High ambient temperature, humidity, heat index, and stroke risk in a Mediterranean region

Background: Studies assessing the relationship between high ambient temperatures and stroke risk yielded conflicting results, while potential associations of humidity and heat stress with stroke risk remain underexplored. Objective: To explore the association of ambient temperature, humidity, and heat index (HI) with the risk of total stroke (combined ischemic-intracerebral hemorrhage [ICH]), ischemic stroke (IS), ICH, and transient ischemic attack (TIA) among older adults during the warm season in the Mediterranean region. Methods: In this time-stratified case-crossover study, we utilized data from the Israeli National Stroke Registry. We included all first stroke and TIA events among individuals aged ⩾18 years that occurred during the warm season (2014–2019). Temperature and relative humidity were assessed using high-resolution satellite-based models and monitoring stations, respectively, based on patients’ residential addresses. A heat index (HI) was calculated based on a combination of temperature and relative humidity data. Conditional logistic regression models with Distributed Lag Non-Linear Models (DLNMs) were used with adjustment for potential confounders, including air pollution. The HI models were stratified by participants’ demographic and health characteristics. Results: The sample included 22,269 individuals with a first stroke (mean age 72 ± 14 years; 55% males) and 8728 individuals with a first TIA (mean age 69 ± 14 years; 52% males) during the warm season. Higher temperature (32°C vs. 27°C), particularly on the stroke event date, was associated with increased risk of total stroke (odds ratio (OR) = 1.32; 95% confidence interval (CI) = 1.23–1.41). The strongest association for relative humidity (90% vs. 70%) was observed 2 days before the stroke event (OR = 1.09; 95% CI = 1.06–1.12). An HI of 100°F was associated with an approximately 40% higher risk of total stroke compared to HI of 80°F on the event day (OR = 1.39; 95% CI = 1.32–1.47). Associations were slightly weaker for ICH, possibly reflecting its relatively smaller sample size, whereas associations with TIA were similar in magnitude to those observed for IS. No evidence of effect modification was observed across subgroups defined by sociodemographic characteristics or comorbidities. Significance: High temperatures combined with high humidity are associated with an immediate increase in the risk of stroke and TIA, even in a region where the population is acclimatized and most buildings are air-conditioned. Preparedness and prevention strategies may be crucial for reducing stroke risk during periods of heat stress.