Each year, environmental pollutants lead to the death of an estimated 1.7 million children under five, the World Health Organisation (WHO) said in a new report on Monday.

The causes include unsafe water, lack of sanitation, poor hygiene practices and indoor and outdoor pollution, as well as injuries.

The new numbers equate to these pollutants being the cause of one in four deaths of children one month to five years old, media reported.

"A polluted environment is a deadly one -- particularly for young children," Margaret Chan, the WHO director-general, said in a statement. 

"Their developing organs and immune systems, and smaller bodies and airways, make them especially vulnerable to dirty air and water."

Infants exposed to indoor or outdoor air pollution, including secondhand smoke, have an increased risk of pneumonia during childhood as well as an increased risk of chronic respiratory diseases -- such as asthma -- for the rest of their lives, the report stated.

The global body also highlighted the increased risk of heart disease, stroke and cancer from exposure to air pollution.

However, new report highlights that the most common causes -- diarrhea, malaria and pneumonia -- of child death are preventable through interventions like insecticide-treated bed nets, clean cooking fuels and improved access to clean water, are already available to the communities most affected, media added. 

Other potential pollution prevention solutions mentioned in the report are removing mould and pests from housing, removing lead paint, ensuring sanitation and good nutrition at schools and using better urban planning to create more green spaces in cities.