If you had planned to call in sick today to have your senses checked out at the doctor's, please cancel your scheduled appointment.

You were not the only one who smelled the haze last night.

The 3-hour Pollutant Standards Index (PSI) had been slowly increasing since Tuesday afternoon before peaking at 87 at 8pm last night.

Even though it went down subsequently to 83 by 10pm, it was still the highest level this year.

Plausible-sounding explanations

The National Environment Agency (NEA) said there are a few possible causes for a burning smell in the air.

One reason could be due to "transboundary smoke haze from forest and peat fires in the region, the occurrence of local fires or other localised sources of burning".

Moreover, NEA said it was difficult to pinpoint the cause or source of the smells due to the transient nature of it and added that smells may not always be correlated with higher PSI or PM2.5 readings.

The Meteorological Service Singapore (MSS), on the other hand, said scattered hotspots were detected over Vietnam, Laos and Cambodia on Tuesday.

The PSI is an index of daily air quality levels and computed on the concentration of fine particulate matter (PM2.5), the dominant pollutant during haze episodes, along with other pollutants.

MSS said on its website: "Localised, thin smoke plumes were observed in the vicinity of some of the hotspots", adding that in southern ASEAN, wet weather continued to subdue hotspot activities, with isolated hotspots detected in Brunei.

Another reason NEA gave was the possibility of local vegetation fires.

But the Singapore Civil Defence Force (SCDF) told Channel NewsAsia that the number of vegetation fires in the first three months of 2016 was lower than the previous two years.

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