Impact of heavy rainfall on ambulance response time in Japan.

Ambulance response times (ART) are critical determinants of both acute and chronic patient outcomes. However, little is known about the impact of extreme weather events such as heavy rainfall on ART. The objective of this study was to examine the association between heavy rainfall and ART. ART was calculated as the interval between the receipt time of an emergency call and the arrival of the ambulance at the patient’s location. We analyzed 170,195 dispatch records collected during the rainy seasons (June–September) from 2012 to 2018. Each record was matched with hourly rainfall data corresponding to the call time. A linear threshold model was applied to identify rainfall thresholds and estimate delays in ART across 15 cities in the Kyushu region of Japan. The median ART was 8 min, with the majority of dispatches (59.3%) attributed to acute illness. Hourly rainfall was significantly associated with longer ART, increasing by 0.84 min on average (95%CI 0.48, 1.25) for each additional 1 mm of rainfall exceeding the threshold of 34 mm. The impact varied by case severity: severe cases (hospitalization ≥ 3 weeks) had a lower threshold of 10 mm, with an estimated delay of 0.33 min (95% CI 0.33, 0.64), while mild cases (no hospitalization) had a higher threshold of 35 mm and a longer delay of 1.63 min (95% CI 0.64, 2.32). In the Kyushu region of Japan, there was a significant delay in ART when rainfall intensity reached 30 mm/hour. Transports involving severe patients were more sensitive to heavy rainfall events. These results highlight that extreme weather such as heavy rainfall can reduce the performance of emergency medical services.