# Combating Pathogens Using Carbon-Fiber Ionizers (CFIs) for Air Purification: A Narrative Review
## 4 Discussion
### Defining the Testing Protocols and Parameter Reporting for a Consistent Research Approach  
Among the factors hindering a comparison of results across studies, inconsistent and/or insufficient reporting on parameter space (defining the conducted study) and on the testing protocols that were adopted might be the most significant obstacle. Naturally, when the field of study is relatively broad, a subset of all experimental conditions is often examined. Even so, the metric space comprising all relevant parameters should be defined, and the spans of parameters covered in each study should be carefully and accurately reported. Firstly, such an approach facilitates comparisons of experiments, their repeatability by other groups, and reaching valid conclusions. Secondly, detailed data reporting allows for the construction of complex conceptual models, such as the ones developed in Refs. [66; 67], to study the intricate interconnections of groups of parameters. Moreover, subsets of parameters can be used to infer the influence of particular parameters on system behavior. Regarding the application of small air ions to remove and inactivate pathogens from indoor air, the most important parameters are ion polarity (bipolar, positive, and negative air ions--NAIs), polarity ratio (bipolar ionization typically produces different amounts of positive and negative ions), ion concentration, concentration of ions compared with the concentration of bioaerosols (ratio), type of ionizing devices used, particularly in connection with the production of ozone as a byproduct, and exposure time (due to the short lifetime of the air ions, exposure time can be taken to coincide with the time the source is turned on). For example, the dielectric barrier discharge (DBD)-based ionizer type uses voltage to tear apart the air molecules, thereby creating ions of unpredictable chemical composition (possibly also harmful to humans). In contrast to that, the corona discharge is considered a clean technology because it produces electrons that bond with the existing air molecules (not harmful in any way). All of the aforementioned parameters have to be measured, controlled, and reported. A certain set of parameters may be of interest for a particularexperiment, but the reporting must be as accurate as possible. The ozone concentration is routinely monitored in all of the experiments in our group, although we use the CFI with \"zero ozone production\" declared by the manufacturer. Testing protocols (the physics part) depend on the type of experiment conducted. In an open or partially open interaction volume, ions need to be steered by the airflow into the experimental volume. To work with the microorganisms, we design customized chambers (often like the well-known Goldberg rotating chamber) and setups that provide highly controlled experimental conditions, thus allowing reliable and accurate measurements even when a relatively small quantity of pathogens is used. All of these details are thoroughly described, which we recommend as a good practice. Regarding the biological parameters, the ones necessary to report are microorganism type, size of the microorganism, as well as the average sizes of aerosol droplets and pathogen surface potential. The size of microorganisms and their surface charge significantly influence their interaction with the CFI. Smaller microorganisms, such as viruses (20-300 nm), have a higher surface area-to-volume ratio, allowing more extensive contact with the CFI, which can disrupt their surface coats. Bacteria (0.5-5 um) and larger fungal spores (2-100 um) also interact with the CFI, but their larger size means that gravitational settling plays a more prominent role in their deposition. Additionally, the surface charge of microorganisms affects their attraction to the CFI; negatively charged microbes are more likely to be attracted to a positively charged CFI, enhancing their deposition and potential inactivation.