A thick layer of toxic foam has once again coated parts of a sacred river near New Delhi as the Indian capital battles an acrid and noxious smog that has settled across the city.

The white froth, a mixture of sewage and industrial waste, has formed over sections of the Yamuna River – a tributary of the holy Ganges River – which flows about 855 miles (1,376 kilometers) south from the Himalayas through several states.

The pungent foam contains high levels of ammonia and phosphates, which can cause respiratory and skin problems, according to experts. Its latest arrival has coincided with hazardous levels of pollution that have sickened many of New Delhi’s more than 20 million residents and forced primary schools and some offices to close.

A similar looking mixture has appeared in a canal in India’s southern Tamil Nadu state, according to the Press Trust of India. Video published by the news agency on Thursday shows strong gusts of wind carrying the froth onto the roads and into the paths of cars and motorcycles.

For decades, sections of the Yamuna have been plagued by the dumping of toxic chemicals and untreated sewage. In several sections, the river appears dark and sludgy, while plastic waste lines its banks.

The river is most polluted in areas surrounding Delhi, owing to the area’s dense population and high levels of waste. Only 2% of the river’s length flows through the capital, but Delhi contributes about 76% of the river’s total pollution, according to a government monitoring committee.

The mixture is a regular sight on the Yamuna and despite its toxicity many villagers downstream continue to use the water to bathe and even drink, experts say. Pictures from September also showed toxic foam forming on the Yamuna.

Hindu devotees are often seen performing rituals in the river, surrounded by the dense foam. Every year, many gather on the Yamuna’s banks to celebrate Chhath Puja, a festival dedicated to the sun god Lord Surya, some wading through the foam to bathe and pray.

This week, the toxic foam in New Delhi was joined by a throat-searing blanket of smog that prompted many panicked residents to buy air purifiers and wear face masks to minimize exposure to the fumes.

New Delhi has ranked as the most polluted city in the world for several consecutive days this week, according to Swiss air quality company IQAir. On Thursday, the city had an air quality index (AQI) of 517 – a level considered hazardous, according to the company.

By comparison, the world’s least polluted city, Oslo, has an AQI of just three. China’s capital Beijing, which used to frequently feature on the world’s most polluted list, has in recent years taken big steps to clean its air and has a current AQI of 25 – a number considered “good.”

Two other Indian cities – Kolkata and Mumbai – both ranked on IQAir’s list of the world’s 20 most polluted cities, with AQI numbers of 205 and 102, both considered unhealthy.