The World Health Organization has said that around 1.7 million children - or a quarter of all global deaths of children under five - are due to unhealthy or polluted environments.
Dirty water and air, second-hand smoke and lack of adequate hygiene lead to fatal cases of diarrhoea, malaria and pneumonia, WHO said in its report Inheriting a sustainable world: Atlas on children's health and the environment.
Margaret Chan, director-general said: "A polluted environment is a deadly one - particularly for young children. Their developing organs and immune systems, and smaller bodies and airways, make them especially vulnerable to dirty air and water."
The report said that harmful exposure can start while the foetus is in the womb.
Increased exposure to bad air after they are born raises the risk of pneumonia as well as their lifelong risk of chronic respiratory diseases such as asthma. Air pollution also raises the lifelong risk of heart disease, stroke and cancer.
Children are also at higher risk of diarrhoea and pneumonia in households that do not have safe water and sanitation or are polluted with smoke from fuels like coal or dung used for cooking and heating.
Reuters reports that children are also exposed to harmful chemicals through food, water, air and products.
WHO expert on public health, Maria Neira urged governments to do more to make places safe for children.
"Investing in the removal of environmental risks to health, such as improving water quality or using cleaner fuels, will result in massive health benefits," she said.