The risk of hospitalization associated with foehn winds and temperature in the mountainous region of Switzerland.

Background: Foehn winds are intense warm winds, common in mountain regions, but their health impacts and potential to exacerbate existing heat-related risks remain poorly understood. This is particularly concerning for rural areas where older, heat-vulnerable individuals live. Objective: We investigated the independent and combined association of foehn winds and temperature with cause-specific emergency hospitalizations in Switzerland. Methods: We analyzed daily temperature, foehn winds intensity, and cause-specific hospitalizations near eight foehn wind-observing meteorological stations in Switzerland (1998-2019). We performed a case-time series analysis to examine the association between foehn winds intensity and hospitalization risk with and without temperature adjustment, and whether foehn winds amplify cold and heat-related hospitalizations with an interaction term between foehn winds and temperature. Results: Foehn winds intensity showed small and no consistent association with hospitalizations in temperature-adjusted (0.4% [95% confidence interval: -1.1%, 1.8%] per 6 hours increase in daily full foehn winds intensity) and nonadjusted models (0.8% [-0.5%, 2.3%]). However, foehn winds may amplify heat-related hospitalization risk with a 14% (-3%, 33%) increase in risk at the 99th temperature percentile (vs. temperature of minimum risk) on foehn days, compared with -2% (-8%, 6%) on non-foehn days. The association was larger for females, older adults, and for hospitalizations due to respiratory and mental health causes. Conclusions: While foehn winds did not directly impact hospitalizations, they may contribute to an amplification of heat-related health risks, especially for females and older adults. Further research is needed to assess their effects in other regions, climates, and vulnerable populations.