India's top court Tuesday gave authorities two days to devise a plan to tackle choking levels of smog in Delhi which have prompted warnings of a health "emergency" in the world's most polluted capital.

On Tuesday, the concentration of PM2.5 -- the fine particles linked to higher rates of chronic bronchitis, lung cancer and heart disease -- was at 372, a level considered "hazardous" by US embassy measures but down from Monday.

Here are five things to know about Delhi's smog problem:

What causes hazardous levels of smog?

New Delhi's air quality usually worsens with the onset of winter, particularly after the Diwali festival when millions of revellers let off heavily polluting firecrackers and cooler temperatures trap those pollutants.

The capital is also affected by dusty winds from the arid west, the burning of crop stubble in farms around the city and smoke from fires used in poorer neighbourhoods for heating and cooking.

A 2014 World Health Organization survey of more than 1,600 cities ranked Delhi as the most polluted.

Why is it getting worse?

Delhi's population continues to grow rapidly as tens of thousands of migrants arrive in the capital each year.

Dust-generating construction sites, industrial emissions as well as a dominance of coal-fired power plants all contribute to the increasingly noxious air.

Car sales have also soared as incomes rise with nearly 10 million vehicles jostling for space on Delhi's roads, adding to smog left behind by thousands of diesel-guzzling trucks that rumble across the capital every night.

What's being done to tackle it?

Authorities in Delhi on Monday closed schools for three days, banned all construction work for five days and temporarily closed a coal-fired power plant near the capital.

It also banned fire crackers, but said they would be allowed at religious events -- though not at weddings.

Authorities are also considering cloud-seeding to produce rain, a technique Beijing used to clear the air ahead of the 2008 Olympic Games.

Last year a higher levy on diesel trucks was brought in, while temporary driving restrictions were introduced in January that took around a million cars off the roads for two weeks.

Last month, the city government also announced plans to install air purifiers and a mist-making device at major intersections to curb choking pollution.

But the Delhi government is powerless to stop the burning of crop stubble in the nearby states of Haryana and Punjab, which takes place at the end of each harvest.

What's the economic impact?

Between five and 10 percent of workers in the capital have called in sick over the last week with respiratory problems, according to a survey published Monday by Indian business body ASSOCHAM.

ASSOCHAM also warned that persistently poor air quality could hit tourism.

"Environment and air pollution related issues might hurt brand India," said ASSOCHAM's general secretary, D.S. Rawat.

A study by the World Bank in 2013 said air pollution and other environmental degradation costs India $80 billion per year, about 5.7 percent of GDP.

Why does it matter?

Health advisories warn of "serious risk" of respiratory problems for residents of Delhi and say all outdoor physical activity should be avoided.

People with heart or lung disease, older adults and children should stay indoors and keep activity levels low.

According to an American study published in February, air pollution kills more than 5.5 million people around the world each year, with over half of those deaths occurring in fast-growing China and India.

Source: AFP