The Central Pollution Control Board (CPCB) defended its air quality monitoring methodology on Tuesday, explaining why official Air Quality Index (AQI) readings are capped at 500 despite private apps showing levels as high as 1,800 in Delhi after Diwali. The clarification was issued amidst questions about the significant difference between government data and readings provided by private air quality monitoring platforms.
The CPCB stated that the 500-point ceiling is an intentional feature of the National Air Quality Index (NAQI), following international practices. It explained that readings beyond 500 do not alter the health advisory, as all such levels fall under the 'Severe' category. The board further clarified that the official index uses scientifically validated 24-hour rolling averages to assess sustained exposure to pollutants, while private apps often capture hourly fluctuations through low-cost sensors, leading to spikes in pollution levels.
The NAQI has six categories: 'Good', 'Satisfactory', 'Moderately Polluted', 'Poor', 'Very Poor' and 'Severe'. An AQI reading of 500 or above indicates broadly similar health risks and hence does not warrant a separate category. The CPCB added that beyond this level, the AQI remains in the 'Severe' category, and extrapolation would not alter the assigned health impact category.
The discrepancies between official readings and private apps were evident during Diwali night when social media posts showed private air quality apps reporting AQI levels between 1,500 and 1,800 in Delhi, while the CPCB's official readings remained below 400. The CPCB stated that private apps may report higher AQI during brief pollution spikes but show lower figures when pollution levels subsequently decline. It emphasized that the NAQI provides a consistent and reliable indication of the likely health impacts of sustained exposure to pollutants, which is the scientifically relevant measure for public health advisories.