Commuters cover their mouths as they wait to cross a road in the haze in Singapore on June 20. Singapore's air quality deteriorated to "hazardous" levels late on Wednesday as smoke from slash-and-burn land clearing in Indonesia enveloped the city-state, inflaming tensions between the Southeast Asian neighbours.

The Pollution Standards Index (PSI) soared to a record high of 321 at 10 p.m., up from 290 just an hour earlier and below 200 earlier in the day. A PSI reading above 300 indicates "hazardous" air quality, while a reading between 201 and 300 means "very unhealthy". Reuters/Edgar Su