Why vast parts of Delhi still lack reliable pollution data - and how ‘data shadow’ leaves millions in city uncounted

A staggering 74% of the capital lies beyond this immediate monitoring ambit. When the radius is expanded to 5 kilometres, a quarter of Delhi's area still falls outside the network, leaving peripheral and outer zones in the southwest and northwest particularly underserved.

The problem, as detailed in the CSE’s report ‘Air quality monitoring: addressing data shadow regions,’ extends beyond Delhi’s borders. An analysis of 29 other NCR cities found a similar pattern of uneven distribution. Twenty-one of these cities possess just a single monitoring station each, despite official guidelines recommending a multi-faceted monitoring system.

"Delhi is one of India's few regions meeting or exceeding its target number of stations, but the widening gap between demand and actual monitoring infrastructure underlines the urgent need to expand or redistribute them more equitably," the CSE report stated. "The city's outer zones are home to rapidly expanding populations and carry high-exposure risks but remain largely invisible in the monitoring network."

Official guidelines, based on the Indian Standard 5182, stipulate the number of monitors required based on population size. For a city with a population between 100,000 and one million, the ideal network would include thousands of monitors for various pollutants. The CSE analysis found that towns with fewer than 100,000 people require a minimum of four stations for particulate matter (PM) alone.

Experts argue that the current sparse network is inadequate for reliable risk assessment and policy-making. Anumita Roychowdhury, executive director of research and advocacy at CSE, emphasised that the regulatory network must be representative of diverse land-use patterns.

"But a high-density network can be very expensive. So, it's necessary to support regulatory monitoring with a multilayered system, including satellite-based pollution mapping and sensor-based local assessment of exposure," Roychowdhury was quoted as saying by TOI.

This vulnerability of a limited network was highlighted by Sunil Dahiya, founder of the think tank EnviroCatalysts. He pointed out that having only one station in many NCR cities leaves them as "blind spots" and makes readings susceptible to localised manipulation, such as the sprinkling of water to temporarily alter air quality data. "We need a robust network of regulator-grade stations coupled with low-cost sensors to give granular data for cities and rural areas," Dahiya added, as cited by TOI.

While six new stations have been proposed for locations including JNU, IGNOU and the Delhi cantonment, the findings present a stark picture: the world’s most polluted capital city is still largely flying blind when it comes to understanding the full scope of the toxic air its residents breathe.