Effects of extreme heat on the onset of dementia and all-cause mortality: A 3-year follow-up study in a large population-based cohort in Japan.

INTRODUCTION

Long-term effects of repeated extreme heat exposures on dementia onset remain unclear.

METHODS

We analyzed 2016 to 2019 Japan Gerontological Evaluation Study data, including 57,178 dementia-free, independent adults (≥ 65) residing ≥ 30 years in the same municipality. Dementia and mortality were identified through public long-term care insurance. Extreme heat exposure was defined as cumulative (sum over 1-3 years, capturing the combined effect of multiple years) or point (days in a single year, estimating each year's independent effect) above the 90th or 99th percentile of local climate. Three-level random-effects logistic regression was applied.

RESULTS

Cumulative exposure to extreme heat over 1 to 3 years increased the odds of dementia and the combined outcome of dementia or mortality. Time-lagged analyses showed significant effects for 1-year and 3-year prior exposures but inconsistent or reverse effects for 2-year exposure.

DISCUSSION: Extreme heat may accelerate dementia onset or death. Heat adaptation policies for older adults are warranted.

HIGHLIGHTS: Evidence is scarce on the impact of repeated extreme heat exposure on dementia onset. Extreme heat relative to local climate norms was associated with higher odds of dementia onset and all-cause mortality in older adults. Adverse effects appear rapidly, within 1 to 3 years of exposure. Heat adaptation strategies should be incorporated into dementia prevention frameworks.