The World Health Organization says almost a quarter of all disease around the world is caused by environmental exposures that can be avoided. A new report shows children are particularly at risk. It finds more than 33 percent of disease in children under the age of five is linked to the environment.

The World Health Organization says this is the most comprehensive study done on how preventable environmental hazards contribute to a wide range of diseases and injuries. The study looks at 102 diseases and finds 85 of them are strongly influenced by environmental factors.

The lead author of the report, Annette Pruess-Ustun, says practically everyone is affected in some way. But, she says not everyone is affected in the same way. There are great disparities between the poor and rich countries.

"Three point five million deaths a year are from three diseases; lower respiratory infections, malaria, and diarrheal diseases," said Annette Pruess-Ustun. "The great majority of this burden touches children in developing countries. So, this highlights the great inequalities. Another example. Four million deaths a year are combined from cancers and cardiovascular diseases. Now here we have the reverse. The affects mostly adults in the developed world."

Pruess-Ustun says in the developing world children are getting sick and dying from unclean water, bad sanitation, and indoor smoke pollution. In the developed world, she says chemical exposures and the occupational stresses that arise from modern life are behind cancer and cardiovascular disease.

The report estimates more than 13 million deaths annually are due to preventable environmental causes. It says the four main diseases influenced by poor environments are diarrhea, lower respiratory infections, unintentional injuries and malaria. It says preventing environmental risk could save as many as four million lives a year, mostly in developing countries.

WHO Director of Public Health and Environment Maria Neira says cheap and effective tools are available that can reduce death and disease from environmental causes.

"We are talking about reducing air pollution indoor and outdoor," said Maria Neira. "We are talking about better access to clean water. We are talking about the prevention of chronic and acute respiratory infections through the use of better fuels. And, we are talking about even the prevention of some non-communicable diseases as cancer by regulating the exposure to certain chemicals or improving the working environment and making sure that all of these will be contributing to our health."

The World Health Organization says studies show the cost-benefits of cleaning up hazardous environments are huge. For example, it says investing in good water and sanitation gives an eight-fold return and says investing in the reduction of indoor smoke pollution gives a seven-fold return.

WHO notes half of the world's population uses solid fuels such as wood and dung. It estimates 1.5 million people, mainly women and children, in developing countries die from indoor smoke pollution. It says buying a better stove that costs about $5 and using better fuel, such as gas, will prevent most of these deaths.