PSI stands for Pollutant Standards Index, which is a type of air quality index used to indicate the level of pollutants in the air. It is a number reported on a scale of 0 to 500, and it considers six air pollutants: sulphur dioxide (SO2), particulate matter (PM10), fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3). During haze periods, PM2.5 is the key pollutant that is factored into the PSI. The PSI is based on a scale devised by the United States Environmental Protection Agency (EPA) to provide a way for broadcasts and newspapers to report air quality on a daily basis. In Singapore, the National Environment Agency updates the PSI on its website every hour, and it is a moving 24-hour average. The PSI is used to determine the air quality levels in a particular area, and it enables the public to take necessary precautions to protect their health.