SINGAPORE — Air quality worsened late last night, with the three-hour Pollutant Standards Index (PSI) creeping into the unhealthy range.

The three-hour PSI reading hovered between 61 and 69 for most of the day, before rising to 74 at 9pm. It went up to 83 an hour later, and reached 96 at 11pm. The reading hit 100 at midnight. At 1am, it was 102.

A PSI reading of 0 to 50 is considered “good”, 51 to 100 is classified as “moderate”, while 101 to 200 is “unhealthy”.

The air quality reporting was tweaked in April to incorporate levels of fine particulate matter, or PM2.5, which were previously reported separately from the PSI.

The National Environment Agency (NEA) said in an update on the haze situation that the total number of hot spots detected in Sumatra decreased yesterday to 82 from 194. The reduced hot-spot count was due to partial satellite coverage.

The NEA said occasional slight haze may be experienced in Singapore if the winds blow from the south-west. It expects the overall air quality for today to be in the moderate range.