Reading environmental data
How to read environmental data
Most misleading environmental claims are built from accurate numbers. The failure is almost never fabrication — it is a figure quoted without the thing it was measured against, over a period it did not cover, or from a method that answers a different question. Seven questions catch most of it.
The seven questions
In order. A number that survives all seven can be used; one that fails an early question rarely needs the later ones.
1. What is the limit? A reading on its own is a description. A reading against a published limit is a finding. "PM2.5 was 180" says less than "PM2.5 was three times the 24-hour standard of 60" — and only the second can be argued with.
2. Who published the limit, and is it law? India's NAAQS are notified and enforceable. WHO guidelines are health targets. IMD's heatwave criteria define an event and regulate nothing. Three very different kinds of threshold, routinely quoted in the same breath.
3. Measured or modelled? See the separate page. It decides what the number can be used for.
4. Over what period, and where? An annual mean, a 24-hour mean and an instantaneous reading are three quantities. A city figure, a station figure and a worst-monitor figure are three more.
5. What is the denominator? Shares, rates and per-capita figures conceal their base. A percentage of a small number moves easily.
6. What is missing? A gap in a series is a gap, not a zero. A station that stopped reporting is not a clean station, and an empty day should stay empty.
7. Compared with what? Almost every claim of improvement or deterioration depends on a baseline. State it, or the claim cannot be checked.
- Baseline
- The reference a change is measured from. Changing it changes the answer without changing anything real.
- Denominator
- The base a share or rate is computed on. Absent denominators are where most misleading percentages live.
- Reporting floor
- The lowest value a method returns. A reading at the floor means "at or below", never zero.
Every reading against its published limit
This is the sentence the rest of this site is built on, and these are the limits it means.
Air — PM2.5, 24 hours
60 µg/m³
CPCB, NAAQS 2009 — notified, enforceable
River — dissolved oxygen
> 5.0 mg/L
PWQC, Environment (Protection) Rules 1986
River — BOD
< 3.0 mg/L
PWQC, Environment (Protection) Rules 1986
Heat — departure from normal
4.5 °C
IMD — a definition, not a standard
Rain — legal limit
No legal threshold.
No statute sets one
Fire — legal limit
No legal threshold.
No statute sets one
Two of the six have no limit at all. That is not a gap in this site's work — it is a fact about Indian environmental regulation, and saying so is more useful than inventing a benchmark for a reading to fail.
Where claims usually break
Four patterns, all of them built from true numbers.
The averaged incomparable. Two indices with different governing pollutants, two forest figures at different canopy thresholds, two heat counts on different baselines — averaged into one number that means nothing.
The seasonal quoted as annual. A stubble-burning share measured in a peak week, applied to the year. A river maximum from after the monsoon, quoted as the river's condition.
The threshold that is not a threshold. A policy target quoted as a legal limit, or a definitional criterion quoted as a safety standard.
The missing denominator. A percentage change with no base, a share with no total, a per-capita figure with no population year.
What this number cannot tell you
- These questions cannot make a bad dataset good. They can only tell you what a dataset is fit for.
- They cannot resolve a genuine disagreement between two credible methods. Where one exists, both should be printed.
- They cannot substitute for reading the source. Every one of them is answered in the publication the number came from, which is why this site links to it.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- National Ambient Air Quality Standards, 2009 Central Pollution Control Board · A notified, enforceable limit
- Primary Water Quality Criteria Central Pollution Control Board · Criteria by designated best use
- WHO global air quality guidelines, 2021 World Health Organization · A health guideline, not a law — the distinction question 2 is about
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.
environmental deaths One annual table, one cause per death — and the largest killer in it is the one nobody writes about. Stand in it Eco Action Seven questions are a syllabus. This is the programme that runs them on real readings.