The Air quality Index (AQI) at Alandur and Velachery Central Pollution Control Board stations on Wednesday (May 26) was respectively 47 and 73, a release from the Central Pollution Control board on Wednesday evening said. Air Quality Index is calculated based on the average concentrations of pollutants like Nitrogen Dioxide (NO2), Ammonia(NH3), Sulphur Dioxide (SO2), Carbon Monoxide(CO), Ozone, PM10 and PM 25 measured over a standard time interval.An A