The PSI used in Singapore is the same as the API used in Malaysia. The Inter-Ministry Task Force was set up in Sep 1994. When a PSI reading is released during the smoke haze periods, it is for air quality for the immediate past three hours. It therefore does not correspond to what you may observe at the point when you hear the PSI announcement. Poor visibility does not necessarily mean higher particles levels in the air .
Poor visibility can be due to many reasons e.g. presence of mist, sun reflection etc.