Characterizing the effects of extreme heat events on all-cause mortality: A case study in Ahmedabad city of India, 2002-2018

Recent years have seen a rise in extreme heat event (EHE)-related deaths in India. However, the impact of specific temperature thresholds on health risks remains understudied. Using Distributed Lag Non-Linear Models (DLNM), we explored the link between EHEs, defined by various temperature thresholds, and mortality risk in Ahmedabad, India, from 2002 to 2018, considering a 21-day lag. We observed a 'J'-shaped exposure-response curve, identifying a Minimum Mortality Temperature (MMT) of 26 degrees C for Ahmedabad. Notably, a higher and sustained risk of all-cause mortality was associated with T-max > 35 degrees C. EHEs definition of T-max similar to 40 degrees C (95th percentile) increased all-cause mortality risk by 30% (Relative Risk (RR): 1.30, 95% Confidence Interval (95% CI): 1.26-1.35), with substantially higher risk at T-max of 45 degrees C (RR: 3.08, 95% CI: 2.47-3.83). Analysis of attributable fractions (AF) indicated T-max >= 85th percentile contributed most to total mortalities, with an AF of 3.58% (95% CI: 3.20-3.96). Gender-stratified analysis revealed higher risk of EHE-related deaths for females. The highest mortality risk was identified on the same day of exposure and persisted longer during more intense EHEs. The activation of city's heat action plans should consider the significantly elevated mortality risk below the current threshold (similar to 40 degrees C) and the persistent risk during high-intensity EHEs.