A Delhi-based climate research organisation, Climate Trends, has found that none of the 11 major Indian cities - Ahmedabad, Bengaluru, Chandigarh, Chennai, Delhi, Kolkata, Lucknow, Mumbai, Pune, Varanasi, and Visakhapatnam - had safe air quality levels between 2015 and 2025. An AQI of 0-50 is considered safe. Delhi was the most polluted city during this period, the study found.