Apparent temperature and heatstroke-related emergency ambulance dispatches in the Kanto area of Japan: When does the tragic relationship first begin?

摘要

Background: Rising heatstroke cases in recent years has prompted the urgency to understand how atmospheric temperature affects heatstroke-related ambulance services in Japan. Aims: A time-series study was conducted to examine the association between apparent temperature and daily heatstroke related ambulance dispatches within the Kanto metropolitan area in Japan. Methods: A total of 12,907 heatstroke-related emergency dispatch cases occurring from 2000 to 2009 in five major cities -Saitama, Chiba, Tokyo, Kawasaki, and Yokohama - were analyzed. Generalized Additive Models and Zero-inflated Poisson regressions were used to estimate the effects of daily maximum 3-hour apparent temperature (AT) on dispatch frequency from May to September, with adjustment for seasonality, long-term trend, weekends, and public holidays. Linear and non-linear exposures were considered. Effects on days when AT first exceeded its median in the warm season were also investigated. City-specific estimates were combined using random effects meta-analyses. Results: The adjusted relationship between AT (2-day average) and heatstroke-related dispatches had a "j-shape". Significant risk increase began from 21°C with a combined relative risk (RR) of 1.22 (95% confidence interval: 1.03-1.44), increasing to 1.50 (1.42-1.58) at peaked AT. When linear exposure was assumed, combined RR was 1.44 (1.38-1.49) per degree increment. Associations were significant on days when median AT was initially exceeded in a particular season, especially in Tokyo. Over two-thirds of these initial hot days were in June. There were more heatstroke-related dispatches on weekends, but not on public holidays. Conclusions: Significant increase of heatstroke-related dispatches began rather early in June suggesting the harmful effect of the initial warming of weather despite the generally mild temperature. Risk increase that began early at low perceived temperature implies the need for early prevention.