I just finished a report about that topic. What I understand is…
The PSI (Pollution Standard Index) was created in the 1970s. It was an daily index based on different pollutants, but not PM2.5.
wiki > The Pollutant Standards Index, or PSI, is a type of air quality index, which is a number used to provide the public with an easily understandable indicator of how polluted the air is. Initially PSI was based on five air pollutants, …
In 2014, the PSI was revised and named AQI (Air Quality Index) now including PM2.5 pollution.
wiki > … since 1 April 2014 it has also included fine particulate matter (PM2.5).
Recently -no specific date found- technology has been developed to read and publish Real Time Air Quality Index (RTAQI)
Some websites for reference : aqicn.org/here/
All indexes are true, and use the same raw data, but…
The old PSI (still used by Taiwan EPA) is an index calculated during the last 24 hours! and does not include PM2.5 (proven to be dangerous to health)
Many governments, especially South East Asia, use daily average numbers and indexes to lower the numbers and not scare the people.
Governments also used different standards to define healthy, moderate and unhealthy… the same tune, to pretend everything it’s ok.
As I was writing the report, I displayed the situation in my city and compared the different standards…
The Taiwan EPA PSI was 82… yellow color… moderate
The hourly PM2.5 index was 7… red color… high
The Real Time AQICN was 134… orange color… unhealthy for sensitive groups
Different indexes, different standards, different recommendations,… Who knows?
Conclusion: just trust the real time raw data provided by AQICN and WAQI.