As the Hindu festival of Diwali kicks off on 10 November, the Indian capital of Delhi, already blanketed in choking smog, is bracing for pollution to worsen. Over the past week, children struggling to breath the acrid air have flooded hospital emergency departments, and schools have been forced to close. Why is Delhi’s air pollution so bad right now?

Since 3 November, the air quality index (AQI) — a measure of eight pollutants, including fine particulate matter, ozone and sulfur dioxide — in Delhi has remained consistently above 99. Anything above 150 is classified as unhealthy. On 6 November, the AQI surpassed 500, the top of the scale.

The city’s daily concentration of fine particles less than 2.5 micrometres in diameter (PM_2.5) — which can enter the bloodstream, leading to poor health — remained above 200 micrograms per cubic metre between 3 November and 9 November. World Health Organization (WHO) guidelines recommend concentrations stay below an average of 15 micrograms per cubic metre over 24 hours.

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