The air quality index stood at “hazardous” levels of 497, touching nearly 700 in parts of the city of 20 million.

Haze shrouded India’s capital city again on Monday, with residents braving dangerous air quality to return to work after a weekend of clearer air and better weather.

The air quality index of the US Embassy in New Delhi stood at “hazardous” levels of 497 as of noon local time (06:30 GMT), with levels of airborne PM 2.5 – particles that can reach deep into the lungs – touching nearly 700 in parts of the city.

That is more than 10 times the recommended safe limit of 60 for PM 2.5.

A dip in wind speed and temperature is making air denser, trapping pollutants and worsening air quality, said Vivek Chattopadhyay, a senior programme manager at New Delhi-based non-profit Centre for Science and Environment.

Last week, pollution reached record levels causing a public outcry.

Calling Delhi a “gas chamber”, Delhi Chief Minister Arvind Kejriwal declared a public health emergency.

The city government in the metropolis of over 20 million has put restrictions on the use of private cars with an “odd-even” system – allowing cars to run on alternate days, depending on whether their licence plate ends in an odd or even number.

The scheme, which includes a two-day waiver for a religious festival, has helped little, prompting environmentalists to call for urgent action.

“The chief minister (of Delhi) needs to declare an emergency,” said Bharati Chaturvedi, founder of the Chintan environmental advocacy group.

“If this was the plague, he would have declared an emergency.”

Every year, as India’s winter season approaches, farmers in Delhi’s neighbouring Punjab and Haryana states, where agriculture is a mainstay, burn off rice field stubble in preparation for the sowing season.

The smoke from fields mixes with vehicle exhaust fumes and construction dust, making Delhi the world’s most-polluted capital.

India’s Supreme Court last week chided authorities for their failure to curb the pollution and asked the city government, its neighbouring states and the federal government to work together to help improve air quality.