Thermal sensation and heat stress were evaluated in a plastic greenhouse, with and without evaporative cooling, under arid climatic conditions in Riyadh, Saudi Arabia. Suitable thermal comfort and heat stress scales were selected for the evaluation. Experiments were conducted in hot sunny days to measure the required parameters (i.e., the dry and wet bulb temperatures, globe temperature, natural wet bulb temperature, and solar radiation flux) in the greenhouse. The results showed that in the uncooled greenhouse, workers are exposed to strong heat stress and would feel very hot most of the day time; they are safe from heat stress risk and would feel comfortable during night. An efficient evaporative cooling is necessary during the day to reduce heat stress and to improve the comfort conditions and is not necessary at night. In the cooled greenhouse, workers can do any activity: except at around noon they should follow a proposed working schedule, in which the different types of work were scheduled along the daytimes based on the heat stress value. To avoid heat stress and to provide comfort conditions in the greenhouses, the optimum ranges of relative humidity and air temperature are 48–55% and 24–28°C, respectively. 1. Introduction 1.1. Background Human thermal comfort is defined as a condition of mind, which expresses satisfaction with the surrounding environment. In the arid regions, evaporative cooling is often applied to reduce the inside greenhouse air temperature in summer. Greenhouse environment is usually designed according to the crop growth requirements ( : 20–30°C; RH: 70–80%) which, in many situations, may not be suitable for humans that are working in the greenhouse [1]. This is mainly because relative humidity in the evaporatively-cooled greenhouses is much higher than outside. High air humidity may provide discomfort sensations and heat stress in the greenhouse. On the other hand, evaporative cooling and air stream can improve the comfort conditions in the greenhouses in hot summer [2]. Discomfort and heat stress reduce productivity of the greenhouse workers and may lead to more serious health problems, especially for aged workers [2]. Therefore, greenhouse workers should take care when they enter the greenhouse in hot summer to protect their health from heat and/or sunstroke [1]. Factors affecting human thermal comfort and heat stress level can be classified according to [3] as (i) environmental factors such as the dry bulb temperature of air and its relative humidity ( and RH), air current speed, and the mean radiant temperature of the
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