The 24‑hour PSI is Singapore’s daily air‑quality indicator. It is calculated from six pollutants—PM2.5, PM10, ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide—and represents the average air quality over the previous 24 hours.

The 1‑hour PM2.5 reading measures the hourly average of fine particulate matter (≤2.5 µm). PM2.5 is the main haze pollutant and is used for quick decisions about outdoor activity.

The National Environment Agency (NEA) advises that when the 24‑hour PSI reaches the unhealthy range, healthy people should cut down on prolonged or strenuous outdoor exertion. Elderly people, pregnant women, children and anyone with chronic lung or heart disease should minimise or avoid such activity altogether.

For immediate outdoor plans, NEA recommends checking the 1‑hour PM2.5 reading and following its personal guide. For activities scheduled for the next day, the 24‑hour PSI forecast and associated health advisories should be consulted.

Understanding the difference between the daily PSI and the hourly PM2.5 helps residents decide whether to postpone a jog, keep children indoors, or simply enjoy a normal day, keeping health risks in check during haze episodes.