###### Abstract
Other studies recommend demand controlled filtration (DCF) with ACRs of 4 h\({}^{-1}\) during 'at rest' times of the cleanroom to save energy [15; 16; 17]. However, with a reduced ACR, the risk of an undetected exceedance of the bioburden increases. In order to control this risk, the continuous monitoring of not only particles but also the number of MCPs in the cleanroom air by use of real-time viable particle counters, could be highly beneficial. This would enable an immediate detection of an increased bioburden, and the ability to react accordingly. By this, it would enable ACR reduction and DCF without enhancing the risk of undetected exceedance of bioburden limits. BFPC systems work using a laser-induced fluorescent principle [18]. Other than classic laser particle counters, which detect and classify particles by scattered light analysis, a BFPC also measures the wavelength of the scattered light. Substances found in all microorganisms (bacteria and fungi) are dipicolinic acid, nicotinamide adenine dinucleotide (NADH), and riboflavin [19; 20; 21]. They create a fluorescent signal that can be detected by the device. The different wavelengths distinguish viable particles from others. However, 'dead' microbes are detected, as they also contain these substances [22]. One drawback of the measuring principal is the potential for false detection. Some substances, which do not belong to the group of viable particles, still cause a fluorescent signal in the BFPC, and are thus detected as viable. One of these substances is 2-propanol, which is widely used in aseptic manufacturing for disinfection purposes. The effect of a false MCP detection by the use of 2-propanol has been observed in previous studies [19; 23]. A direct comparison of BFPC-measured values with CFUs counted on agar plates or strips is hardly possible, as described by several scientists [19; 24; 25; 26]. The reason for this is the dependency on possible growth on agar plates with CFU methods. However, not all microbes will grow on agar, and standard methods for incubation do not support the growth for all microbes. For this reason, the measured values of BFPC systems are typically much higher than the values measured by conventional microbial tests. This is, in fact a big advantage of the BFPC systems, as an immediate overall MCP result can be obtained and knowledge of the general bioburden status of a cleanroom can be provided. Though the specific species of microbes is not known at this point, this is still valuable not only during pharmaceutical operations, but also for an evaluation of the operational readiness of a cleanroom in preparation for commissioning and qualification [25]. It would minimize the time required for finding bioburden leaks in the cleanroom installation [2, 19, 27]. A detriment is that industry-wide recommended action limits for MCPs measured by BFPC systems are not available yet [22]. Consequently, the derived research question for this experimental study is: can BFPC systems be used to enable minimized ACRs with the aim of saving energy without compromising on cleanroom air quality, and what can be an appropriate action limit for MCPs? This question led to a study design with three different ACRs combined with three different cleanroom conditions with regard to operator garments and behavior.