A recent meta-analysis provides compelling evidence that higher outdoor temperature is associated with lower blood pressure (BP), with each 1 °C higher temperature associated with 0.26 mmHg lower SBP and 0.13 mmHg lower DBP . The same meta-analysis examined the association of indoor temperature and BP but found only four eligible studies for SBP and insufficient studies for analysis of indoor temperature and DBP. In this issue of Journal of Hypertension, Zhao et al.  present results that begin to address this gap using a large, population-based study of adults living in England. Consistent with earlier findings , Zhao et al.  found that higher indoor temperature was associated with lower SBP and DBP. Importantly, they showed that these results persisted after adjusting for likely confounding factors including: age, body mass, sex, ethnicity, health behaviors, hypertension treatment, outdoor temperature (accounting for seasonal effects), and markers of socioeconomic status that may influence access to heating – education, household income, and area deprivation. The fact that results remained after adjusting for confounding factors increases confidence that these findings may represent not only an association but potentially a causal effect.
Even if the effects observed by Zhao et al. are causal, a question remains whether they are large enough to have utility for clinical practice or population health. For context, a meta-analysis showed that on average, monotherapy medication reduces BP by 9.1 and 5.5 mmHg for SBP and DBP, respectively . Data from a large environmental sciences study of six European cities showed that within a city (i.e., not due to geographic differences) indoor temperatures varied 6.9 °C in Prague up to 17.0 °C in Athens and 17.6 °C in Helsinki . Figure 1 shows the results of translating Zhao et al.  unadjusted and fully adjusted model results per 1 °C into the total difference in BP potentially observable within a city. The results are particularly striking for DBP in which the potential difference in DBP associated with a large indoor temperature variation, such as that found in Athens or Helsinki, exceeded the average impact of a monotherapy medication.
The article by Zhao et al.  is an example of observational work at its best. They leveraged a large, population-based sample, addressed missing data through multiple imputation, and adjusted for likely confounding factors. Now we need randomized experimental designs or clinical trials to test whether changing indoor temperatures can help reduce BP to aid its management and improve population health.
Conflicts of interest
There are no conflicts of interest.
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