"Not only do pollutants harm babies' developing lungs, they can permanently damage their developing brains, and thus, their futures", Anthony Lake, UNICEF's executive director, said.
The UNICEF report also states that South Asia has the largest proportion of babies living in areas where air pollution is at least six times higher than worldwide limits (10 micrograms per cubic metre).
A further four million were at risk in East Asia and the Pacific.
The report entitled Danger in the Air: How air pollution can affect brain development in young children also notes that breathing in particulate air pollution can damage brain tissue and undermine cognitive development - with lifelong implications and setbacks.
The effects lasted a lifetime, it said.
"Protecting children from air pollution not only benefits children", Lake added.
It called for wider use of face masks and air filtering systems, and for children not to travel during spikes in pollution.
Some schools in the city were closed but there was criticism when they re-opened, with parents accusing the authorities of disregarding their children's health.
Lastly, be aware about the air pollution levels near your area.
In China, where air pollution has cut life expectancy in the industrial north by three years, the government has imposed production curbs on industry to counter a smog crisis that rivals India's - but progress has been patchy.
Meanwhile a separate study by scientists at hospitals in London found that the British city's polluted air was leading to lower birth weights, linked to higher infant mortality and disease later in life.
Satellite imagery used to compile the data also revealed that the issue was growing in African cities, Unicef said.