In Singapore, the National Environment Agency uses the Pollutant Standards Index (PSI). It has five categories from “Good” to “Hazardous.”
Singapore's Pollutant Standards Index
The National Environment Agency updates the PSI on its website every hour, and it is a moving 24-average. This means the Pollutant Standards Index tells you what the air quality has been like, on average, over the past 24 hours.
In contrast, IQAir AirVisual reports Singapore air quality data every hour, or in real-time. In this way, if there is a sudden dangerous spike in air pollution, the value will reflect this.
IQAir AirVisual’s default index is the U.S. Air Quality Index (AQI), which is one of the world’s most stringent. When Singapore’s PSI says the air is “moderate,” the U.S. AQI can deem it “unhealthy for sensitive groups,” including children and the elderly.
The US AQI and the Singaporean PSI are both calculated using the same six pollutants, and both use the dominant pollutant – the pollutant with the highest Index value – to determine the overall Index value. During haze, particulate matter 2.5 (PM2.5) is usually the highest and therefore determines the overall AQI or PSI value most of the time.
When PM2.5 density is between 13-55 micrograms per cubic meter (µg/m³), the Singapore PSI says the air quality is “Moderate.” But the US AQI considers roughly half of the Singapore “Moderate” category (35.5-55.4 µg/m³) to be “Unhealthy for Sensitive Groups,” offering a warning to children, the elderly and people with heart and lung disease.