The study examined lags of zero to 13 days, calculated region-specific relative risks with conditional quasi-Poisson regression and combined those estimates using a random-effects meta-analysis. It also stratified the results by age and sex. Across the full study population, the cumulative relative risk for emergency hospitalization was 2.21 at the 99th-percentile temperature, with a 95% confidence interval of 2.00 to 2.43.

At the first-percentile temperature, the cumulative relative risk was 1.24, with a 95% confidence interval of 1.09 to 1.40. Thus, the model estimated a larger relative risk at the heat extreme than at the cold extreme, while identifying an association with increased emergency use at both ends of the temperature distribution. Age and sex patterns differed markedly for the heat extreme.

Among males aged 15 to 49 years, the cumulative relative risk was 4.24, with a 95% confidence interval of 2.93 to 6.15. For men aged 50 to 64, it was 4.20, with a 95% confidence interval of 3.46 to 5.10. These were the largest heat-associated relative risks reported in the stratified results, indicating that the observed association was concentrated particularly among younger and middle-aged men.

The pattern among women was different. The highest heat-associated relative risk for women occurred among those aged 65 and older, where the cumulative estimate was 1.87 with a 95% confidence interval of 1.64 to 2.14. The study therefore did not identify one heat-related risk profile for all adults.

Instead, its estimates varied by demographic group, with the largest heat-related associations among younger and middle-aged men and a smaller but still elevated association among older women. Cold-associated susceptibility was most pronounced among older adults. For males aged 65 and older, the cumulative relative risk was 1.28, with a 95% confidence interval of 1.06 to 1.53.

Among females aged 65 and older, it was 1.29, with a 95% confidence interval of 1.04 to 1.58. These findings show that the cold extreme was associated with increased acute kidney injury emergency hospitalization in both older men and older women, contrasting with the concentration of the strongest heat associations among younger and middle-aged men. The study’s design enabled investigators to evaluate both the geographic and temporal dimensions of temperature exposure.

Regional estimates were assessed over lags ranging from zero to 13 days, while space-time stratification and conditional quasi-Poisson regression were used to estimate relative risks for emergency acute kidney injury admissions associated with daily mean temperature. Random-effects meta-analysis then pooled the region-specific results. The figures associated with the article included the relationship between daily mean temperatures and emergency hospitalizations using regional data.

Because this was an observational case-crossover study, the results establish statistical associations rather than demonstrating that temperature alone caused the additional hospitalizations. The confidence intervals also show variation around the stratified estimates, particularly the wide interval for the heat-associated relative risk among males aged 15 to 49. The findings are consequently most relevant as evidence of population-level vulnerability patterns associated with temperature extremes in Japan during the study period.

Overall, the nationwide analysis linked both hot and cold conditions with greater acute kidney injury emergency hospitalization, but not with the same strength across demographic groups. Its central result is a divergence in susceptibility: the highest heat-associated relative risks occurred among younger and middle-aged men, whereas the strongest cold-associated estimates occurred among adults aged 65 and older of both sexes. The authors said these patterns support considering demographically targeted prevention during temperature extremes, while emphasizing that understanding the underlying biological and social mechanisms is necessary for developing effective public-health interventions in a changing climate.

Subject of Research: Age- and sex-specific associations between ambient temperature extremes and acute kidney injury emergency hospitalizations in Japan

Article Title: Association between ambient temperature and acute kidney injury: a nationwide space-time-stratified case-crossover study in Japan

Article References: Asanbek kyzy, A., Nishimura, H., Nawa, N., Fushimi, K., & Fujiwara, T. (2026). Association between ambient temperature and acute kidney injury: a nationwide space-time-stratified case-crossover study in Japan. Journal of Exposure Science & Environmental Epidemiology. https://doi.org/10.1038/s41370-026-00985-1

Image Credits: AI Generated

DOI: 10.1038/s41370-026-00985-1

Keywords: acute kidney injury, ambient temperature, extreme heat, extreme cold, Japan, emergency hospitalization, age differences, sex differences, case-crossover study, climate change, Association, between