Most of the affected children - 220 million - live in South Asia.
A special study conducted by the United Nations Children's Fund (UNICEF) shows that hundreds of thousands of children die a year from diseases caused by air pollution.
"We protect our children when we protect the quality of our air", said UNICEF executive director Anthony Lake.
"Together, outdoor and indoor air pollution are directly linked to pneumonia and other respiratory diseases that account for nearly one in 10 under-five deaths, making air pollution one of the leading dangers to children's health", reads a press statement from UNICEF.
Commenting on installation of giant outdoor air purifiers at five highly polluted locations, he said: "It is true that five purifiers won't really matter for entire Delhi but there are local areas where the pollution-level is really high".
Beyond the children living in the most toxic air, about 2 billion children in the world, constituting the vast majority, live in places where air pollution exceeds the level that the WHO considers unhealthy, the report said.
Risky levels of air pollutants are known to increase the risk of asthma, stroke and lung cancer - and scientists have found children are more vulnerable to the effects of air pollution.
"Air pollution affects poor children the most", Nicholas Rees, a UNICEF specialist on climate and economic analysis who wrote the report, told Reuters. The call came in response to a new report issued Monday by UNICEF, which called on world leaders to reduce the air pollution the organization says impacts around two billion children globally.
Minimize children's exposure: Sources of pollution such as factories should not be located within the vicinity of schools and playgrounds. As urban air quality declines, researchers say, people living in cities face increased risks of heart disease, stroke, lung cancer and acute respiratory illnesses.
PM2.5 particles and droplets are considered to be the most harmful kind of air pollution because they are fine enough to evade the body's natural filters, penetrate the lungs and enter the bloodstream.
Burning of solid fuels for household cooking, heating and lighting is a major cause of household, or indoor, air pollution.
Children are known to be more susceptible than adults to both indoor and outdoor air pollution as their lungs, brains and immune systems are stil developing.
You can view air quality from the US Embassy, Indian government monitoring stations as well as other Airveda monitors set up around your city and rest of the country.