Impact of wildfire smoke on air pollution-related premature mortality in rapidly growing Kathmandu Valley

The dangers of urban population growth in highly polluted regions are starkly illustrated by the Kathmandu Valley, where nearly half of the days each year exceed the national PM 2.5 air quality guideline of 40 μg/m 3 . Wildfire smoke, particularly during the pre-monsoon season, is a major contributor to air quality degradation. Using diverse datasets, including in-situ observations, satellite data, and machine learning classification, we identified 311 wildfire smoke days between 2003 and 2023, with smoke events contributing significantly to the valley's persistent poor air quality. While PM 2.5 concentrations showed no significant trend over the study period, premature adult deaths rose dramatically from 1,000 in 2004 to 12,600 in 2021. The exponential increase in mortality was largely influenced by rapid population growth from 0.52 million to 4.1 million adults in the valley, rather than solely by worsening air quality. The stable but consistently poor air quality became lethal at a larger scale simply because more people were exposed, with wildfire smoke alone contributing to an average of 70 adult deaths annually. Despite the mortality rate per 100,000 people showing only modest increases, the sheer growth in the exposed population has transformed a persistent air quality problem into a major public health crisis. Our findings demonstrate how population growth in regions with chronically poor air quality can dramatically amplify public health impacts even without air quality deterioration, emphasizing the urgent need for air quality management in rapidly urbanizing regions. • Wildfire Smoke Significantly Contributes to Poor Air Quality and Mortality • Despite stable PM2.5 concentrations in Kathmandu Valley, premature adult deaths increased dramatically • Rapid urbanization necessitates immediate and effective air quality management strategies