Air pollution soared in New Delhi on Thursday to hazardous levels rarely seen outside winter months as sand blown from deserts enveloped the Indian capital in a once-in-a-decade phenomenon.
Doctors warned the grit carried by hot summer winds posed serious health risks to the city of 20 million and there was little to do "but pray for rain".
Smog spikes during winter in Delhi, already one of the world's most polluted cities, where air quality eclipses the World Health Organisation's safe levels on any given day.
Pollution levels usually ease in summer, providing some relief from the smog as temperatures soar to mid-40 degrees Celsius (113 Fahrenheit).
But the city woke up to stifling heat and smog 20 times safe levels Thursday, as strong winds blew dust from deserts in western India and beyond across the plains where Delhi lies.
"It is a very unusual phenomenon seen once in a decade or so. The dust is not settling down and the sky is obscure," said Mahesh Palawat, vice president of meteorology and climate change at SkyMet Weather, a private forecaster.
"It is quite different from winter pollution. It is the dust this time that is the culprit. It may cause breathing problems to many," he told AFP.
Levels of PM10 — dust and larger particles with a diameter of 10 micrometres — exceeded 900 per cubic metre in parts of Delhi on Thursday, according to the US embassy website. The WHO considers 50 the maximum safe level for PM10.
Readings from India's own pollution monitor on Thursday detected PM10 closer to 1,300 — or 26 times safe exposure levels.
Summer winds have been especially violent this season, experts say, with hundreds killed by powerful dust storms and freak winds across India's north.
The sand blowing into the capital is colliding with dust from open construction sites and car and factory exhaust, compounding the crisis.
Usually during winter, smog chokes Delhi as cool air traps pollutants close to the ground.
During these months it is PM2.5 — microscopic particles that can lodge deep into the lungs — that cause the most harm.
Ozone levels, usually a lesser concern given the magnitude of air pollution woes in Delhi, are also causing particular grief this summer, according to a recent report by the Centre for Science and Environment.
"The surprise this year is the number of days when the daily air quality index has shown ozone also as a dominant pollutant along with the particulate matter," said the centre's executive director Anumita Roychowdhury.
Arvind Kumar, a lung surgeon at a medical facility in New Delhi, said the smog threatened all, but children, the elderly and those with asthma were at particular risk.
"If such a situation was seen in the western world, cities would have been evacuated but for us we just need to pray for rain and the dust to settle down," he told AFP.