As Indians awake to smoke-filled skies from a weekend of festival fireworks, Delhi's worst season for air pollution begins - with dire consequences.
A new report from the UN children's agency, UNICEF, says about a third of the two billion children in the world who are breathing toxic air live in northern India and neighbouring countries, risking serious health effects including damage to their lungs, brain and other organs.
Of that global total, 300 million children are exposed to pollution levels more than six times higher than standards set by the World Health Organization, including 220 million in South Asia.
|New Delhi's dirty air threatens public health|
The alarming numbers are hardly a surprise. Delhi's air pollution, among the world's worst, surges each winter because of the season's weak winds and countless rubbish fires set alight to help people stay warm.
Even days before the city erupted in annual firework celebrations for the Hindu holiday of Diwali, recorded levels of tiny, lung-clogging particulate matter known as PM2.5 were considered dangerous at well above 300 micrograms per cubic metre.
On Monday, Delhi residents were advised to stay indoors, with health warnings issued for the young, elderly and those with respiratory or heart conditions.
Since being identified as one of the world's most polluted cities, Delhi has tried to clean its air in recent years.
It has barred lorries from city streets, required drivers to buy newer cars that meet higher emissions standards and carried out several weeks of experimental traffic control, limiting the number of cars on the road.
But other pollution sources, including construction dust and cooking fires fuelled by wood or kerosene, continue unabated.
Children face much higher health risks from air pollution than adults. Children breathe twice as quickly, taking in more air in relation to their body weight, while their brains and immune systems are still developing and vulnerable.
"The impact is commensurately shocking," with 600,000 children below the age of five across the world dying each year from air pollution-related diseases, Anthony Lake, UNICEF executive director, said in the report released on Monday.
"Millions more suffer from respiratory diseases that diminish their resilience and affect their physical and cognitive development.
The report was released in the run-up to the COP22 UN climate change summit in Marrakech starting on November 7.
Of the two billion children worldwide breathing unhealthy air, the report puts 620 million of them in South Asia - mostly in northern India.
Another 520 million children are breathing toxic air in Africa, and 450 million in East Asia, mainly China, according to the report, which combined satellite images of pollution and ground data with demographic patterns to determine which populations fell into the highest risk areas.
A WHO report released in September indicated that 90 percent of the world's population breathes polluted air and that three million deaths each year are linked to exposure from indoor and outdoor pollution.