Exposure to ambient fine particulate matter (PM2.5) is identified as one of the leading risk factors for morbidity and mortality in India. Here we estimate ambient PM2.5 exposure and its 18-year (1998-2015) trend in 109 Indian cities using satellite data and further classify them into six vulnerable classes (from index 1 for low vulnerability to index 6 for extreme vulnerability). PM2.5 exposure has shown a rapid increase in Delhi and the cities in Uttar Pradesh, Bihar, Jharkhand, West Bengal, Punjab, Haryana, Rajasthan, Madhya Pradesh, Chhattisgarh and Odisha. Amongst the cities with a population of more than 0.5 million (as per the 2011 census), Thiruvananthapuram is the least vulnerable and Aligarh is the most vulnerable city based on 18-year statistics. Only 27 cities are identified as 'low' to 'moderately' vulnerable to ambient air pollution. The median incremental rate of the annual PM(2.5 )exposure has increased by 57.9% (from 0.9 to 1.15 mu g m(-3) per year) with the night-light counts (a proxy for urbanization rate) increasing from < 20th percentile to > 80th percentile. 51 out of the 60 Indian cities chosen for the 'smart city' mission are highly vulnerable to PM2.5 exposure (vulnerability index > 2) and thereby face challenges to achieve the core objective of the mission (i.e., a sustainable environment). Our results will facilitate prioritizing a clean-air action plan for the cities based on their vulnerability rankings to achieve the maximum health benefit for the exposed population.