On 9 September 2019, there was again slight haze in Singapore, possibly due to an increase in hot spots in Sumatra, Indonesia.[29] These slightly hazy conditions persisted.[30] At 4pm on 14 September 2019, the 24-hour Pollutant Standards Index (PSI) entered the "unhealthy" range of above 100 for the first time since 2016[31], starting from 103 in western Singapore. The PSI steadily climbed up[32] with all parts of Singapore registering "unhealthy" PSI readings at 1am the next day, peaking at 124 from 5am to 8am in western Singapore.[33] Despite this, the air quality improves over the next two days, resulting in the PSI dropping back to "moderate" levels.[34]