children."
Robin Hewings, Cancer Research UK's tobacco control manager, said: "This research shows the terrible toll tobacco takes on the health of smokers and non-smokers. The ban on smoking in public places has massively reduced people's exposure to secondhand smoke in the workplace and it has encouraged people to make their cars and homes smoke-free.
"Successful campaigns to encourage people not to smoke indoors need to be continued while measures that make teenagers less likely to start, such as the removal of displays of cigarettes in shops, must be pursued."
Öberg, M., Jaakkola, M., Woodward, A., Peruga, A., & Prüss-Ustün, A. (2010). Worldwide burden of disease from exposure to second-hand smoke: a retrospective analysis of data from 192 countries The Lancet DOI: 10.1016/S0140-6736(10)61388-8