CNA Explains: Why PSI and AQI readings differ – and which one should you follow during haze season?

Countries also split their index bands differently, said Associate Professor Liya E Yu from the National University of Singapore’s (NUS) Department of Civil and Environmental Engineering.

For example, she noted that the US AQI is a six-band system as it devotes a specific band to “Unhealthy for Sensitive Groups”. 

There is no equivalent band in Singapore’s PSI system or Malaysia’s Air Pollutant Index.

Singapore’s PSI, however, provides separate health advisories for different population groups, including children, older people and those with chronic heart or lung conditions.

Singapore’s PSI, the AQI and Malaysia’s API serve the same general purpose – to communicate air pollution levels and their potential health implications to the public, said Provost’s Chair Professor Rajasekhar Balasubramanian from NUS’ Department of Civil and Environmental Engineering.

But since they differ in calculation methods, concentration breakpoints for each air pollutant, and health categories, their values should not be compared directly without accounting for these methodological differences, he added.

Dr Yu similarly advised against comparing figures from different systems as it would lead to confusion.

Instead, people should focus on the health advisories or air quality descriptions that accompany each system’s grading framework to get guidance on activities and plans amid air quality conditions, he said.