Background. In this study we have investigated the effect of cold exposure on young healthy subjects, with particular emphasis on the hematological system. Study design. The lightly clothed subjects were seated in a thermally controlled climatic chamber for three hours. Methods. One group (control) was exposed to three hours at 28 °C, while the other group was first exposed to one hour at 28 °C, and then two hours of decreasing temperature, ending at 6 °C. Mean body temperature decreased, blood

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... pressure increased, and metabolism increased by 60 % in the cold exposure group compared to the control group. Blood samples were taken through an arterial catheter every hour during the exposure, one hour before and 1.5, 3 and 25 hours after the exposure. Results. Red blood cell counts increased, due to hemo-concentration. In the cold neutrophil counts did not increase during the day, but were stable at a lower level compared to the controls. Conclusions. It is suggested that the cold induced decrease in neutrophil count is caused by an activation and increased adhesion of the neutrophils to the endothelial wall. This finding is supported by the cold exposure-induced increases in IL-8 and Thromboxane B2 in LPS-stimulated blood, and an increase in the plasma concentration of P-selectin.