In 2016, 600,000 children were estimated to have died from acute lower respiratory infections caused by polluted air.
Air pollution is one of the leading threats to health in children under 5, accounting for almost one in 10 deaths among this age group, the report reveals.
"This is inexcusable. Every child should be able to breathe clean air so they can grow and fulfil their full potential" said WHO Director-General Dr. Tedros Adhanom Ghebreyesus in a statement.
Air pollution also effects neurological development and cognitive ability and can trigger asthma and childhood cancer, the report says. Children exposed to excessive pollution may also be at greater risk of chronic conditions such as cardiovascular disease in adulthood.
"Air pollution is stunting our children's brains, affecting their health in more ways than we suspected. But there are many straightforward ways to reduce emissions of dangerous pollutants," said Dr. Maria Neira, director of the Department of Public Health, Environmental and Social Determinants of Health at the WHO.
According to the WHO, children are more susceptible to pollution because they breathe more often, taking in more pollutants, and are closer to the ground, which is where some pollutants have higher concentrations.
In few places is pollution more pertinent than India's most populous city, Delhi, where residents are bracing themselves for peak crop-burning season and the annual Hindu festival of Diwali, both of which add to a thick, toxic smog cast over the city.
"Air pollution is one of the leading risk factors for the national burden of disease in India," the report states, adding that researchers tracked more than 1,000 women in India throughout pregnancy and found a direct correlation between increased exposure to pollution and premature, underweight babies.
Air conditions are deteriorating quickly in the country's capital; 29 monitoring stations in the city on Monday recorded "very poor" air quality while four stations recorded air quality as "severe." Delhi is now the second most polluted major city in the world, according to air quality tracker AirVisual, second only to Lahore in Pakistan.