People fly kites at Singapore's Marina Bay area as the three-hour Pollutants Standard Index (PSI) of Singapore hit 128 at 11 a.m. local time and reached the "unhealthy" level, Aug. 26, 2016. (Xinhua/Then Chih Wey)

SINGAPORE, Aug. 26 (Xinhua) -- Haze is back in Singapore with the three-hour Pollutant Standards Index (PSI) reaching 215 Friday afternoon, the highest of this year, said Singapore's National Environment Agency (NEA).

A reading above 100 means that the air quality is "unhealthy," and a reading above 200 is "very unhealthy." It is considered "hazardous" if the reading hits 300.

A strong burning smell were reported over many places since early Friday morning, and it is likely caused by the burning in Indonesia as the latest satellite image on Friday showed some hotspots in central Sumatra, said the NEA.

As of 2 p.m. local time (0600 GMT), the three-hour PSI reading hit the highest level this year at 215, in the very unhealthy range, while the one-hour concentrations of PM 2.5 was 167 µg/m3 in the western region.

The visibility has also worsened especially in the northern and western parts of Singapore.

The PSI incorporates six types of pollutants - sulphur dioxide, particulate matter that is 2.5 micrometres in diameter or smaller (PM2.5), particulate matter that is 10 micrometres in diameter or smaller (PM10), ozone, nitrogen dioxide and carbon monoxide.

A man does exercise at Singapore's Marina Bay area as the three-hour Pollutants Standard Index (PSI) of Singapore hit 128 at 11 a.m. local time and reached the "unhealthy" level, Aug. 26, 2016. (Xinhua/Then Chih Wey)

Of the six, PM2.5 is considered particularly hazardous as the small size of the particles enters the human lungs more easily. A reading below 55 is "normal", and it is considered "elevated" and "high" if the PM 2.5 was between 56 to 150, and 151 to 250, respectively. If the PM 2.5 reading surpassed 250, it means the pollution in the air is "very high."

The situation has since then moderated, with the one-hour PM 2.5 reading dropped to 85 µg/m3 in the central region in Singapore at 6 pm, followed by 84 in the west, 77 in the south, and 69 in the north and east.

For the rest of Friday, The agency predicted that the prevailing winds would fluctuate and would be expected to blow from the west or southwest.