India's ambient air quality has declined significantly in the last 24 hours, with numerous northern cities reporting "destitute" and "severe" levels of pollution. Compared to October 20, most northern and central regions had a significant change from "satisfactory" and "moderate" to "poor" and "very poor" classifications, indicating the early start of India's winter smog season, analysis of entral Pollution Control Board's (CPCB) national Air Quality Index (AQI) bulletin for October 20 & 21 reveals

At least 60 of the more than 270 cities examined had AQI readings classified as "poor" or worse. The number of cities in the "very poor" range (AQI 301-400) virtually quadrupled overnight, going from 7 on October 20 to 13 on October 21. Alarmingly, one Haryana city, Jind, reported "severe" pollution with an AQI of 421, suggesting conditions that might harm even healthy people and have a significant impact on those with respiratory problems. Other Haryana cities that rank among the most polluted include Bahadurgarh (368), Rohtak (376), Sirsa (353), and Gurugram (370). 

The National Capital Region (NCR) remains at the centre of the crisis. Delhi's AQI increased from 345 ("destitute") to 351, with PM2.5 being the most prevalent pollutant at 35 of 39 measuring locations. Nearby cities—Ghaziabad (324), Noida (320), Manesar (320), and Bhiwadi (364)—mirrored Delhi's deteriorating pattern, indicating that the regional air shed is already heavily polluted despite relatively calm weather circumstances. 

Particulate pollution has worsened significantly across the Indo-Gangetic belt. Lucknow (250), Kanpur (229), Patna (226), and Begusarai (203) all reported "poor" air, mostly due to PM2.5, showing an increase in fine particulate matter from crop waste burning and automobile emissions. Several Bihar and eastern Uttar Pradesh cities that rated "satisfactory" or "moderate" air quality on October 20 fell into the "poor" category within 24 hours. 

While northern and central India struggled with poisonous air, the southern and northeastern states mostly remained in the "good" to "satisfactory" range. Cities like Bengaluru (97), Kochi, Mysuru (41), Thrissur (48), Madikeri (25), and Gangtok (34) continued to have comparatively clean air. However, minor increases in PM2.5 and PM10 concentrations were seen in some coastal locations, including Chennai (142) and Puducherry (138), which were most likely caused by buildings and localised transportation emissions. 

Across India, PM2.5 and PM10 were the dominant pollutants, accounting for more than 80% of recorded AQI values. Sporadic increases in ozone (O₃) and carbon monoxide (CO) were recorded in isolated locations, including Baddi (180, O₃) and Mandikhera (87, CO), indicating secondary pollutant production in industrial clusters. 

The CPCB encourages inhabitants in "destitute" and "severe" AQI regions to limit their outdoor activities, wear N95 masks when outside, and avoid intense activity. To prevent further deterioration, local governments are recommended to implement dust control measures, limit open burning, and monitor construction emissions. 

According to the data, India's battle for clean air remains uneven, with some places improving and others deteriorating. As winter approaches and pollution levels rise, consistent, coordinated measures across transportation, industry, and agriculture will be critical to avoiding another season of hazardous air.