Objectives Cardiovascular mortality has been linked to changes in outdoor temperature. However, the mechanisms behind these effects are not well established. We aimed to study the effect of outdoor temperature on blood pressure, as increased blood pressure is a risk factor for cardiovascular death.

Methods The study population consisted of men aged 53–100 years living in the Boston area. We used a mixed effects model to estimate the effect of three temperature variables: ambient, apparent and dew point temperature (DPT), on repeated measures (every 3–5 years) of diastolic (DBP) and systolic blood pressure (SBP). Random intercepts for subjects and several possible confounders were used in the models, including black carbon and barometric pressure.

Results We found modest associations between DBP and ambient and apparent temperature. In the basic models, DBP in association with a 5°C decrease in 7-day moving averages of temperatures increased by 1.01% (95% CI −0.06% to 2.09%) and 1.55% (95% CI 0.61% to 2.49%) for ambient and apparent temperature, respectively. Excluding extreme temperatures strengthened these associations (2.13%, 95% CI 0.66% to 3.63%, and 1.65%, 95% CI 0.41% to 2.90%, for ambient and apparent temperature, respectively). Effect estimates for DPT were close to null. The effect of apparent temperature on SBP was similar (1.30% increase (95% CI 0.32% to 2.29%) for a 5°C decrease in 7-day moving average).

Conclusions Cumulative exposure to decreasing ambient and apparent temperature may increase blood pressure. These findings suggest that an increase in blood pressure could be a mechanism behind cold-related, but not heat-related, cardiovascular mortality.