Journal of the National Institute of Public Health
Online ISSN : 2432-0722
Print ISSN : 1347-6459
ISSN-L : 1347-6459
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Evaluation of regional differences in heatstroke occurrence using a regression model of heatstroke occurrence probability
Kei SHIMONOSONO , Asuka TAKEDA, Kaoruko SEINO, Dai SIMAZAKI, Jun TOMIO
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Keywords: heat stroke, probability of heatstroke occurrence, regional differences, emergency transport data, wet-bulb globe temperature
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2025 Volume 74 Issue 2 Pages 119-129

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Abstract

【目的】本研究は熱中症救急搬送者数が増加する日最高外気湿球黒球温度（WBGT）を都道府県別に示すことを目的に，より精度の高い熱中症警戒情報の発出基準の策定に資するべく，その指標として熱中症発生確率（Px）を定義して熱中症発生の地域差・時期・年齢差を評価した． 【方法】Pxは日最高外気WBGTを1ºC単位で区分し，当該区分となる全日数に対して人口100万人あたりx人以上の熱中症救急搬送者が発生した日数の比率と定義した．便宜的にPxが50％となるときの日最高外気WBGT（Wx）が人口100万人あたりx人以上の熱中症救急搬送者が発生する閾値と考えることができる．2010～2023年における熱中症救急搬送者数と屋外のWBGT等のデータを用い，x=1, 2, 5, 10の4条件で月別・年齢別にPxを算出した．これらの結果を基に二項ロジスティック回帰分析により，3種類のPxの回帰モデル（全年齢・全期間モデル，全年齢・月別モデル，年齢別・月別モデル）を作成し，陰性的中率，陽性的中率，AUC（Area of Under the Curve）の指標で回帰モデルの妥当性を検証した．最終的に都道府県別にWxを算出し，熱中症発生の地域差を評価した． 【結果】東京都を対象として月別・年齢別にPxを算出し，それぞれ50％以上となるときの日最高外気WBGT を分析すると，9月，6～8月，5月の順，成人，少年，高齢者の順に高くなった．Pxの回帰モデルについては，月別・年齢別モデルではデータの不均衡により陽性的中率が0％となる場合があったものの，全年齢・全期間モデルと全年齢・月別モデルでは得られるWxは妥当性があるとされた．都道府県別のWxについては，比較的寒冷な地域ほどWxが低くなることが明らかとなった． 【結論】PxとWxを用いることで熱中症救急搬送者数が増加するときの日最高外気WBGTについて地域差ならびに時期を考慮できるとともに，高齢者に限れば年齢も考慮可能であることも示された．近年では自治体独自に屋外のWBGTを測定して自動的に警報を発出するシステムを構築している例もあり，本研究の成果はこのような自治体において警報発出基準を定める際に利用できる可能性がある．

Translated Abstract

[Objectives] This study was performed to elucidate the daily maximum WBGT values for each prefecture that correspond to the number of people transported to hospitals for heatstroke and to evaluate regional, seasonal, and age differences in heatstroke occurrence by defining heatstroke occurrence probability (Px). [Methods] Px was defined as the ratio of the number of days when the number of heatstroke emergency transport patients per million people was x or more to the total number of days in the relevant category, which was divided into categories of 1ºC for the daily maximum WBGT. The daily　maximum WBGT when Px is 50% (Wx) can be considered as the threshold value for the number of emergency heatstroke transport patients per million people. Px was calculated for each month and age group under the following four conditions: x = 1, 2, 5, and 10. Furthermore, we created three types of regression models for Px using binary logistic regression analysis (an all-ages, all-period model; an all-ages, monthly model; and an age-specific, monthly model) and verified the validity of the regression models using indices of negative predictive value, positive predictive value, and area under the curve. Finally, we evaluated regional differences in heatstroke risk using Wx by prefecture. [Results] When Px for Tokyo was calculated by month or age group, and the daily WBGT maxima were analyzed when they were 50% or higher, the highest maxima were found in the following orders: September>June to August>May; and adults>children>the elderly. When a regression model was created, the positive predictive value of the monthly and age-specific models was sometimes 0% owing to data imbalance; however, the Wx obtained from the all-age, all-period model and the all-age, monthly model was considered valid. Finally, when Wx was evaluated by prefecture, Wx was clearly lower in relatively cold regions. [Conclusion] Px and Wx values revealed that regional differences and seasonal variations in daily maximum WBGT should be taken into account during heatstroke emergency transport, and age should also be taken into account if analysis is limited to the elderly. In recent years, some local governments have built systems that automatically issue warnings based on WBGT measurements. The results of this study may be useful for local governments when setting warning criteria.

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