<p>Operating rooms of room classes H1a and H1b, which are operated according to ÖNORM H6020, are operated with a&#xa0;slight overpressure compared to the surrounding rooms due to normative requirements in order to prevent the penetration of potentially contaminated air. However, this operating condition carries the risk that infectious germs could escape from the operating room into the surrounding areas. Since these operating rooms in healthcare facilities play a&#xa0;central role in the treatment of patients, airborne particles that are harmful to health must be abstracted from the room air as completely as possible. A&#xa0;research project initiated in response to the 2019 coronavirus disease (COVID-19) pandemic investigated the extent to which operation with different negative pressures affects the integrity of the protective zone of the operating rooms. In this series of tests, the H1a operating room of the Operation Innovation Center (OPIC) on the FH Campus Vienna was operated at a&#xa0;negative pressure of approximately −6 Pa instead of the usual positive pressure of approximately +1 Pa. Particle measurements were taken to determine whether the above-mentioned change in operating pressure had a&#xa0;negative impact on the particle prevention behavior of the protective zone. The measurements demonstrated that the protective zone, with an area of 8 m², remained stable both in terms of the number of specified particles and their spatial location. These findings provide a&#xa0;sound basis for future planning and the operation of ventilation systems in operating rooms under varying pressure conditions.</p>