Association between nighttime heat exposure and emergency hospitalization for out-of-hospital cardiac arrest: A time-stratified case-crossover study in Japan.
Nishimura H, Nawa N, Fushimi K, Fujiwara T • International journal of biometeorology • 2026
Nighttime heat exposure was associated with increased emergency hospitalizations for out-of-hospital cardiac arrest in Japan (RR 1.090, 95% CI 1.047-1.135), with higher risks among people aged ≥75 years and females in northern areas, and in locations with lower air conditioner penetration.
Key Findings
Results
Nighttime heat exposure was associated with a higher risk of emergency hospitalization for out-of-hospital cardiac arrest (OHCA) in Japan.
The pooled relative risk was 1.090 (95% CI: 1.047-1.135).
The study identified 135,898 OHCA hospitalizations from a nationwide administrative database.
The study period covered May to September, 2011-2021.
Nighttime heat exposure was defined as days when minimum temperature exceeded the region-specific 92.5th percentile, with daily maximum temperature adjusted for as a proxy for daytime heat.
Results
The association between nighttime heat and OHCA hospitalization was stronger among people aged ≥75 years and among females living in northern areas.
Among people aged ≥75 years the RR was 1.295 (95% CI: 1.117-1.500).
Among females in northern areas the RR was 1.352 (95% CI: 1.145-1.596).
Analyses were stratified by age, sex, and region.
Results
Lower air conditioner penetration in a location was associated with higher nighttime-heat-related risk among females and those aged ≥75 years.
Location-specific air conditioner penetration rate was included as a meta-regressor to account for geographical heterogeneity in RRs.
In locations with lower AC penetration, females and those aged ≥75 years had higher RRs.
Methods
The study used a time-stratified case-crossover design with two-stage analysis to estimate nighttime heat effects.
Location-specific RRs were estimated using conditional quasi-Poisson regression over lag 0-6 days.
A random-effects meta-analysis then pooled the location-specific RRs.
The study was motivated by the few existing studies on adverse health effects of nighttime heat exposure.
What This Means
This study looked at whether very hot nights are linked to more people being taken to hospital after cardiac arrest outside the hospital in Japan. Using national hospital records from May to September over 2011-2021 (135,898 cases), the researchers compared days with unusually warm nighttime temperatures to other days in the same month for the same area, while accounting for daytime heat.
Hot nights were associated with about a 9% higher risk of emergency hospitalization for cardiac arrest. The increase was larger for people aged 75 and older and for women living in northern parts of Japan, where risks were roughly 30% to 35% higher. Risks for women and older adults also appeared higher in places where fewer households had air conditioning.
This research suggests that nighttime heat may be an important health risk beyond daytime heat, particularly for older people and in areas where cooling is less common. Because this is an observational study based on administrative data, it shows association rather than proving cause, but the findings may be relevant for heat-health planning and for thinking about access to cooling at night.
Nishimura H, Nawa N, Fushimi K, Fujiwara T. (2026). Association between nighttime heat exposure and emergency hospitalization for out-of-hospital cardiac arrest: A time-stratified case-crossover study in Japan.. International journal of biometeorology. https://doi.org/10.1007/s00484-026-03342-x