Heat and Cold Extremes and Urban Mortality in Greece: An Event-Based Assessment Using Cumulative Thermal Stress Indices

Extreme temperatures increasingly threaten public health, yet temperature–mortality relationships vary substantially across regions and are often obscured by average exposure–response models. This study investigates heat- and cold-related mortality in five climatically diverse Greek cities—Athens, Thessaloniki, Larissa, Patra, and Heraklion—during 1992–2024 using an event-based framework that integrates cumulative thermal stress with synoptic atmospheric conditions. Heat and cold events were defined using the Excess Heat Factor and Excess Cold Factor, combined with persistence criteria and Spatial Synoptic Classification air masses. Mortality responses were assessed through daily mortality ratios, regression analyses, and event severity categories. Dry Moderate air masses dominated across cities, accounting for more than 60% of all days in each city, indicating that extremes typically reflect departures from generally mild background conditions. Linear associations between cumulative thermal stress and mortality were weak overall, with correlation coefficients generally below |0.15| for cold events and below 0.20 for heat events. However, severe heat events produced substantial mortality increases, with mean mortality ratios reaching 1.69 in Larissa and exceeding 1.30 in all cities, despite relatively low event frequency. In contrast, cold-related mortality was often linked to frequent lower-severity events, particularly in Thessaloniki (more than 200 cold events) and Athens. These findings demonstrate that mortality risk concentrates in discrete high-impact episodes rather than increasing linearly with thermal stress, underscoring the value of event-based approaches for locally tailored adaptation and early-warning strategies.