Each year, more than five million children under the age of 14 die from illnesses caused by their environment, the WHO warned.

The worst killers among children were diarrhoea, malaria and acute respiratory illnesses.

"These deaths could be avoided by cleaning up the places where children spend time, whether at home, at school or in their neighbourhood," the text stressed.

In a bid to raise awareness of the environment's impact on children's health, the WHO has dedicated this year's World Health Day on Monday to tackling the problem.

According to the WHO, some 40 percent of all children under five suffer from health problems related to their environment.

The key risk factors are unclean water, inadequate sanitation and waste disposal, poor food hygiene, poor housing, child abuse and neglect, home accidents and unsafe use of dangerous chemicals.

To drive the point home, the WHO text also stressed the huge economic burden of environment-linked disease among children.

Chronically-sick or disabled children cannot regularly attend school and are unlikely to grow up to be productive members of society, the statement said.

It argued that by investing in a healthier environment, countries would see immediate results in falling healthcare costs.

It gave the example of Mexico where it claims the state could save two billion dollars (1.86 billion euros) per year in healthcare costs by cutting airborne pollution by just 10 percent.

SPACE.WIRE