China's capital Beijing maintained an 'orange' pollution alert, the second-highest level, on Monday, closing highways, halting or suspending construction and prompting a warning to residents to stay indoors.

The choking pollution was caused by the 'unfavourable' weather, the Ministry of Environmental Protection said on Sunday. Emissions in northern China soar over winter as urban heating systems are switched on and low wind speeds have meant that polluted air has not been dispersed.

In New Delhi, the U.S. embassy's monitoring station recorded an air quality index of 372, which puts air pollution levels well into 'hazardous' territory. A thick smog blanketed the city and visibility was down to about 200 yards (metres).

Air quality in the city of 16 million is usually bad in winter, when coal fires are lit by the poor to ward off the cold. Traffic fumes, too, are trapped over the city by a temperature inversion and the lack of wind. However, the government has not raised any alarm over the current air quality and no advisories have been issued to the public.

In Beijing, a city of 22.5 million, the air quality index in some parts of the city soared to 500, its highest possible level. At levels higher than 300, residents are encouraged to remain indoors, according to government guidelines.

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