When haze drifts over Singapore, people check the air‑quality index more often than usual. The government’s own numbers help decide whether schools stay closed, masks are handed out, or outdoor activities are cancelled.

The National Environment Agency (NEA) publishes two official indicators: a 24‑hour Pollutant Standards Index (PSI) and a 1‑hour PM2.5 reading. These are the figures the agency recommends for daily use.

Both the PSI and the international Air Quality Index (AQI) turn raw pollutant concentrations into sub‑indexes, but they use different concentration breakpoints and health‑category bands. "The main difference between the indexes lies in the thresholds used for their various air‑quality categories," explained Professor Steve Yim, director of the Centre for Climate Change and Environmental Health at Nanyang Technological University.

Online platforms such as IQAir present a live ranking called AQI+. The service applies US AQI breakpoints to hourly measurements, a method that does not follow Singapore’s PSI calculation. IQAir therefore produces numbers that can look higher or lower than the official readings.

Local experts say Singapore residents should rely on NEA’s PSI figures as the primary reference. "An online site may display a precise‑looking AQI number without necessarily providing a more accurate picture of local air quality," warned Professor Rajasekhar Balasubramanian, Provost’s Chair Professor in NUS’s Department of Civil and Environmental Engineering. He added that NEA’s data come from government‑grade monitors and internationally recognised measurement methods.

The 24‑hour PSI offers a stable picture of overall exposure, while the 1‑hour PM2.5 reading captures short‑term spikes. "The 24‑hour PSI provides a stable indicator of overall pollution exposure, while the 1‑hour PM2.5 concentration reflects a more responsive indication of short‑term changes," said Professor Balasubramanian. The hourly figure is useful for quick decisions, such as whether to exercise outdoors during a sudden haze surge.

In practice, use the NEA PSI for a reliable, day‑long assessment and check the 1‑hour PM2.5 when you need to know if the air is safe right now. Third‑party AQI numbers can be informative, but they may not match Singapore’s official standards.