Choking winter smog blanketed the Indian capital New Delhi on Tuesday, rivaling some of the worst air pollution episodes in Chinese cities and at one point touching more than 31 times the international recommended limit for particles of dust and toxic liquids.
Security guards and pedestrians wheezed audibly in the streets, while the sun and sky were hidden from view by a grey-brown cloud of smog judged by the standards of the World Health Organisation to be very dangerous to human health.
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According to the Delhi Pollution Control Committee, so-called PM10 particulate matter (particles with a diameter of less than 10 micrograms) peaked at 1,560 micrograms per cubic meter of air at 11am at its Anand Vihar monitoring station.
The WHO's "guideline" limit is 50 micrograms averaged over 24 hours, although any level above zero is regarded as a health risk. Particulates include sulfate, nitrates, ammonia, salt, carbon, dust and water.
Anumita Roychowdhury, an executive director at the Center for Science and Environment (CSE), a non-government group, said smog had become a feature of Delhi's cool winters, while Tuesday's episode was particularly serious because there was very little wind to shift the pollution away from the city.
"This shows that the pollution levels are always so bad that the moment nature is not on your side you see the actual levels in your face," she said.
The 22m residents of greater Delhi have enjoyed a brief respite from smog in recent years following the conversion of public transport from diesel to compressed natural gas a decade ago. Power stations and polluting industries have also been moved out of the city centre.
Vehicle pollution, however, continues to worsen, with 1,400 vehicles a day being added to the city's fleet of cars, trucks and two-wheelers. Ms Roychowdhury said more than half of cars sold now were powered by diesel, which contributes to particulate pollution, compared with only 4 per cent in 2000, probably because diesel is subsidized whereas petrol is not.
She called for the Delhi government to emulate other cities around the world in dealing with short-term "pollution emergencies", for example by restricting the use of cars. In the longer term, Delhi needed to reduce the use of personal vehicles and improve fuel quality and vehicle efficiency.
Also on Tuesday, the International Council on Clean Transportation, another environmental research group, called on India rapidly to adopt low-sulfur fuels and stricter vehicle emission standards to improve air quality and public health.
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"Policy action is urgently needed to control vehicular pollution that now causes nearly 40,000 premature deaths each year," the group said as it launched a report on India.
"Our study suggests strongly not just that India can afford ambitious policies on clean vehicles and fuels, but that it can ill-afford not to pursue such policies," said co-author Anup Bandivadekar. Ms Roychowdhury said India was "seven to 12 years behind Europe" on standards for fuels and vehicle emissions.