Air quality index (AQI) reports daily air quality and its elevated level is associated with public health risks (Szyszkowicz 2019). Based on different national quality standards and dose-response relationships of pollutants, countries have different air quality indices (Zhang et al. 2020; Sofia et al. 2020). The Indian national air quality index considers eight pollutants (PM10, PM2.5, NO2, SO2, NH3, CO, O3, and Pb) with a 24-hourly averaging period. It is subdivided into six categories i.e., good (0–50), satisfactory (51–100), moderately polluted (101–200), poor (201–300), very poor (301–400), and severe (401–500) as shown in Fig. 1 (Perera 2018; Ghorani-Azam et al. 2016). The sub-indices for individual pollutants at a monitoring location are calculated using its 24-hourly average concentration value (8-hourly in case of CO and O3) and health breakpoint concentration range. The worst sub-index is the AQI for that location (https://app.cpcbccr.com/AQI_India/). An increment in AQI causes acute and chronic mode health concern especially in the older age people and in children (Januszek et al. 2020; Pant et al. 2020). Due to the COVID-19 pandemic confinement, there is a significant reduction in the level of such toxic pollutants globally (Selvam et al. 2020; Singh and Chauhan 2020).
Fig. 1 Indian national air quality index—category and range