Physical and mental performance is impaired at raised body temperature. If protection ensemble is used in a CBRN(E) environment thermal unbalance can occur. Body heat storage has been simulated for different geographical places, times of the year as well as protection levels. The times it takes to reach three body heat storage levels have been estimated. The levels are 3 kJ/kg (impaired vigilance), 5 kJ/kg (endurance several hours) and 10 (7) kJ/kg (endurance less than 1 (2) hours). Furthermore the amount of sweat lost has been calculated. If impermeable CBRN-ensemble is worn it is essential that the activity level is kept at a minimum level. A permeable protection suit allows medium heavy activities, also during summer in Central Europe, with low risk of heat casualties. If the protection level is reduced by opening the closures at neck, wrists and ankles even a heavy task (500 W) can be performed without causing heat exhaustion. Acting in conflict areas as the Balkans, Iraq, Afghanistan, Somalia, Tchad and DR Congo, the endurance is limited during summer even if a permeable suit is worn in combination with a ballistic protection vest. If possible, work should be done during the dark hours. The risk of dehydration is more pronounced when the heat load is relatively low while great heat load is associated with lower risk because of limited action time. If extern cooling is used endurance is improved. Different techniques are discussed as well as cooling efficiency.