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 by the World Health Organisation (WHO).
PM 2.5 particles can include dust, soot and smoke and are so small they can lodge deep in the lungs and enter the bloodstream.
Bangkok's air quality index (AQI) on January 22 was 163, according to data from AirVisual. Anything above 150 is considered unhealthy.
Like many growing Asian cities, Bangkok is plagued by vehicle fumes, dust from construction sites and emissions from industry but the burning of stubble and undergrowth in fields in surrounding rural areas is believed to contribute to much higher pollution levels in the dry, winter months.
Last January, the city closed schools for a week because of pollution.