The slight haze yesterday was due to a build-up of particles in the air, said the National Environment Agency (NEA).
The build-up led to air quality entering the unhealthy range in parts of Singapore in the morning for the first time since late September. It improved later in the afternoon to enter the moderate band, but re-entered the unhealthy range in the early evening.
The particles could have come from various sources such as vehicle traffic emissions or smoke from fires in the region, said air pollution researcher Erik Velasco.
An NEA Facebook post yesterday morning noted that the haziness earlier in the day was taking time to clear as winds have remained light since Tuesday night.
The 24-hour Pollutant Standards Index (PSI) reading at 10pm ranged from 88 to 104. A reading of 51 to 100 is in the moderate range while one from 101 to 200 is in the unhealthy band.
The 24-hour PSI first entered the unhealthy range at 10am in the east with a PSI reading of 101.
The one-hour PM2.5 concentration reading ranged between 29 and 50 micrograms per cubic m at 10pm. It was between 36 and 66 micrograms per cubic m at 10am.
A PM2.5 reading of 0 to 55 is in the normal band, while one of 56 to 150 falls in the elevated band. Such readings measure the concentration of tiny particles in the air.
Dr Velasco told The Straits Times that several factors could have caused air quality to tip over into the unhealthy band. He noted there were a few hot spots in Lampung, South Sumatra, on Tuesday and said that some smoke from fires there may have reached Singapore due to the wind direction.
However, he said that in conditions of calm winds and a lack of air turbulence, "pollutants tend to be trapped close to the surface, increasing their ambient concentrations".
Dr Velasco added that no extreme pollutant levels like those seen in September were expected, but "if the levels remain high, the authorities may consider reducing or limiting some industrial activities to control emissions while the meteorology changes".
Air quality became unhealthy for the first time since 2016 on Sept 14 and persisted for days.
Haze shrouded Singapore on many days in September because of smoke from forest fires in Sumatra. Rain in late September improved air quality.
An NEA forecast at the beginning of this month said that a few mornings in early November could be slightly hazy as particulate matter accumulates in the atmosphere under light wind conditions. This would usually improve later in the morning as winds strengthen.