Associations of indoor and outdoor temperatures and their difference with home blood pressure: The Masuda Study

Ambient temperature and blood pressure (BP) are closely related; however, few studies have examined the association of out-of-office BP with indoor or outdoor temperature. The effect of the difference between indoor and outdoor temperatures on BP also remains unknown. Therefore, this study aimed to investigate the association of indoor and outdoor temperatures and their difference with home BP. We studied healthy 352 participants (mean age, 49.8 years; 46.0% women) from a population-based cohort using 2-year data on temperature and self-measured home BP. We measured home BP and indoor temperature at the same time in the morning and evening every day. Outdoor temperature during the same period was based on national data. We observed 82,900 home BP measurements in the morning and 66,420 in the evening. In the mixed-effects model adjusted for age, sex, and possible confounders, indoor temperature was inversely associated with systolic and diastolic BP in the morning and evening. A 1 °C increase in indoor temperature reduced systolic and diastolic BP by 0.37 and 0.22 mmHg, respectively, in the morning and by 0.45 and 0.30 mmHg, respectively, in the evening (all P- values<0.001). The magnitude of associations was stronger for indoor than outdoor temperature. Similarly, a 1 °C increase in indoor temperature above outdoor temperature decreased systolic and diastolic BP by 0.33 and 0.12 mmHg, respectively, in the morning and by 0.45 and 0.26 mmHg, respectively, in the evening independent of outdoor temperature (all P -values <0.001). In conclusion, controlling indoor temperature is important to stabilize home BP levels.