Systematic exposure bias in city-level air quality reporting revealed by 38 years of Indian observations.

Accurately representing urban air quality remains a central challenge in managing global exposure and communicating environmental risk. In rapidly urbanising regions such as India, Air Quality Index (AQI) reporting often masks intra-city variability, limiting both scientific interpretation and public relevance. Here we analyse a 38-year air-quality record (1987-2024) from 64 consistently monitored Indian cities to quantify long-term deterioration, identify shifts in dominant pollutants, and assess the representativeness of existing AQI computation. The share of Poor-Severe days has risen from 15% to over 35%, with degradation spreading from the Indo-Gangetic Plain to southern and coastal regions. Particulate matter remains dominant in over 85% of observations, while sulphur dioxide influence has declined by 95% and nitrogen dioxide has doubled. A consultation with 450 stakeholders across 23 States revealed strong support for exposure-based reporting. We show that conventional averaging can misrepresent exposure by up to ten percentage points and introduce an exposure-weighted framework that corrects this distortion, improving the fidelity, transparency, and global transferability of city-scale air-quality assessment.