The white foam is a mixture of industrial waste and sewage, and covers much of the Yamuna River – the largest tributary of the Ganges.

This is not the first time toxic foam has covered rivers in the million-strong city of New Delhi, he wrote CNN World.

The foam should contain large amounts of ammonia and phosphoric acid, which can cause breathing difficulties and skin problems, according to experts.

The foam comes at a time when the city is battling severe air pollution, which is said to have made many people sick, and forced some schools and offices to close, according to CNN.

Spread with the wind

The Yamuna River has for decades been characterized by heavy pollution due to the discharge of toxic chemicals and waste. In some areas of the river, the water is dark and murky, with large rivers filled with plastic waste on the banks.

This river stretches from the Himalayas to Allahabad, and is more than 130 miles long.

Although only two percent of the river is in the capital, New Delhi accounts for 76 percent of the pollution, according to a investigation carried out by Indian authorities.

Despite the huge pollution and toxic foam, many people still wash and drink from the river.

New Delhi was this week ranked the city with the worst air quality in the world, according to IQair – a Swiss company that measures air quality. Their measurements showed an air quality index of 517 – meaning the air was toxic.

In second place are Norway and Oslo, with an index of three.