News · Longevity & Aging
Blood pressure read slightly lower on extreme heat days, across 4.2 million US clinic visits
Heat strains the heart, yet on the hottest days blood pressure at routine check-ups came in a little lower, more so in older adults. That matters for how readings are interpreted in summer.
- An observational records study of 4,221,866 summer clinic visits at 2,554 US community health centers.
- Extreme heat days were defined by local temperature thresholds, matched to each clinic's location.
- On those days, the top blood pressure number was about 0.7 points lower, and 1.1 points lower in people 65 or older.
- The dip was largest on the day of the heat and the day after, and faded by the third day.
- The shifts are small for any one person, but they can tip borderline readings in summer.
Heat waves are hard on the heart. Hospital admissions and deaths from heart disease rise when temperatures spike, and as summers get hotter, doctors are paying closer attention to what heat does to everyday measures of cardiovascular health. Blood pressure is the most basic of them, and it turns out to move in a direction some might not expect.
A study of more than 4 million clinic visits across the United States, published in JAMA Network Open, found that on extreme heat days blood pressure readings came in slightly lower, especially in older adults.
How the body copes with heat
MedlinePlus explains the starting point: your body normally cools itself by sweating. That system has limits, and the page warns that during hot weather, especially when it is very humid, sweating just isn’t enough to cool you off.
Sweating costs fluid, and to push heat out through the skin the body also widens the blood vessels near the surface. Both of those can nudge blood pressure down, which is why a lower reading on a hot day is physiologically plausible even though heat puts the heart under strain.
How the heat and blood pressure study worked
The researchers used electronic health record data from a national network of 2554 CHCs across 40 US states during the summer months of 2019 to 2023. Community health centers care largely for low-income and underserved patients, who are often the most exposed to heat and the least able to escape it.
Previously, how heat moves blood pressure in routine clinic care had been poorly described, especially in large and diverse groups of patients. Here, every visit with a blood pressure reading was matched to the temperature at the clinic’s location on that day and the days before, using a detailed temperature map that resolves conditions block by block rather than city by city. That lets the study compare the same kinds of patients, in the same clinics, on hot and ordinary days. Extreme heat was operationalized as days exceeding location-specific thresholds, so a hot day in Minneapolis was judged against Minneapolis summers, not Phoenix ones.
What happened to readings on the hottest days
Across 4,221,866 visits, the pattern was consistent. Same-day extreme heat exposure was associated with lower systolic BP and diastolic BP, by about 0.7 and 0.3 points respectively. Systolic is the top number, diastolic the bottom one.
Older adults showed the biggest shift. Decreases were larger among adults aged 65 years or older, with the top number about 1.1 points lower. The effect was short-lived: BP reductions peaked on the day of exposure and the following day and were attenuated by day 3.
Why small blood pressure shifts in heat still matter
A drop of less than one point will not change anything for most people. The consequences sit at the edges. Blood pressure treatment decisions are often made on readings close to a threshold, and a summer check-up that comes in a point or two lower could make someone’s usual pressure look better controlled than it is.
For older adults on blood pressure medicines, the concern runs the other way. Medicines that lower pressure, combined with heat that also lowers it, can leave some people lightheaded and prone to falls. The authors conclude the findings may be relevant for clinical BP measurement and cardiovascular risk assessment, which is a quiet way of saying that a summer reading deserves a second look.
What clinic records cannot show
The study matched heat to the clinic, not to where patients lived or how much time they spent outdoors. Blood pressure was measured in routine care, not under research conditions, and the authors note that differences in resting time and technique across thousands of clinics add noise.
They also acknowledge that heat may change who comes in. If some patients skip routine visits on the hottest days, the people who do attend may differ from those who do not, and that alone could shift the average. Only about one visit in 40 happened on an extreme heat day.
What this changes for blood pressure checks in summer
For most people, a slightly lower reading in a heat wave is nothing to act on. For anyone whose blood pressure is being adjusted, it is worth knowing that a hot-day reading may understate their usual level, and worth mentioning if a check-up falls in a heat wave.
For people on blood pressure medicines, particularly older adults, the practical advice is the familiar heat advice: drink enough, avoid the hottest part of the day, and tell a doctor about dizziness rather than changing doses alone. As heat waves grow more common, this kind of evidence is likely to shape how clinics interpret summer readings.
People also ask
What did the study find?
Same-day extreme heat exposure was associated with lower systolic blood pressure (-0.7 mm Hg) and diastolic blood pressure (-0.3 mm Hg). Decreases were larger among adults aged 65 or older (systolic -1.1 mm Hg against -0.6 mm Hg in younger adults). Reductions peaked on the day of exposure and the next day and were attenuated by day 3.
Why would heat lower blood pressure?
To shed heat, the body widens blood vessels near the skin and loses fluid through sweat. Both can lower blood pressure. The study measured the pattern, not the mechanism, but this is the likely explanation.
Is lower blood pressure in heat a good thing?
Not necessarily. For people on blood pressure medicines, especially older adults, a further drop in heat can bring dizziness or falls. It also means a summer reading may look better than someone's usual level.
What counted as extreme heat?
Days hotter than a threshold set for each location, so an extreme day in Phoenix is hotter than one in Seattle. Temperatures came from a detailed map matched to where each clinic sits.
Who are community health center patients?
People using a national network of clinics that care largely for low-income and underserved communities, which makes this one of the largest and most diverse looks at the question.
Should I change my medication in a heat wave?
Not on your own. If you take blood pressure medicine and feel lightheaded in hot weather, talk to your doctor about what to do. This is general information rather than medical advice.