Air
What is Delhi's AQI?
The Air Quality Index is a single number between 0 and 500 that converts several pollutant concentrations into one scale. It is not an average of them. CPCB calculates a sub-index for each pollutant and the AQI is the worst of those sub-indices, so the number you read is the one pollutant currently doing the most harm — in Delhi, almost always PM2.5.
One number, eight pollutants, worst case
The index exists because eight concentrations in four different units are not something anybody can act on before leaving the house.
CPCB's National Air Quality Index, published in 2014, takes measured concentrations of up to eight pollutants — PM2.5, PM10, nitrogen dioxide, sulphur dioxide, carbon monoxide, ozone, ammonia and lead — and converts each into a sub-index on a common 0 to 500 scale using breakpoints set for that pollutant.
The AQI for a station is then the highest sub-index among them, and the pollutant that produced it is named as the governing pollutant. This is deliberate. Averaging would let a clean reading on one pollutant hide a dangerous one on another, and an index designed to tell people whether to go outside cannot do that.
A station's AQI is only calculated if at least three pollutants are reporting, and one of them has to be PM2.5 or PM10. A station with two sensors working does not produce a number.
- Sub-index
- One pollutant's concentration expressed on the 0–500 scale. Eight pollutants can produce eight sub-indices at one station.
- Governing pollutant
- The pollutant whose sub-index is the highest, and therefore the one the AQI is reporting. Naming it turns a number into a cause.
- Worst monitor
- How this site reads the city: the highest station AQI of those reporting, named. Not a city average — see below.
The six bands, and the one number that is a limit
The bands are CPCB's own. Only one line in this table is a legal standard — the rest are labels for how bad it is.
Good
0 – 50
CPCB National Air Quality Index, 2014
Satisfactory
51 – 100
CPCB National Air Quality Index, 2014
Moderately polluted
101 – 200
CPCB National Air Quality Index, 2014
Poor
201 – 300
CPCB National Air Quality Index, 2014
Very poor
301 – 400
CPCB National Air Quality Index, 2014
Severe
401 – 500
CPCB National Air Quality Index, 2014
PM2.5, 24-hour mean — the legal standard
60 µg/m³
CPCB, NAAQS 2009
The band names carry no legal force. An AQI of 400 breaks no law by itself. What is enforceable is the National Ambient Air Quality Standards of 2009 — the last row above — and the index is a way of communicating how far past them the air has gone.
This matters when a number is quoted. "Delhi's AQI was 400" is a description. "Delhi's PM2.5 was six times the 24-hour standard" is a finding against a published limit, and it is the one that can be argued with.
How to read it
Two AQI numbers for the same city on the same evening are usually both correct.
Delhi has more than forty continuous monitors. CPCB publishes a city figure that averages across them, weighted by a two-kilometre population grid. This site publishes the worst monitor of those reporting, named, alongside the unweighted mean as a cross-check.
The two answer different questions. A city average tells you about the city. A worst monitor tells you what somebody living in Bawana is breathing, and on a bad evening in Delhi the gap between the cleanest and the dirtiest station is larger than the gap between a good day and a bad one.
AQI is also a rolling figure, not an instant one. Sub-indices are computed on averaging periods set per pollutant — 24 hours for the particulates, eight hours for ozone and carbon monoxide — so an AQI published at 8pm is describing a window, not that minute.
What this number cannot tell you
- It cannot tell you which pollutant you are breathing unless the governing pollutant is named beside it.
- It cannot be averaged with another city's AQI. Two indices built from different governing pollutants are not the same quantity.
- It cannot be converted back into a concentration. The mapping runs one way, and different concentrations can land on the same index value.
- It says nothing about indoor air, which is where most people in Delhi spend most of the day.
Why the index only goes to 500
The scale was designed with 500 as the top of the worst band because concentrations above it were not expected to be routine. Delhi has reached the ceiling. When a station's sub-index saturates, the index stops distinguishing between bad and much worse — which is an argument for reading the underlying concentration rather than the index on the worst days of winter.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- National Air Quality Index — CPCB Central Pollution Control Board · The index, its breakpoints and the six bands
- National Ambient Air Quality Standards, 2009 Central Pollution Control Board · The enforceable limits the index is measured against
- Real-time Air Quality Index from various locations data.gov.in, mirroring CPCB · The per-station feed this site reads
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
Next
The reading this explains, the explanations next to it, and the one place to stand in it.
show different numbers? Four reasons two honest sources give you a different AQI for the same city at the same minute. Stand in it CityScapes A day in Delhi reading the city's own air, waste and water in the places that produce them.