The Singapore air quality index is the Pollution Standards Index (PSI) – it is an alternative to the Air Quality Index (AQI) that is used in Singapore. Many other countries now commonly use their own versions of an Air Quality Index as a common metric for air quality but Singapore’s National Environment Agency (NEA) uses a variant PSI, which was devised by the United States Environmental Protection Agency. The PSI used in Singapore gives a score for air pollution based on the density of six pollutants: PM2.5, PM10, carbon monoxide, ozone (O3), nitrogen dioxide (NO2), and sulphur dioxide (SO2). The NEA provide 24-hour PSI readings (with a rolling average for the past 24 hours) to help residents manage their own exposure to air pollution.4 The PSI scale ranges from 0 to 300+, from “Good” to “Hazardous”, with a colour coded scale: green, blue, yellow, orange and red. Singapore’s NEA reports that its PSI has ‘has remained in the ‘Good’ and 'Moderate' range for much of 2019’.5