###### Abstract  
This experimental study examines the use of a real time viable particle counter in pharmaceutical cleanroom operations under ISO class 8/Annex 1 Class C 'in operation' conditions. The used measuring system, a Biofluorescent Particle Counter (BFPC), continuously detected particles >1.0 \(\upmu\)m as well as microbe carrying (viable) particles (MCPs). In addition, classic air samplers for the evaluation of Colony Forming Units (CFU) were installed to enable a comparison of counter-provided values with classic, agar-based methods of microbial air sampling required by regulatory guidelines. A test room comparable to typical cleanrooms in the pharmaceutical industry was operated under three different air change rates (ACR). At these ACRs, the operators wore three different garments. With test repetition, 18 experiments were conducted simulating different pharmaceutical process conditions. The results show correlations of measured particles to MCPs and MCPs to CFUs. Furthermore, albeit an industry-wide used and regulatory accepted minimum ARC value of 20 h\({}^{-1}\), the study results indicate that an ACR of 10 h\({}^{-1}\) is capable of keeping cleanrooms within regulatory limits for particles and CFUs, and thus provides the opportunity to save energy for cleanroom operations. The implementation of real time viable particle counters can be used to achieve a quality advantage for ISO class 8 cleanrooms for a continuous, documented control of the cleanroom status, and thus enable a reduction of ACRs with the aim of energy saving. Further long-term studies should validate this. Biofluorescent Particle Counter (BFPC); real time viable particle counter; air change rate; ISO 8; Class C; energy saving; pharmaceutical production; cleanroom 'in operation'; annex 1; aseptic guide 24 June 2022410 August 20224 June 202410 August 2022410 August 2022410 August 2022410 August 2022410 August 2022410 August 2022410 August 2022410 August 202  
avoid deviations from cleanroom specifications, on the other hand they are faced with the need to save energy as much as possible. A reduction of the air change rate (ACR) would be a possible approach, as various studies have shown [7; 10; 11]. While particles of various sizes can be detected in real time with commonly used laser particle counters, the regulatory accepted detection of CFUs, using microbial air samplers (MAS) with agar plates or strips, requires subsequent incubation for several days before results are available. This time lag puts manufacturers at risk. If the results are unexpectedly high, the produced batches may be contaminated, and as the production is finished by this time there is no chance to avoid that. As a consequence, manufacturers are still reluctant to reduce air change rates [1], as they would have to rely on particle count only for cleanroom control until CFU results are available. This is despite a good correlation between airborne particles and microorganisms [12]. BFPCs could help to close that gap and offer continuous control of airborne microorganisms. From the authors' experience, the largest areas in sterile pharmaceutical production facilities are the preparation and supporting areas for product manufacturing prior to sterile filtration and filling. These preparation areas require a Class C environment according to the EU GMP-guide Annex 1, and ISO 8 (Class 100,000) according to the FDA aseptic guide [4; 5]. In the same guide, the FDA states: 'Air change rate is another important cleanroom design parameter. For Class 100,000 (ISO 8) supporting rooms, airflow sufficient to achieve at least 20 air changes per hour is typically acceptable' [5]. Though this is not a strict regulatory expectation, many pharmaceutical companies apply the value of 20 h\({}^{-1}\) as the minimum required ACR for their aseptic preparation and supporting areas. Recent studies have shown that the regulatory 'in operation' limits for particles and CFUs can be achieved with a significantly lower ACR of 10 h\({}^{-1}\)[10; 13; 14].