Delhi was the world's most polluted major city on Saturday morning, according to Swiss group IQAir's live ranking, as the Indian capital faced severe pollution forecasts a day before Diwali, the Hindu festival of lights. India has ordered schools in and around Delhi to offer online classes, and encouraged people to work from home ahead of Diwali on Sunday, which is usually marked by the widespread use of firecrackers that further worsen the air quality. IQAir ranked Delhi first among the world's major cities at 10:35am (0505 GMT), with a US air quality index reading of 581, double that of Lahore in neighbouring Pakistan, at 285. IQAir's reading, based on concentrations of air pollutants, is not directly comparable to India's official index, which uses a different scale and methodology. Read: Diwali celebrations light up the city sky India's Central Pollution Control Board put Delhi's air quality index at 363 at 10am, in the "very poor" category. Friday's air quality in Delhi was the worst pre-Diwali air in eight years, local media said, citing official data. Pollution ​is an annual problem in many parts of northern India from the onset of winter, as ⁠cold, heavy air traps vehicle emissions, construction dust, and smoke from crop-waste burning.