Northern haze

Chiang Mai spent several days this month as the most polluted city on the planet. Indeed, its PM2.5 level — a measurement of inhalable particulate matter at individual particles diameters of 2.5 micrometres and smaller — reached 226 micrograms per cubic meter (μg/m³).

Pollution levels peak in northern Thailand during the burning season — normally in March and April — when crop waste burning and forest fires fill the air with particles, creating a haze so dense that Doi Suthep mountain, on the western edge of the city, is obscured.

The predictability of the annual surge in pollution and lack of effective preventative action have angered city…