Why is AQI so bad in North Indian cities? Why are Southern and Western cities better?

Air Pollution: A recent study has revealed that between 2015 and November 2025, the air quality in any major city of India was not recorded at ‘safe level’. During this entire period, Delhi remained the most polluted city in the country. This analysis was released by ‘Climate Trends’ (Delhi-based climate research organization) on Friday (28 November) titled ‘Air Quality Assessment of Major Indian Cities (2015-2025)’.

The analysis found that pollution levels were most severe and persistent in northern cities. In comparison, the air quality level in southern and western cities was relatively better. For this study, annual average Air Quality Index data of 11 cities was analyzed, including Ahmedabad, Bengaluru, Chandigarh, Chennai, Delhi, Kolkata, Lucknow, Mumbai, Pune, Varanasi and Visakhapatnam. Air quality is generally considered at a safe level when the AQI is between 0 and 50.

What are the findings of the analysis?

The latest analysis shows that among 11 cities, Delhi had the worst air quality between 2015 and November 2025. The annual average AQI level in the national capital was highest (above 250) in 2016 and has seen a decline since 2019. Although it is still far from the safe level. AQI will be 180 in 2025. After Delhi, Lucknow, Varanasi, Ahmedabad and Pune are next, where AQI levels remained high for a long time. “Lucknow and Varanasi started out at extremely high levels (often above 200). Although both cities have shown steady improvement since 2019, their AQI values will remain above healthy limits until 2025,” the analysis said.

Compared to these cities, air quality levels were much better in Chennai, Chandigarh, Visakhapatnam and Mumbai. But even in these cities the air quality did not remain at a healthy level. The analysis revealed that the Air Quality Index (AQI) in Chennai, Chandigarh, Visakhapatnam and Mumbai was between 80 and 140. Although Bengaluru had the best air quality among 11 cities, it did not reach safe AQI levels between 2015 and November 2025. According to the analysis, the AQI level in the city remained between 65 and 90 in most of the years.

Why is the situation bad in northern cities?
Air quality levels are higher in northern cities – such as Delhi, Lucknow and Varanasi – compared to western and southern cities – such as Chennai, Chandigarh, Visakhapatnam, Mumbai and Bengaluru – especially during winter due to the region’s geographical location. The problem is that these northern cities are landlocked from all sides, because the Indus-Gangetic plain is surrounded by the Himalayas. Due to this the pollutants get trapped and cannot spread. Additionally, the analysis said, “Dense urban structures within cities create additional ‘surface roughness’, a frictional effect that further slows wind speed and limits dispersion.”

The situation is even worse in winter
Rain during the summer monsoon and strong westerly winds help disperse pollutants, but the situation becomes worse in winter. Delhi is particularly affected during this period because it is located in the middle of a vast, flat valley surrounded by the Himalayas to the north. The analysis said, “During winter (December-February) the air in the planetary boundary layer (the lowest part of the atmosphere) is thin because cold air is denser near the Earth’s surface. Cold air becomes trapped beneath the warm air above, forming a kind of atmospheric ‘lid’. This phenomenon is called winter inversion. Since vertical mixing of air occurs only in this layer.” “It happens inside, so the emitted pollutants do not get enough space to spread into the atmosphere.”