GENEVA, July 27 (Reuters) - Four million children under the age of five die every year due to environmental hazards including polluted air or water, or exposure to chemicals, the World Health Organisation (WHO) said on Friday.

Poisonings, acute respiratory infections, diarrhoeal diseases and malaria carried by mosquitoes which thrive in dirty water account for most of the toll, the United Nations agency said in a technical report.

"This is something that intuitively we have always recognised, but we never put a number to it," WHO expert Jenny Pronczuk told a news briefing.

Some 30 percent of illnesses and deaths in children due to disease can be attributed to environmental factors, according to the report.

But chemicals have different effects as children grow, and often the effects of exposure to toxins while in the womb only emerge later in adolescence, the WHO said in the report drawn up by 24 scientific experts.

"For example if you look at lead exposure, the effect will be different if the child was exposed in utero because the lead of the mother goes into the bones of the child," Pronczuk said.

Africa is the region with the most environmental-related diseases, followed by parts of south east Asia, she added.

AlertNet news is provided by