First check what the number measures
PM2.5 describes fine particles generally 2.5 micrometres and smaller, according to the US Environmental Protection Agency. A concentration tells you how much material is present in a volume of air, commonly expressed in micrograms per cubic metre. AQI means Air Quality Index: it translates pollutant information into an index and categories. These are related but different quantities. A PM2.5 concentration of 50 is not automatically an AQI of 50.
An AQI needs a named system
India’s government explanation describes a national AQI with six categories and pollutant-specific sub-index calculations. AirNow reports the US AQI. An index value therefore needs the name of the system used to calculate it. When comparing apps, check whether they identify the same standard. A number converted under one system should not be read using another system’s category chart without checking the conversion rules.
Current does not necessarily mean a single instant
AirNow explains that its current AQI uses NowCast calculations based on multiple hours of monitor readings. The calculation uses longer averages when conditions are stable and shorter averages when they change quickly. A daily figure and a current figure can describe different time windows. An apparent disagreement may start with the period being measured rather than an error in either display.
Check the pollutant and the location
AirNow calculates current AQI for ozone and particle pollution. India’s framework considers several pollutants, including PM2.5, PM10, nitrogen dioxide and ozone. Read which pollutant each figure represents and which location it describes. A value beside a named station and a value labeled for a wider city need an explanation of how the wider figure was assembled. A city label alone does not show that both apps used the same station data.
A comparison that avoids a false conclusion
Imagine one app labels a number PM2.5, 24-hour average, while another labels its number current US AQI. This is an illustrative example, not a reading from Karachi or Delhi. The values are not directly interchangeable: the measurement, time window and conversion may differ. Compare raw concentrations over the same period first, then index values calculated under the same standard. Check the update time as well.
What to check before sharing a screenshot
Identify the units, index standard, pollutant, time window, location and update time. If an app does not disclose them, its number is harder to interpret. This guide explains the display; it does not calculate today’s AQI, diagnose exposure or replace official local health guidance. The India Gate photograph is a city file image from January 2024. Its appearance is not a measurement of pollution or evidence of present conditions.