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The effects of environmental hyperthermia (exposure to a hot, dry microclimate) on the human body were investigated with particular reference to certain clotting parameters in healthy subjects and patients at risk of thrombosis. The study covered 70 volunteers, 10 of them clinically healthy (6 males and 4 females) aged 37.7 +/- 9.7 and 60 patients at risk of thrombosis aged 18-60 and divided according to pathology as follows: 26 with ischaemic cardiopathy, 22 with metabolic disorders (12 diabetics, 8 with dyslipidaemia, 2 with hyperuricaemia) and 12 with obliterating arteriopathies of the lower extremities (Fontaine stage 2 and 3). The following standardised protocol was adopted: 2 hours exposure in a controlled climate chamber (40 degrees C, 40-50% humidity, standard air speed 4 m/min, barometric pressure 760 mmHg) for a total of 8 exposures (2 per week for 1 month). This approach was adopted in order to assess not only the effect of each single exposure but also the role of any adaptation to heat. Three blood samples were taken from each subject for each session: the first in basal conditions in a comfortable environment, the second at the end of the 2 hour exposure; the third 30 minutes after the end of the session. Simultaneously samples of arterial blood were taken for pH assays and a spleen echography was performed in basal conditions and at the end of the session for each subject. Each blood sample was tested for several parameters essentially attributable to blood concentration for a broader view of the biological effects of exposure to heart (Ht, blood protein, Nat, K+). The clotting factors under specific study were also assessed (platelet count and volume, beta-thromboglobulin, PF4, von Willebrand Factor VIII, thromboxane B2, fibronectin). Body weight, blood pressure and oral temperature were also measured in all subjects before and after each session. In all subjects both healthy and at risk of thrombosis oral temperature increased (1 +/- 0.4 degrees); on average blood pressure was already higher in basal conditions in the patient group; body weight fell by 900 +/- 120 G in both groups. Ht and blood protein increased significantly in both groups while electrolyte changes were insignificant and blood pH showed a tendency towards acidosis. Clotting parameters revealed a tendency towards thrombophilia in all subjects: platelet count and volume were already higher in the patient group in basal conditions and increased after exposure to hyperthermia. Beta-thromboglobulin, FP4, Factor VIII, thromboxane B2 and fibronectin all increased.(ABSTRACT TRUNCATED AT 400 WORDS)