An intrinsic characteristic of forestry work is that it is an outdoor activity, exposing its workers to the prevailing climate conditions. For a large majority of forest workers this implies being exposed to warm conditions while performing physically strenuous work. The overall aim of the thesis was to study effects of heat stress on the health and productivity of forest workers using manual working methods. Studies were made to evaluate the ISO heat stress measurement methods in forestry and the effects of dehydration on the health and productivity of forest workers in manual thinning. The studies were conducted in North East Zimbabwe in September-October 1996. Three metabolic rate assessment methods of ISO 8996 were compared. Large variations between the assessments were found depending on the method chosen. Differences between the ISO heat stress indices in heat stress risk assessment were found as well as unacceptable variations in the assessment of allowable exposure times with ISO 7933. It seems therefore questionable that today's standard methods are able to guide manager of forest operations in reliable heat stress risk management. The consumption of a fluid level assuring full hydration resulted in a significant lower percentage of heart rate reserve used, as well as a considerable reduction of time consumption when compared to consumption of a fluid level leading to mild dehydration. It was found that during harvesting, the responses of the forest workers on the fluid consumption levels were affected by their physical condition as well as their work manner and that changes in work manner had occurred between the fluid consumption levels for some workers. Moreover an accumulating effect of inadequate fluid consumption over days was found on the time consumption. As these results were obtained in mildly warm climate conditions, it is recommended to extend ILO's recommendation to drink at least 5 litres of water per work day during heavy forestry work also to temperate climate conditio
