India’s official air-quality scale shows readings above 200 pose clear breathing discomfort to most people on prolonged exposure.

Readings above 400 and up to 500 are classified as “severe” and affect healthy people while also seriously impacting those with existing diseases.

The scale however does not go beyond 500 – a cap set more than a decade ago when the National Air Quality Index was launched., external

So why was that threshold introduced?

“It was assumed that the health impact would be the same no matter how much higher it goes because we had already hit the worst,” says Gufran Beig, founder director of SAFAR.

He admits that the 500 cap was originally set to avoid creating panic as crossing that mark signalled an alarming situation requiring immediate mitigation.

But this approach effectively flattens the data – anything above 500 is treated the same on official monitors, even if the real concentration is far higher.

“International organisations and portals don’t impose such a cap,” says Mr Beig. “That’s why we see numbers going much higher on global platforms.”

The BBC has reached out to India’s pollution control board for a comment.

Beyond the artificially imposed threshold, there’s also a difference in defining hazardous air.

For example, the World Health Organization (WHO) guidelines classify PM2.5 levels – particulate matter smaller than 2.5 micrometers – above 15 micrograms per cubic metre in a 24-hour period as hazardous, while in India, the threshold is 60 micrograms.

Experts say that globally, there’s no universal AQI formula. US, China and the European Union each apply their own pollutant thresholds.

“WHO provides guidelines and every country has built its index based on its own adaptability, weather and local environment,” says Mr Beig. Comparing India’s AQI to WHO or US standards can thus be misleading.

Then there is also a difference in the instruments used.

The Indian pollution control board uses Beta Attenuation Monitors (BAMs), which physically measure the mass of particles in the air and are calibrated to strict, standardised metrics for every reading.

In contrast, portals like IQAir rely on sensor-based monitors, says Abhijeet Pathak, a scientist who formerly worked for India’s pollution control board.