## 4 Discussion
Hence, a derivation based on the particle/viable ratio is obviously not a feasible approach to find a BFPC limit value for cleanrooms. The maximum number of CFUs found during 'in operation' phases throughout all tests with pharmaceutical garments 18G + 19G was 10 CFU/m\({}^{3}\), being ten times lower than the regulatory limit of 100 CFU/m\({}^{3}\). An exceedance of regulatory CFU/m\({}^{3}\) limits was observed during the tests with NCG. The corresponding MCP/m\({}^{3}\) values measured by the BFPC in these test phases were around 1000/m\({}^{3}\). For that reason, an MCP limit for the BFPC, used in the given cleanroom in this experimental study, can be set at 1000 MCP/m\({}^{3}\). BFPC values measured below this number are not likely to lead to an exceedance of the regulatory limit value of 100 CFU/m\({}^{3}\). The good responsiveness of the BFPC on peak generation of MCPs as triggered in the test runs with NCG also shows the applicability of the system to detect bioburden leaks during ongoing production or for cleanroom commissioning and qualification. This possible application has been proposed by [PERSON] et al. [36] and [PERSON] et al. [19]. Continuous viable particle control may also be used to reduce the number and frequency of microbial air sampling. [PERSON] et al. summarized the results of a meeting with the FDA to get authority feedback on regulatory acceptance of BFPCs for cleanroom control in which the FDA expressed a positive position [3]. However, these systems are regarded as an additional tool for quality assurance and are (today) not seen as a full alternative to classic CFU analysis. This study provides data which support the aim of air sampling reduction. As reported by other authors [19; 23], the effect of a false viable detection by fluorescent substances such as 2-propanol also became visible during these experiments. Though spray bottles were used in the back of the detection systems and close to the exhaust air grid, the generated fluid particles had a clear effect on particle detection as well as MCP detection by the BFPC. Therefore, manufacturers should avoid the use of disinfection spray bottles in rooms where such detection systems are used. Energy savings by means of reduced ACRs has been described by various authors [6; 11; 37]. In a previous publication by [PERSON] et al. [10], which focused on a regulatory limit values for particles >0.5 \(\upmu\)m and >5 \(\upmu\)m, it was stated that ISO 8 cleanroom operations can be performed with lower ACRs than 20 h\({}^{-1}\). The results of this study support that outcome, as the BFPC results correlate well with particles and may also be used to continuously control the bioburden status of a cleanroom with early warning possibilities in case of high microbial values. This is in line with the study results of [PERSON] et al. [11], who found that 'lower flow rates reduce unwanted turbulence and the magnitude of velocity fluctuations, making the ventilation system more efficient at removing contaminants... '. If enhanced particle or heat generation within a cleanroom can be excluded as a reason for higher ACRs, manufacturers should test and qualify their individual cleanrooms with an ACR of 10 h\({}^{-1}\) BFPC systems cannot, today, replace regulatory required CFU recording by classic air sampling but can support manufacturers in lowering ACRs in cleanrooms without compromising the control of air quality. This would reduce the energy demand for cleanroom operations.