"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.