Why India caps pollution reading at 500 when the air is far more toxic
For millions of people in northern India, the November air tastes like ash, the sky looks visibly hazy, and even stepping outside feels like a challenge.
Most people’s morning routine starts with checking how bad the weather is. But what they see depends entirely on which monitor they use.
Government-backed apps like SAFAR and SAMEER reach 500, the upper end of India’s AQI (air quality index) scale, which converts complex data on various pollutants such as PM2.5, PM10, nitrogen dioxide, sulfur dioxide, carbon monoxide and ozone into a single number.
But private and international trackers such as IQAir and open-source tracking platform AQI routinely show much higher numbers; Most of the time it exceeds 600, and on some days it exceeds 1,000.
This contradiction causes people to ask the same question every year: Which numbers should they trust? So why doesn’t India officially report air quality above 500?
India’s official air quality scale indicates that values above 200 pose a clear risk of breathing discomfort for most people with prolonged exposure.
Readings above 400 and up to 500 are classified as “severe” and affect healthy people as well as those with existing illnesses.
But the scale does not go beyond 500; This was an upper limit set more than a decade ago. National Air Quality Index has been published.
So why was this threshold introduced?
“It was assumed that the health impact would be the same no matter how high it got, because we had already been through the worst,” says Gufran Beig, founding director of SAFAR.
He admits that the 500 limit was initially set to avoid creating panic, because exceeding this limit indicates an alarming situation that requires immediate mitigation.
However, this approach effectively flattens the data; Everything above 500 is treated the same on official monitors, even if the actual concentration is much higher.
“International organizations and portals do not impose such an upper limit,” says Mr. Beig. “That’s why we see the numbers being much higher on global platforms.”
The BBC has reached out to India’s pollution control board for comment.
Beyond the artificially imposed threshold, there is also a difference in defining hazardous weather.
For example, World Health Organization (WHO) guidelines classify PM2.5 levels above 15 micrograms per cubic meter (particulate matter smaller than 2.5 micrometers) in a 24-hour period as hazardous, while in India the threshold is 60 micrograms.
Experts say there is no universal AQI formula worldwide. The United States, China, and the European Union each impose their own pollutant thresholds.
“WHO provides guidelines, and each country has created its index based on its own adaptability, weather and local environment,” Mr. Beig says. Therefore, comparing India’s AQI with WHO or US standards may be misleading.
There are also differences in the instruments used.
India’s pollution control board uses Beta Attenuation Monitors (BAMs), which physically measure the mass of airborne particles and are calibrated against stringent, standardized measurements for each reading.
In contrast, portals like IQAir rely on sensor-based monitors, says Abhijeet Pathak, a scientist who previously worked for India’s pollution control board.
Sensor-based monitors use laser scattering and electrochemical methods to estimate the number of airborne particles.
“The sensor is a different tool and it is not possible to calibrate them every time they are run,” says Mr. Pathak, adding that “sensor-based air quality monitoring is yet to be approved by the Indian government.”
According to him, India’s air quality framework has not been comprehensively revised since 2009 and he, along with other environmental scientists and activists, has called for the scale to be recalibrated based on the latest technology.
“The National Air Quality index will have to be revised if we want to include sensor-based data,” says Mr. Pathak.
Mr Beig says removing the cap is also crucial: “Most of the existing literature shows that health symptoms continue to worsen as pollution levels increase.”
After all, India’s AQI doesn’t stop at 500 because pollution stops there. Since the system is built with a ceiling, it stays at 500.
