Measurement explainer
AQI vs PM2.5: these numbers are not interchangeable.
PM2.5 is a pollutant concentration, usually reported in µg/m³. AQI is a communication index derived from pollutant sub-indices. One describes how much particulate matter is measured; the other compresses air-quality information into a health-oriented category.
Concentration
PM2.5 = 80 µg/m³
A physical concentration measurement/average for one pollutant and one location over a defined period.
Index
AQI = 180
A unitless communication number derived from pollutant sub-indices. The overall Indian AQI is determined by the worst qualifying sub-index.
How an AQI is constructed
Pollutant concentration
PM2.5, PM10, NO₂, O₃…
Pollutant sub-index
Each pollutant maps to its own index
Overall AQI
Worst qualifying sub-index determines AQI
A raw PM2.5 number is therefore not interchangeable with an AQI number. They answer different questions and use different units.
Why averaging time matters
A current PM snapshot is not automatically a CPCB PM2.5 sub-index.
CPCB's National AQI methodology uses 24-hourly average values for PM2.5 and PM10. That is why Delhi Air Health refuses to feed an ambiguously averaged or instantaneous PM number straight into the AQI breakpoints and label the output “official AQI.”
Why two apps can disagree
Often they are not showing the same thing.
Different station
Two apps may choose different monitors, especially when you are between monitoring locations.
Different AQI system
Indian CPCB AQI and other countries' AQI systems use different breakpoints and sometimes different pollutant rules.
Different update / averaging window
One service may use a recent reading while another displays a longer rolling average or lagged feed.
Different dominant pollutant
An overall AQI may be driven by PM10, ozone or another pollutant even when the app foregrounds PM2.5.
Primary reference
CPCB — About National Air Quality Index ↗Keep learning
Related explainers
Particle size → lung deposition
What is PM2.5?
Why fine particles can reach the small airways and alveolar region, and how that links to respiratory and cardiovascular effects.
Not just a bigger number
PM2.5 vs PM10
How fine and coarse particle fractions differ in size, deposition and what a monitor is actually reporting.
Minute ventilation matters
Why exercise changes exposure
Running and basketball make you breathe more air per minute, changing particle dose even when the monitor reading is unchanged.