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Background: Higher cardiovascular mortality in winter may be partly explained by increased blood pressure (BP) because of cold exposure. However, this winter excess mortality is higher in countries with moderate winter than those with severe winter climate.

Objectives: Although higher BP in low outdoor temperatures has been reported, the magnitude of association of indoor temperature with ambulatory BP remained unclear. We aimed to compare the associations of indoor and outdoor temperature with ambulatory BP.

Methods: From repeated measurements on two consecutive days during colder months (October-April) among 868 elderly individuals, we assessed the association of indoor and outdoor temperatures with ambulatory BP using multilevel analysis with random intercept for each individual.

Results: Correlation between indoor and outdoor temperature got weak along with decreasing outdoor temperature. Outdoor temperature was not significantly associated with ambulatory BP. In contrast, a 1[degrees]C lower indoor temperature was significantly associated with 0.22 mmHg higher daytime SBP, 0.18% higher nocturnal BP fall, 0.34 mmHg higher sleep-trough morning BP surge independent of potential confounders including physical activity. The models with indoor temperature showed better fit of the model than those with outdoor temperature. Night-time SBP did not show significant association with indoor and outdoor temperature but with bed temperature.

Discussion: Our results suggest the importance to assess the association of indoor temperature with cardiovascular mortality to determine whether improving housing thermal environment reduces winter excess mortality.

Conclusion: Indoor temperature showed stronger association than outdoor temperature with BP in colder months.

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