The method

How the analysis works

Short method, updated 19 October 2026. Each report identifies its data, period, calculation and coverage. Historical reports keep their own dated methods.

Two measures, two meanings

Measured concentration describes the pollution recorded at monitoring stations. AQI converts pollutant concentrations into an index. These measures have different units and must not be treated as interchangeable.

Annual summaries and special studies state their own inclusion rules. The station count, comparison period and weighting belong to the particular measure. A city summary does not describe every neighbourhood.

The readings and their coverage

The analysis uses hourly CPCB and DPCC monitoring records compiled by the Centre for Research on Energy and Clean Air (CREA). When a report uses an alternative feed, its source and period must be identified. This office runs no monitoring instruments. Errors in our analysis remain our responsibility.

Station identifiers are used to match records. Duplicate records cannot create extra hours. Quality checks and units are pollutant-specific. A daily pollutant value requires at least 16 distinct valid hourly readings within its 24-hour window. Missing readings are not zero pollution.

For carbon monoxide and ozone, the documented-formula daily calculation uses the highest qualifying eight-hour mean wholly inside that window. An eight-hour block needs at least six valid hourly readings. The daily 16-hour requirement also applies. The calculation does not join observations across a gap to invent a continuous block.

The daily window and AQI formula

The current daily AQI window runs from 4 PM IST on the previous day to 4 PM IST on the report date. The start is included and the end is excluded. Earlier reports may use another stated window. A calendar-day result and a rolling-window result must not be compared as if their periods were identical.

For the public AQI calculation, each qualifying pollutant concentration is converted with CPCB's documented formula. A station AQI requires at least three qualifying pollutants, including PM2.5 or PM10. The highest sub-index sets the station's AQI, and the responsible pollutant is named. The daily city AQI is the mean of qualifying station AQIs. Separate concentration or population summaries state their own basis.

An AQI displayed at 500 is an index ceiling. It does not establish that a monitoring instrument cannot measure a higher concentration. Calculating an index from mean concentrations can also differ from averaging hourly indices. Our documented-formula result should be identified as our calculation from the recorded concentrations.

Sources: CPCB's AQI calculation note and CPCB's AQI calculator. The AQI 500 investigation explains the distinction between the index and measured concentration.

Comparisons and uncertainty

The daily year-on-year baseline uses the corresponding date last year, plus or minus seven days. Comparisons use matched station identifiers and disclose their eligible network. Special investigations can use different, explicitly stated panels and periods. Their estimates must not be combined with a daily comparison merely because both are percentages.

A bootstrap interval describes the variation generated by its resampling design. It does not automatically cover instrument error, missing observations or every form of temporal dependence. The station panel, resampling unit and assumptions matter. A range crossing zero does not establish a direction of change. Sensitivity results that change the conclusion must be disclosed.

No single check turns an observational comparison into a causal estimate. Every numerical finding needs its own coverage and uncertainty checks.

Population near monitors

Population estimates describe residents within the stated proximity threshold of eligible monitors. They are not personal exposure measurements and do not mean a monitor represents only those residents. The current population surface, official annual total and allocation method are described in the population methodology linked on this page.

The population headline counts people near monitors in the Poor or worse band, effective AQI above 200. A day with no qualifying Poor monitor is not relabelled using Moderate monitors. Station categories and city summaries retain their actual values. Historical population estimates keep their dated basis.

The long record and the limits of attribution

The 1980 to July 2026 long-record report separates earlier model-based estimates from the later monitoring record. The observing-system change around 2000 is retained and discussed. A smooth chart does not remove that change in evidence, and a historical association does not establish its cause.

Checks, wording and human decisions

R and Python code compute the figures. Language models can assist with code and wording, but their prose is not a measurement or a validation result. Numerical, source, coverage and public-content checks are required. These checks reduce errors; they do not make errors impossible.

Ajay Maken selects the daily film's narratives and approves its final version. Automatic review panels are not part of the daily run. A separate expert review requires his specific request. The full written record can contain findings omitted from his chosen film.

The public record must retain results and limitations needed to interpret its conclusions. Detailed working logs may be kept separately, but a material caveat must not disappear merely to make a shorter report.