###### Abstract  
Forced ventilation is applied in clean rooms, specifically operating rooms, to ensure the health of both the patient and the medical staff. Ventilation reduces the risk of patient contamination, and its parameters are legally prescribed. In addition to preventing contamination, the ventilation system also ensures the creation of a comfortable environment for personnel who spend a large amount of working time in the operating room. This research focuses on the appropriate design of the air flow rate from the distribution element to the operating room. The PMV and PPD indexes were used to evaluate thermal comfort for human beings. The measurements of parameters determining thermal comfort took place in a laboratory with an accurate model of the operating room, including equipment and HVAC system, during the summer months, in cooling mode. Discharge speeds in the range of 0.15-0.175 m/s were evaluated as the most comfortable, with the PPD index ranging up to 22%. There was a significant increase in user dissatisfaction up to the limit of 70% at higher discharge speeds. indoor air quality; health care; occupational health; built environment; thermal comfort; HVAC; special environment; predicted mean vote (PMV); predicted percentage of dissatisfied (PPD); clean room; operating room; airflow velocity +
Footnote †: journal: Applied  
The HVAC system defines not only the air flow in the operating room but also the thermal conditions [21], which are specified by legislation and standards [1; 2; 5; 6; 7; 8; 9]. These standards also prescribe the use of LAFC in operating rooms. In addition to infection prevention, HVAC technical standards state that thermal comfort must be achieved in the OR for all members of the surgical staff as well as for the patient [13; 21]. Thermal comfort is defined as a state when the thermoregulatory member of the human body is minimally stressed or as a state of mind that expresses satisfaction with the internal microclimate [3; 22; 23; 24; 25; 26]. The perception of the thermal environment by people is evaluated by the international standard ISO 7730, standard EN 15 251 and standard ASHRAE 55 [3; 4; 18; 27]. Various models and assessment indices have been created to assess the quality of the indoor environment and thermal comfort. Among the first is the \"Two-node model\", which compares the temperature of the core of the body and the surface of the body for the analysis of energy exchanges between the human body and the surrounding environment [28]. In the publication \"Determining lines of equal comfort\" [29], the concept of thermal comfort was empirically expressed using the effective temperature. Subsequently, these studies were improved, and the \"Standard Effective Temperature\" [30] was introduced. Currently, the most used model is according to Fanger, which represents the PMV (Predicted Mean Vote) and the PPD (Percentage People Dissatisfied) indices for predicting a person's thermal sensation [31]. In these models, thermal comfort is a function of subjective parameters, including thermal resistance of clothing and heat flow from the body due to metabolism, and objective parameters, including mean radiant temperature, air temperature, air flow velocity and air humidity in the room [31; 32; 33]. Several studies on thermal comfort in clean rooms, and specifically in the OR, have reported that achieving thermal comfort during LAFC operation is problematic [6; 7; 8; 9; 16; 17; 21; 34; 35]. Different feelings of comfort and perception of air temperature result in different subjective parameters of thermal well-being. Persons in the operating room differ in the thermal resistance of clothing (CLO) but also in the heat flow from the body due to metabolism (MET). In addition to these parameters, the position of the person in the OR has a fundamental influence on the overall feeling of thermal well-being. By default, the medical staff stands during the procedure in the OR, though some participants can also sit, e.g., anesthesiologist doctor and anesthesiologist nurse.