Schools in India's capital New Delhi reopened on Wednesday after closing for two days due to a spike in air pollution that triggered a public health emergency and prompted protests by residents and environmental groups.

Children, many wearing face masks, returned to schools in New Delhi and its surrounding cities even as air pollution stayed at "unhealthy" levels in most parts of northern India, including the capital city.

India's weather office forecast light rains on Wednesday and a downpour on Thursday. While drizzle could lead to a further deterioration in air quality, torrential rains tend to bring down pollution levels, which hit their highest levels of the year on Sunday.

The overall air quality across the city showed some improvement for the second consecutive day thanks to stronger winds which helped blow away some of the pollutants hanging in the atmosphere.

Authorities in New Delhi banished cars with number plates ending in an odd number on Monday in a bid to reduce the pollution, although politicians in the city of more than 20 million people, bickered over the causes of the pollution and who should take the blame for it.

Environmental experts say any increase in the burning of crop stubble in Punjab and Haryana states - part of India's farm belt that borders Delhi - would lead to an increase in pollution levels. The burning of crop residues have helped turn India's capital into the most polluted major city in the world.

On Tuesday night, more than 1,500 people gathered at India Gate, the war memorial at the centre of Delhi, to protest persistently high pollution levels and demand action from the federal and the state governments.

"People are angry because it's a collective failure and politicians need to come together to chalk out a comprehensive plan to address this crisis," said Vimlendu Jha, an environmentalist who founded the activist group Swechha.

The air quality index measured by the U.S. Embassy in New Delhi stood at "unhealthy" levels of 177 on Wednesday, down from 331 on Tuesday morning.

Anything above 401 is classified as "severe". Air pollution at that level can seriously affect those with existing respiratory illnesses, and even those who are otherwise healthy.

PM 2.5, or tiny particulate matter that is less than 2.5 microns in diameter, are considered particularly dangerous because they lodge deep in the lungs. PM 10 is another pollutant that is inhaled when people breathe through their mouths.