Conditional protective behaviour intentions in response to heat stress and UV radiation: Evidence from a...

Individual heat protection depends on conditional behaviours activated when risk cues cross meaningful thresholds. Unlike UV protection - where decades of preventive communication have ingrained clear behavioural triggers - heat guidance still only vaguely specifies when protective action is required. Behavioural science suggests that risk perception, exposure and motivation interplay to determine protective action. Current research primarily links heat-related risk perceptions and protective behaviours. We extend this work by examining how people plan protective actions conditional on anticipated exposure levels in simulated forecast scenarios. We conducted a population-representative vignette experiment in Germany (n = 2,003). Using days sampled from past weather forecasts in an ecologically valid weather-app format as cues, we investigate when and how people intend to protect themselves against heat and UV exposure. Using composite behavioural indices, weighted regression, LASSO feature selection and spline-based exposure-response models, we identified the cues that drive protective behaviour intentions and the presence of threshold responses. Heat-protective behaviour intentions showed no population-level threshold, rising only gradually. Participants consistently relied on maximum air temperature, rather than perceived temperature, which better reflects thermos-physiological heat stress and includes more climate parameters (standardized β = 0·376, 95% CI 0·295, 0·456 vs. standardized β = 0·077, 95% CI 0·007, 0·148). In contrast, UV protection intentions showed a sharp threshold at UVI 7-8, aligned with WHO messaging. These findings are consistent with the interpretation that current heat communication does not provide sufficiently salient behavioural cues. Establishing clear, actionable heat thresholds is essential to improve public readiness for escalating heat extremes.