Asthma and other respiratory problems in children are likely to be caused by air pollutants. Their lungs have not fully developed even by the age of eight, and exposure to pollutants while they grow can lead to chronic respiratory disease later in life, the WHO study says.
There's also evidence to suggest that cancer and heart disease may be caused by exposure to pollutants during childhood.
In its new study, Principles for Evaluating Health Risks in Children Associated with Exposure to Chemicals, the WHO estimates that 13 million children's deaths every year could be avoided just by improving the environment. Of these, 4 million children die annually from exposure to pollutants in the air, in water and from exposure to chemicals.
The risks increase in children who are neglected and malnourished. One in five children in the poorest parts of the world has a life expectancy of just five years.
(Source: The World Health Organization).