Washing hands with soap can halve the number of young children suffering from pneumonia, the leading killer of youngsters under 5 years old worldwide.

It can also greatly reduce cases of diarrhea and the skin infection impetigo, scientists said on Friday.

Although it may seem simple and logical, researchers from the Centers for Disease Control (CDC) in Atlanta have shown improved hygiene could save many lives, particularly in poor countries.

"One of the things that this research shows it that there are even more health benefits to the simple act of hand washing than we have previously appreciated," said Dr Stephen Luby, the lead researcher and an epidemiologist at the CDC.

"Therefore efforts to promote it become that much more important," he added in an interview.

More than 27,000 children under the age of five, mostly in developing countries, die every day from preventable causes. Pneumonia and other respiratory infections kill an estimated 2 million children each year. Almost three-quarters of those who die are less than a year old.

Hand washing with soap is particularly important in poor countries because there are so many more pathogens in the environment. Children also do not have the same level of nutrition and access to healthcare as in richer nations so they are much more vulnerable.

"Removing the pathogens from a low income household has a greater health benefit than it would in London or Atlanta," Luby added.

He and colleagues compared the impact of routinely washing with soap in 900 households in squatter settlements in Karachi, Pakistan over a year. About 600 households received supplies of regular or antibacterial soap while 300, which acted as a control group received, received school supplies.

The homes were visited weekly to encourage better hygiene.

Cases of pneumonia and diarrhea were cut by 50 percent in families given soap compared to the control group. There was also a 34 percent drop in impetigo. There was no difference in households given the two types of soap.

The research, which was funded by the Procter & Gamble Company, is published in The Lancet medical journal.

Luby said washing hands is particularly important before preparing food and eating, after using the toilet and after cleaning an infant to reduce the amount of pathogens.

"The time has come to shout from the roof tops that hand-hygiene promotion should be a worldwide priority for public health and health care," Didier Pittet, of Switzerland's Hopitaux Universitaires de Geneve, said in a commentary.