A health outcome-informed vulnerability index and two-stage risk matrix approach to assessing future heat risk in Taiwan

Extensive evidence has demonstrated that climate change intensifies extreme heat events and elevates risks to human health. This study assessed future heat-related risk across 19 cities and counties in Taiwan by developing a health outcome-informed vulnerability index and applying a two-stage risk matrix framework. Heat hazard was represented by the Heat Wave Duration Index under the Shared Socioeconomic Pathway 5-8.5 scenario for the baseline year (2023) and near-future periods up to 2037. Vulnerability was derived by applying principal component analysis separately to sensitivity and adaptive capacity indicators that were pre-screened based on their positive correlations with heat-related illness rates (HRIR). The first principal components from the two dimensions were then weighted according to their regression coefficients with HRIR to construct a single health-informed vulnerability index, with adaptive capacity contributing approximately 1.76 times more than sensitivity. Results indicated a general increase in heat risk over time, with hotspots emerging across central, southern, and eastern Taiwan. Among these regions, eastern counties exhibited the highest vulnerability and are projected to escalate to Level 5 risk in 2034–2037. Scenario analysis further demonstrated that improving adaptive capacity, such as air-conditioner availability, can effectively reduce vulnerability and the associated heat risk. The proposed framework supports targeted adaptation planning by providing actionable and indicator-specific strategies to aid local governments in prioritizing resources for heat resilience.