Impact of lifting the mandatory evacuation order after the Fukushima Daiichi Nuclear Power Plant accident on the emergency medical system: a retrospective observational study at Minamisoma City with machine learning analysis

OBJECTIVES: This study aimed to identify factors that delayed emergency medical services (EMS) in evacuation order zones after the 2011 Great East Japan Earthquake and Fukushima Daiichi Nuclear Power Plant accident and to investigate how the lifting of the evacuation affected these factors over time. DESIGN: This research was a retrospective observational study. The primary outcome measure was onsite EMS time. A gradient boosting model and a decision tree were used to find the boundary values for factors that reduce EMS. SETTING: The target area was Minamisoma City, Fukushima, Japan that was partly designated as an evacuation order zone after the 2011 Fukushima disaster, which was lifted due to decreased radiation. PARTICIPANTS: This study included patients transferred by EMS from 1 January 2013 through 31 October 2018. Patients who were not transported and those transported for community events, interhospital patient transfer and natural disasters were excluded. OUTCOME MEASURES: This study evaluated the total EMS time using on-site time which is the time from arrival at the scene to departure to the destination, and other independent factors. RESULTS: The total number of transports was 12 043. The decision tree revealed that the major factors that prolonged onsite time were time of day and latitude, except for differences by year. While latitude was a major factor in extending on-site time until 2016, the effect of latitude decreased and that of time of day became more significant since 2017. The boundary was located at N37.695° latitude. CONCLUSIONS: The onsite time delay in EMS in evacuation order zones is largely due to regional factors from north to south and the time of day. However, the north-south regional factor decreased with the lifting of evacuation orders.