OBJECTIVE--The effect of indoor temperature control on summer and winter ambulatory blood pressure levels at work was studied. METHOD--Ambulatory systolic blood pressure (SBP) and diastolic blood pressure (DBP) were monitored once in summer and once in winter in 101 healthy normotensive subjects aged 28-63 years, engaged in similar physical work, from two plants with and three without air conditioning. Subjects were interviewed about health related habits, and measurements of environmental and occupational conditions were obtained. RESULTS--After controlling for possible confounders, mean SBP and DBP during work were significantly higher in winter than in summer (delta SBP = 3.4 mm Hg, P = 0.035; delta DBP = 3.3 mm Hg, P < 0.003). The seasonal change in SBP and DBP showed an independent association with the presence or absence of air conditioning of the industrial plants (SBP: beta = 0.194, P < 0.0001; DBP: beta = 0.054, P = 0.038). The percentage of subjects with increases of more than 10 mm Hg from summer to winter was higher in plants without than with air conditioning. CONCLUSIONS--(1) In normotensive subjects ambulatory working BP varies by season, with higher values in winter. If validated for hypertensive subjects, it may be necessary to tailor drug treatment to these variations. (2) The findings make it clear that drawing valid conclusions from comparisons of BPs between groups requires controlling for several factors, particularly season of the year. (3) Climatic conditions in the industrial plant influence the magnitude of seasonal variations in BP. Work in plants without air conditioning places a considerable added load on the employee's cardiovascular system.
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