CPCB explains why official AQI is capped at 500
The board stated that the 500-point ceiling is an intentional design feature of the National Air Quality Index to communicate health risks.
The Central Pollution Control Board (CPCB) on Tuesday defended its air quality monitoring methodology, addressing why official AQI readings do not exceed 500. This clarification follows reports of private monitoring apps showing levels as high as 1,800 in Delhi after Diwali, according to the Times of India.
The CPCB said the 500-point limit is a deliberate part of the National Air Quality Index (NAQI) and aligns with international practices, such as the US EPA AQI scale. The board noted that any reading above 500 stays within the 'Severe' category and does not change the associated health advisory.
The official index uses scientifically validated 24-hour rolling averages to measure sustained pollutant exposure. In contrast, the board said private apps often use low-cost sensors that capture hourly fluctuations, which can result in sharp spikes in pollution levels.
The NAQI consists of six categories: 'Good', 'Satisfactory', 'Moderately Polluted', 'Poor', 'Very Poor', and 'Severe'. The board stated that because all levels above 500 represent similar health risks, a separate category is not required.
The CPCB further explained that while private apps might show higher readings during brief peaks, they may also show lower figures when pollution declines. This is because the NAQI continues to reflect the exposure from the previous 24 hours to provide a reliable indication of health impacts.
The clarification follows social media posts showing private app readings between 1,500 and 1,800 in Delhi on Diwali night, while official CPCB readings remained under 400.
