Toxic Haze Covers India's New Delhi

(BBC) Schools in Delhi and surrounding areas have moved classes online and construction has been banned as the Indian capital grapples with hazardous air quality.

On Monday morning, a toxic haze engulfed the city, affecting visibility and delaying flights and trains.

The city's air quality index (AQI) - which measures PM2.5, fine particulate matter that can clog lungs, and other pollutants - was more than 30 times the limit recommended by the World Health Organization. Exposure to this level of pollution can cause serious health problems, especially in children and elderly people.

Toxic air is a recurring problem in Delhi and its suburbs, especially during winters.

The problem is attributed to a mix of factors such as industrial emissions, vehicle exhaust, dropping temperatures, low wind speeds and the seasonal burning of crop stubble in neighbouring states.

Average AQI in Delhi early on Monday was 471, according to the government's Safar app.

India's pollution watchdog classifies AQI levels of 101–200 as moderate, 201–300 as poor, 301–400 as very poor, and anything above 400 as severe. Government-backed apps cap readings at 500, even though private and international monitors often record much higher levels.

The sudden dip in air quality levels since Saturday came after the capital had shown some improvement over the past week - when it oscillated between "poor" and "very poor".