El Nino Watch
An AQI turns pollutant levels into one score. The Thai AQI and the US AQI use different scales: the same air can read 100 on one and 105 on the other, so compare like with like.
Koh Samui watch
Thai AQI (Pollution Control Department, Air4Thai): 0-25 very good, 26-50 good, 51-100 moderate, 101-200 starting to affect health, 201 or more affects health. A 24-hour PM2.5 of 37.5 µg/m³ corresponds to Thai AQI 100.
US AQI (used by many apps, and by Open-Meteo in this dashboard): 0-50 good up to 301-500 hazardous, with different breakpoints.
That is why IQAir, AQICN and Air4Thai can show different numbers for the same place and hour. The app's risk level uses the PM2.5 concentration itself (µg/m³), not an index.