Of the 2 billion children who inhale pollution, the daily breath of 300 million is toxic far beyond safe levels. It's the most recent report highlighting the growing health problem air pollution poses.
Almost one in seven children worldwide live in areas with outdoor air so polluted that it can cause serious physical damage, including harming their developing brains, according to a study by the United Nations' children's agency UNICEF.
The study, released on Monday, uses satellite imagery to show for the first time how many children are exposed to outdoor pollution that exceeds global guidelines set by the World Health Organization (WHO), along with where they live across the globe.
About 2 billion children live in areas where outdoor air pollution exceeds minimum air-quality guidelines, while around 300 million of whom are exposed to pollution six times higher than this minimum - mostly in South Asia, according to the report.
Vulnerability to air pollution
Children are more susceptible than adults to outdoor and indoor air pollution because their brains, lungs and immune systems are still developing and their respiratory tracts are more permeable, says UNICEF.
Air pollution is directly linked to pneumonia and other respiratory diseases that account for almost one death in 10 in children under the age of five - making air pollution a leading factor in child mortality, said the agency.
The effects of air pollution on a child can have lifelong health implications, with studies showing that the lung capacity of children living in polluted environments can be reduced by 20 percent - similar to the effect of growing up in a home with secondhand cigarette smoke.
Indoor and outdoor pollution
The UNICEF study separated air pollution into two kinds, outdoor and indoor. Outdoor air pollution - which includes vehicle emissions, fossil fuels, dust and burning waste - is a growing problem. Recent estimates indicate that urban outdoor air pollution has risen by 8 percent globally between 2008 and 2013, according to the report.
The study found that unless action is taken to control outdoor air pollution, this will become the leading cause of environment-related child death by 2050.
The study also looked at indoor air pollution - which involves burning of solid fuels for household cooking, heating and lighting. The report said indoor air pollution is linked to diseases which kill more children than outdoor air pollution, and thus kills more children globally, especially in Africa and Asia.
One example cited in the report is Ulaanbaatar, the capital of Mongolia - there, air pollution is at its worst during the harsh winters where temperatures reaching as low as minus 40 degrees Celsius (minus 40 degrees Fahrenheit). Coal, wood and other solid fuels are used by 70 percent of the country's population to keep warm.
The high use of fuels, further aggravated by a weather phenomenon in the winter that traps air pollutants near the ground, makes Ulaanbaatar, a city of only 1.2 million, one of the cities with the worst air quality in the world - at times worse than Beijing and Delhi, said the report.
Outdoor air pollution may become the leading cause of environment-related child death by 2050, according to research
How to tackle air pollution
UNICEF published the study a week before the annual UN climate talks, with the upcoming round - COP22 - to be hosted by Morocco from November 7 to 18.
Nicholas Rees, the report's author, told DW that although COP22 is focusing on greenhouse gases, this is very much related to the air pollution that affects millions around the globe.
"Especially on the eve of COP22, reducing fossil fuels emission will go a long way - not just in terms of preventing the impact of climate change, but also reducing air pollution that impacts children’s health," he told DW.
Among the measures UNICEF is calling on world leaders to take is to cut back on fossil fuel combustion, and invest in energy efficiency and renewable energy sources.
It also wants better urban planning so that sources of pollution such as factories are not located within the vicinity of schools and playgrounds.
Anthony Lake, executive director of UNICEF, said: "Air pollution is a major contributing factor in the deaths of around 600,000 children under five every year, and it threatens the lives and futures of millions more every day."