Extreme Heat Exposure and Blood Pressure in Community Health Center Patients.

Importance: Extreme heat has well-documented cardiovascular effects, yet its acute association with blood pressure (BP) in routine clinical settings remains incompletely characterized, particularly in large, diverse outpatient populations. Objective: To assess the association between extreme heat exposure and BP among patients receiving care at community health centers (CHCs). Design, Setting, and Participants: This cross-sectional study was conducted using electronic health record data from a national network of 2554 CHCs across 40 US states during the summer months (June through August) of 2019 to 2023. Participants included adult patients (≥18 years of age) with ambulatory encounters that included BP measurements. Data were analyzed October 2025 to April 2026. Exposure: Same-day and short-term cumulative exposure to extreme heat, defined using the GridEX dataset (500 × 500 m spatial resolution) linked to clinic locations. Extreme heat was operationalized as days exceeding location-specific thresholds. Main Outcomes and Measures: The primary outcomes were systolic and diastolic BP recorded during ambulatory visits. Associations were estimated using mixed-effects models with patient- and facility-level random intercepts, adjusting for age, sex, race and ethnicity, hypertension and medication status, comorbidities, year, and geographic region. Effect modification by age and hypertension and medication status was evaluated. Distributed lag nonlinear models assessed cumulative associations across an 8-day window for the binary extreme heat exposure indicator. Results: Of 4 221 866 ambulatory encounters (cohort mean [SD] age, 47.6 [17.0] years; 63% female), most were for patients without a hypertension diagnosis (81%). Same-day extreme heat exposure was associated with lower systolic BP (-0.7 [95% CI, -0.8 to -0.6] mm Hg) and diastolic BP (-0.3 [95% CI, -0.3 to -0.2] mm Hg). Decreases were larger among adults aged 65 years or older (systolic: -1.1 [95% CI, -1.3 to -0.9] mm Hg vs -0.6 [95% CI, -0.7 to -0.5] mm Hg among adults aged <65 years; interaction P < .001; diastolic: -0.8 [95% CI, -1.0 to -0.6] mm Hg vs -0.3 [95% CI, -0.3 to -0.2] mm Hg among adults aged <65 years; interaction P < .001). Hypertension status modified the association between extreme heat exposure and systolic BP (omnibus interaction P = .003) but not diastolic BP (omnibus interaction P = .84). In distributed lag analyses, BP reductions peaked on the day of exposure and the following day and were attenuated by day 3, with minimal cumulative differences thereafter (cumulative difference across lags 0 through 7: -0.8 [95% CI, -1.0 to -0.6] mm Hg for systolic and -0.5 [95% CI, -0.6 to -0.4] mm Hg for diastolic). Conclusions and Relevance: In this cross-sectional study, extreme heat exposure was associated with modest acute reductions in BP among adults receiving care at CHCs, with slightly larger reductions among older adults. These findings suggest physiologic responses to heat that may be relevant for clinical BP measurement and cardiovascular risk assessment in outpatient settings.