The Central Pollution Control Board’s (CPCB) air quality monitors could not capture the spike in particulate matter (PM) pollution on Diwali night as the analysers only measure in the range of 0–1,000 micrograms per cubic metre (μg/m³), and treats hourly readings beyond this calibration as “outside range”, the pollution watchdog said on Tuesday in response to criticism of data gaps in pollution monitoring.Delhi recorded its worst Diwali-day air quality since 2021 on Sunday. As it hinted at a review of the system, the watchdog issued the clarification regarding the data gaps after 45 of the 59 monitoring stations had missing hourly readings for at least one pollutant.The Indian Express on Monday reported that the CPCB portal stations recorded blank hourly particulate concentrations as the load of particulate matter, fine particles emitted through burning and combustion, exceeded 1,000 µg/m³. However, Delhi Pollution Control Committee’s continuous monitoring data showed that multiple stations recorded values as high as 5,000 µg/m³ during the same period.In a statement on Tuesday, the pollution watchdog, under the Union Environment Ministry, said, “During extreme episodes, which are occasional like Diwali firecracker bursting, this may appear as a data gap at the particular station concerned, and those hours do not contribute to the 24-hour average of relevant pollutant for concerned station.”While acknowledging that monitoring instruments have advanced considerably, it added that “internal deliberations and consultations with respect to processing of data beyond system design range are underway”, hinting at a review of the existing system.The statutory authority issued a two-page unsigned statement and said that concerns were raised in certain media reports on the reporting of AQI by the CPCB during and after Diwali, including capping of AQI at 500, data gaps at some monitoring stations in Delhi and differences with AQI shown by other platforms.It also sought to defend its existing National Air Quality Index (NAQI) in the face of criticism that it caps the upper limit at 500 by stating that similar methodologies are followed by the United States Environmental Protection Agency and China. “The capping at 500 is thus a design feature of the index as has been evolved in 2015 methodology of NAQI in line with prevailing international practice including US EPA AQI scale,” the CPCB’s said in a two-page.Story continues below this adIt said that the upper limit of 500 represents the highest level of health concern, and therefore AQI above 500 indicates the same health risk and “extrapolation of the index would not alert the assigned health impact category.” The CPCB statement, though, did not offer any explanation on the impacts of exposure to high-levels of pollutants in a short-duration. The NAQI was developed in 2015, it said, to communicate air quality status in easy-to-understand terms.Explaining the NAQI’s methodology, CPCB said that AQI is generated every hour using sub-indices for eight pollutants – PM 2.5, PM 10, Carbon Monoxide, Nitrogen Dioxide, Sulphur Dioxide, Ozone, Ammonia and Lead. Each hour’s AQI comes from a running 24-hour average or eight-hours, in the case of ozone and carbon monoxide. However, a station needs at least 16 hours of data in 24 to generate a sub-index and at least three pollutants are required for an AQI, with one of them PM 2.5 or PM 10. The highest sub-index among the eight pollutants is reflected as the AQI.On the perception that private apps show higher AQI, CPCB said that different platforms draw data from government and non-government sources, including monitoring techniques which are not prescribed under the National Ambient Air Quality Standards, 2009. These include low-cost sensors which have calibration and data quality assurance limitations, the CPCB said.“Their methodologies may also apply breakpoints different from the NAQI. For example, some platforms derive AQI from instantaneous hourly readings, at times from low-cost sensors and other sources. Such values cannot be directly compared with the NAQI, which is based on 24-hour/8-hour averaged, calibrated and quality-assured data,” it added.Story continues below this adThe higher AQI on other apps, as per the CPCB, may hold during short peaks when values rise sharply, but when concentrations fall, they show a lower AQI than the national index, as the latter reflects exposure of preceding 24 hours, it added.A breakpoint is the specific pollution concentration threshold used in AQI calculations. By comparing the actual, measured concentration of a pollutant against these fixed breakpoints, the methodology determines whether the air quality category is good, satisfactory, moderate, poor, very poor, or severe.