F2 and Stroke: A hypobaric environment is known to increase the concentrations of prothrombin and thrombin-antithrombin complex, and to enhance the activity of factor VIIa.29 In addition, low barometric pressure potentially increases the incidence of venous thromboembolism and pulmonary embolism.30,31 It may therefore be possible that low ambient pressure raised the incidences of stroke in women and created the positive association between high rainfall and stroke in this study.