Ambient heat exposure as a risk factor for cataracts: Evidence from a nationwide claims-based study in Japan.

Ambient heat exposure intensified by climate change is an emerging public health concern. Long-term ocular risks, particularly cataract development, remain underexplored. This study investigated whether a history of heat-related illness (HRI) increases the risk of developing cataracts, especially nuclear cataracts, and examined how demographic and clinical factors modify this association. This retrospective matched cohort study used data from a nationwide health insurance claims database in Japan. Cataract cases were matched in a 1:4 ratio with controls according to age, sex, diabetes status, and residential region. Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95 % confidence intervals (CIs) with stratified analyses and sensitivity testing. HRI was significantly associated with higher risks of both all types of cataracts (hazard ratio [HR] = 1.96, 95 % CI: 1.81-2.12) and nuclear cataracts (HR = 2.16, 95 % CI: 1.59-2.94). The association was strongest among non-diabetic individuals (HR = 2.44, 95 % CI: 2.22-2.68) and young adults aged 30-39 years (HR = 2.99, 95 % CI: 1.83-4.91, all types of cataracts; HR = 34.53, 95 % CI: 3.49-341.7, nuclear cataracts). These findings suggest that acute thermal stress may contribute to accelerated lens aging and that HRI is an environmental risk factor for cataract formation. Preventive strategies for populations at risk of extreme heat exposure, including younger adults, should be considered.