The Environment and High Blood Pressure

Colder Ambient Temperature and Winter Season

Colder temperatures increase BP over hours to days as well as over more prolonged seasonal periods. Studies across a wide range of populations and climates have demonstrated an inverse association between BP and ambient temperature during the same and/or preceding few days. In one of the largest studies (n > 500,000) conducted across China, a colder temperature of 10° C was associated with a 5.7 mm Hg increase in systolic BP. The impact was even more robust among older adults, those with smaller body mass indices; but was obviated by household central heating. Systolic BP was also on average 10 mm Hg higher during winter compared with summer. These results resemble our findings among 2078 cardiac rehabilitation patients in Michigan whereby reductions in outdoor temperatures by 10.4° C during the prior 1 to 7 days promoted a 3.6 mm Hg increase in systolic BP. Moreover, in both studies temperatures below 5° C did not prompt further elevations in BP. Similar inverse associations between outdoor temperature and BP have also been reported in other recent studies including patients with cardiovascular disease, individuals living in rural China (e.g., 13% lower hypertension control rate during winter), in a large Dutch population (n = 101,377), and across several locations in Italy.

Independent effects of both winter season and cold temperatures on BP have been reported. Cold exposures measured using personal monitors were shown to be associated with higher systolic BP levels during the daytime, even after adjusting for changes in daylight hours (i.e., season). Conversely, nighttime BP was higher during summer (i.e., warmer days) compared with winter months in this as well as in a few other studies. We recently demonstrated similar findings using personal monitoring. Warmer nights (independent of season) led to higher BP levels several hours later during the following afternoon. Along with a few prior studies, these observations support that there is a highly complex interrelationship involving several exposure-related factors (time of day, duration, indoor versus outdoor temperature levels) that determine the true nature of the ensuing BP changes.

Brief exposure to cold induces a rapid thermoregulatory vasoconstriction, thus raising BP. Although the mechanisms responsible for the more persistent pressor responses during winter are likely similar, they may not be entirely identical ( Table 8.1 ). Further physiologic adaptations (e.g., lower vitamin D, weight gain, reduced activity, and changes in diet/fluid balance) likely play additional roles. Conversely, changes in other meteorologic factors such as humidity and barometric pressure have not been consistently associated with BP.

The overall evidence supports that both colder ambient temperatures (over a few hours to days) as well as winter seasons (over more prolonged periods) lead to clinically-meaningful elevations in BP. It is possible that this plays a role in the known increase in cardiovascular events during winter. From a clinical standpoint, patients with hypertension should be more carefully monitored during colder weather to ensure adequate BP control. In one study, 38% of patients required added antihypertensive medications during winter. Although some epidemiologic evidence suggests that residential heating might mitigate the prohypertensive effects of cold, further studies are needed before making definite recommendations in this regard for the sole purposes of preventing winter-induced hypertension. What if any other practical steps (e.g., space heaters, warmer clothes) individuals with hypertension can take to lessen the ill effects of cold exposures on BP require further investigation.