About 17 million babies worldwide live in areas where outdoor air pollution is six times the recommended limit, and their brain development is at risk, the U.N. children's agency (UNICEF) said on Wednesday.
The majority of these babies – more than 12 million – are in South Asia, it said, in a study of children under one-year-old, using satellite imagery to identify worst-affected regions.
"Not only do pollutants harm babies' developing lungs – they can permanently damage their developing brains – and, thus, their futures," said UNICEF executive director Anthony Lake.
Any air pollution above the World Health Organization's recommended limit is potentially harmful for children, and risks grow as pollution worsens, UNICEF said.
Air pollution has already been linked to asthma, bronchitis, and other long-term respiratory diseases.
"But a growing body of scientific research points to a potential new risk that air pollution poses to children's lives and futures: its impact on their developing brains," UNICEF said.
The report highlighted links found between pollution and brain functions "including verbal and nonverbal IQ and memory, reduced test scores, grade point averages among school children, as well as other neurological behavioral problems."
"As more and more of the world urbanizes, and without adequate protection and pollution reduction measures, more children will be at risk in the years to come."
'Danger in the air'
The ultra-fine particles in city pollution can damage the blood-brain barrier -- a delicate membrane that protects the brain from toxic substances.
Damage to the membrane has been linked to Alzheimers and Parkinson's disease in the elderly.
UNICEF also highlighted the growing risk from minute particles of the iron ore magnetite which is increasingly found in urban pollution.
The nano-particles, which easily get into the blood stream, are highly dangerous to the brain because of their magnetic charge and have also been linked to degenerative diseases.
The author of the "Danger In The Air" report, Nicholas Rees, told AFP that toxic pollution is "impacting children's learning, their memories, linguistic and motor skills."
Delhi closed schools in early November after doctors declared a public health emergency, but quickly reopened them -- provoking anger from parents who accused authorities of "playing with children's health".
The crisis saw large swathes of north India and parts of neighboring Pakistan blanketed in acrid air -- an annual phenomenon as cooler air traps particles near the ground, cause pollution levels to spike.
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.
UNICEF urged more efforts to cut pollution, and also to reduce children's exposure to the poisonous smog which has frequently reached hazardous levels in Indian cities in recent weeks.
It called for a greater use of masks, air filtration systems and for children to avoid traveling when pollution levels are at their highest.
Rees said masks help "but very importantly they have to have good filters and they also have to fit children's faces well. A mask that does not fit the face well won't work."