Unsanitary and polluted environments can lead to fatal cases of diarrhea, malaria and pneumonia, which kill 1.7 million children a year, the WHO reports. The UN agency found that the most common causes are preventable.
The report "Inheriting a Sustainable World: Atlas on Children's Health and the Environment," released on Monday by the the UN's public health arm, noted that globally about 25 percent of deaths of children younger than 5 were caused by unhealthy or polluted environments.
More than 90 percent of the world's population is thought to breathe air that violates quality guidelines set by the World Health Organization (WHO), the report noted.
"A polluted environment is a deadly one - particularly for young children," WHO Director-General Margaret Chan said in a statement. "Their developing organs and immune systems, and smaller bodies and airways, make them especially vulnerable to dirty air and water."
UNICEF reported last year that 2 million children breath bad air.
Increased health risks
The report noted that harmful exposure can start in the womb and then continue if infants and toddlers are exposed to indoor and outdoor air pollution and secondhand smoke.
"This increases their childhood risk of pneumonia, as well as their lifelong risk of chronic respiratory diseases such as asthma," the report found. "Air pollution also increases the lifelong risk of heart disease, stroke and cancer."
In households without access to safe water and sanitation or that are polluted with smoke from unclean fuels such as coal or dung for cooking and heating, children are at higher risk of diarrhea and pneumonia, the report found.
Children are also exposed to harmful chemicals through food, water, air and products around them, according to the report.
"Investing in the removal of environmental risks to health, such as improving water quality or using cleaner fuels, will result in massive health benefits," said Maria Neira, a WHO expert on public health.
Top environmental causes
The top causes were listed as respiratory infections - with 570,000 deaths in children younger than 5 linked to indoor and outdoor air pollution and secondhand smoke - and diarrhea, with 361,000 deaths in children under 5 linked to poor access to clean water, sanitation and hygiene.
The WHO reported 270,000 deaths in children under 1 month could be prevented by access to clean water, sanitation and hygiene in health facilities, and by reducing air pollution.
The agency also reported that 200,000 deaths caused by malaria in children under 5 could be prevented by environmental improvements, including reducing mosquito breeding sites or covering drinking water storage.
There were also 200,000 deaths among children under 5 linked to their environments, including poisonings, falls and drowning - cited as "unintentional injuries."