In India and Pakistan, schools and many businesses in Delhi and Lahore were closed by official order this week as levels of pollutants on some days rose far beyond the most severe category of the US Air Quality Index (AQI), the most commonly used international scale.
The US AQI has six categories, ranging from a “good” score of 0-50 to a “hazardous” score of 301-500. On 19 November the AQI was 22 in Paris, 28 in London, and 38 in New York. On that scale, Delhi reached 1758 at noon on 18 November, its worst day this year. Some monitoring stations in Lahore on 7 November recorded AQI scores above 1900.
Countless smaller cities in the region are also under a vast smog cloud, visible both from the ground and from space, that stretches across northern India and southeast Pakistan. The Pakistani city of Multan, which is similarly locked down, reached an AQI of 2135 on 8 November.
The most dangerous pollutants are PM2.5 particles, which are smaller than 2.5 microns in diameter and thus able to enter the bloodstream. Average concentrations should be no higher than 5 µg/m3 to avoid affecting human health, the World Health Organization advises. Spikes as high as 15 µg/m3 are considered safe if they last no more than four days.
On the cities’ worst days this month, …
