Background:

With climate change, temperatures are increasing. Heat-associated health events disproportionately affect certain subpopulations. However, prior research has often lacked information on individual-level health and air conditioning and neighborhood stressors/protections.

Objectives:

To assess whether 1) heat (2-day mean temperature above local 75th percentiles) is associated with increased heart rate and decreased blood pressure, controlling for age, time, season, daily ozone, and daily particulate matter (PM2.5), and 2) associations differ by anti-hypertensive-medication use, renal function, fasting glucose, emotional support, air conditioning ownership and use, normalized difference vegetation index, neighborhood safety, and residence-specific oxides of nitrogen and PM2.5.

Methods:

Health and behavioral characteristics were obtained repeatedly on participants of the Multi-Ethnic Study of Atherosclerosis in six U.S. sites (2000–2010). These were linked with airport temperature, air quality, and satellite- and survey-derived neighborhood characteristics. We used a fixed-effects design, regressing health outcomes on linear temperature splines with knots at the 75th percentiles, interaction terms for each characteristic, and adjustment for month-of-year, age, PM2.5, and ozone.

Results:

Overall, heat was not associated with heart rate. However, for a 2 degree-Celsius increase in heat, systolic blood pressure (SBP) decreased by 1.1 mmHg (95% CI: −1.6, −0.6) and diastolic by 0.3 mmHg (95% CI: −0.6, −0.1). Among non-users of anti-hypertensive medications, heat-associated decreases in SBP were 2.1 mmHg greater among individuals with central air conditioning vs. without. Confidence intervals around the remaining modifiers were wide after multiple-comparisons corrections or sensitivity analyses.

Conclusions:

Outdoor heat is associated with decreasing blood pressure, and cardiovascular vulnerability may vary primarily by ownership of central air conditioning.