Mortality due to cardiovascular diseases (CVDs) rises sharply in winter, a phenomenon known as excess winter mortality. It is partially explained by cold exposure induced hypertension. Nevertheless, existing countermeasures for the prevention of hypertension and CVDs place emphasis only on improving lifestyle habits. Improving life environment has attracted attention recently because people spend 60–70% of their time at home. Therefore, we aimed to quantitatively evaluate the relationship between housing thermal environment and blood pressure (BP) in a real-world setting.
Design and method:
We conducted a nationwide prospective intervention study named the Smart Wellness Housing survey in Japan, as a nonrandomized controlled trial. The intervention was the thermal insulation retrofitting of homes. Participants were recruited by construction companies throughout all 47 prefectures of Japan. Home BP and indoor temperature were measured for 2 weeks before and after the intervention in winter (November-March) of FY 2014 to 2019.
Cross-sectional analyses before intervention involving about 2900 participants (1840 households) showed that systolic blood pressure (SBP) in the morning had significantly higher sensitivity to changes in indoor temperature (8.2 mmHg increase/10°C decrease) than that in the evening (6.5 mmHg increase/10°C decrease) in participants aged 57 years (mean age in this survey). Interaction terms between age/women and indoor temperature were significant, indicating that SBP in older residents and women was vulnerable to indoor temperature change.
The association between BP variability and indoor temperature instability was also analyzed using the morning-evening (ME) difference as an index of diurnal variability and the standard deviation (SD) of 2-week data as an index of day-by-day variability. The analyses showed that the ME difference in indoor temperature was closely correlated with the ME difference in SBP (0.85 mmHg/°C). The SD of indoor temperature was also associated with the SD of SBP (0.61 mmHg/°C).
In the comparison of before and after intervention, insulation retrofitting significantly reduced morning SBP by 3.1 mmHg. In addition, there was heterogeneity in the effect of insulation retrofitting on morning SBP in hypertensive patients compared with normotensive occupants (-7.7 versus -2.2 mmHg).
Hypertension might be not only lifestyle diseases but also life-environment diseases. Residents should keep the indoor temperature high to reduce BP level and keep it stable to reduce BP variability through strategies such as installing high thermal insulation. We expect that these results will be useful to prevent excess winter mortality due to CVDs.