WASHINGTON/ NEW DELHI-Some 300 million children live with outdoor air so polluted it can cause serious physical damage, including harming their developing brains, the United Nations said in a study released Monday.

Globally, about seven million deaths each year are linked to air pollution, 600,000 of them children younger than five, the Unicef report said, citing World Health Organization studies done in 2012 and last year.

"Not only pollutant particles affect lung development, but they are likely to cross the blood brain barrier and disrupt definitively the development of their brain", he said in a statement.

It is also worth mentioning that children have small airways and more permeable respiratory tracts, so they are more likely to suffer blockages or develop infections, as their immune system hasn't fully developed.

About 300 million children in the world are living in areas with outdoor air so toxic - six or more times higher than global pollution guidelines - that it can cause serious health damage, including harming their developing brains, a new United Nations Children's Fund (UNICEF) report has revealed.

Out of that 2 billion breathing toxic air, the report puts 620 million of them in South Asia - mostly northern India.

The findings come a week ahead of the COP 22 in Morocco, where Unicef is calling on world leaders to take urgent action to cut air pollution.

By this morning New Delhi was recording air pollution more than 90 times higher than the World Health Organization's recommendation

According to the report, air pollution has now become a leading killer in kids under age five as it is contributing to more deaths yearly than malaria and HIV/AIDS combined. As north Indian cities enter the season of high air pollution, a new report is warning about the dangers to children.

Around two billion children, meaning more than 25 percent of the entire population, are now living in countries where air pollution is way above the yearly limit established by the World Health Organization (WHO). The most vulnerable children live in low-and middle-income countries, reveals the research.

Children tend to breathe in twice as much air as adults, and have smaller airways that are more easily obstructed by infections.

The toxins in the air interfere with brains still developing and children in regions with poor quality air are at greater risk developmental disability, she said.

Dharmendra rarely steps out into New Delhi's thick, smoggy air anymore. "The multiplier effect of reducing fossil fuel combustion on the wellbeing of children stands to be enormous". Since then, it has done measures to clean its air.

Levels of PM2.5, tiny particulate matter (PM) that reaches deep into the lungs, more than doubled within a few hours to 750 micrograms per cu. metre in the city's worst affected parts, India's Central Pollution Control Board said.

It also supports programmes to increase children's access to quality healthcare and to vaccinate them against conditions like pneumonia.