Authorities in the Thai capital ordered nearly 450 schools to close on Wednesday as pollution levels reached dangerously unhealthy levels. Bangkok, the world's most visited city, saw levels of PM 2.5 particles at 78.3 pg/m3. A level above 35 is considered unhealthy, according to independent air quality monitor AirVisual. Bangkok's air quality index (AQI) on Wednesday was 163, according to data from AirVisual.