An AQI between zero and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor' , 301 and 400 'very poor', 401 and 450 'severe' and above 450 'severe-plus'.
Researchers suggest that either the FGDs at Dadri TPP are malfunctioning or have insufficient capacity, or the coal used has very high sulphur content.