# Combating Pathogens Using Carbon-Fiber Ionizers (CFIs) for Air Purification: A Narrative Review
## 4 Discussion
### Effectiveness of Air Ionization in Comparison with Other Pathogen Mitigation Measures  
To provide a wider perspective on air purification technologies, we give a short comparison of the different available measures. Firstly, the very efficient HEPA filters originated from military requirements for protection against warfare agents of different types [100]. Today, they are used in industrial systems and medical buildings, inside HVAC systems (heating, ventilation, and air conditioning), and beyond. The use of portable HEPA filtration systems is also on the rise for the removal of airborne contaminants in emergency situations [101]. Dense in construction, HEPA filters can collect up to 99.97% of airborne particles with a size of 300 nm. With a particle size smaller or larger, this filter is even more efficient [102]. The two main characteristics of these filters are the pressure drop and the particle collection efficiency, which vary based on the constitution and density of the used fibers as well as the fiber thickness and air velocity [100; 103]. HEPA filters are safe for humans. Exposure to collected microorganisms can happen only during HEPA filter removal. Some microorganisms captured on HEPA filters could survive under normal operating conditions for up to 200 days [104]. Because of their high density, HEPA filters require powerful fans to maintain air circulation throughout the system, which leads to high energy consumption. If the air flow rate is constant, pressure drop increases due to filter clogging [104], so these filters need regular maintenance and replacement. In combination, the high maintenance, high energy consumption, and high cost of HEPA filters prevent their wide use in general. HEPA filter characteristics are summarized in Table 2.  
UltraViolet Germicidal Irradiation (UVGI) kills or inactivates microorganisms by damaging their DNA or RNA. A large number of studies have shown that UVGI is very fast and effective in eliminating viruses [105; 106]. This type of radiation has been used in hospital and industrial systems for the removal of microorganisms from surfaces, air, and liquids. The UVC spectrum of UV radiation shows the strongest decontamination properties [106]. As UV radiation is harmful for humans, indoor spaces can only be decontaminated in the absence of humans, which cannot ensure that harmful particles will not accumulate while people are in the previously treated indoor space. Another approach \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Air contaminants** & & & **Limitations** & **Representative Examples** \\ \hline  & & & - & High cost & \\  & - & Safe for humans & - & High energy consumption & \\ HEPA (High Efficiency & - & High efficacy & - & Regular maintenance and \\ Particulate Air) filters & - & (99.97\% or more) & - & replacement needed & [100; 101; 102; 103; 104] \\  & & & & Exposure to airborne & \\  & & & contaminants during removal or & \\  & & & assessment & \\ \hline UVGI (UltraViolet & - & High efficacy & - & Not safe for humans & [105; 106; 107; 108; 109] \\ Germicidal Irradiation) & - & Fast disinfection & - & Need air flow assistance & [105; 106; 107; 108; 109] \\  & - & Low maintenance & - & Need air flow assistance & \\ \hline  & Ionizers & - & High efficacy & - & Not safe for humans & [18; 28; 37; 61; 99] \\ also producing ozone & - & Fast disinfection & - & & \\ \hline  & - & Good efficacy & - & \\ Ionizers with negligible ozone & - & Safe for humans & - & Better performance with air flow & \\ production & - & Health benefits from & assistance & [10; 62; 63; 110; 111] \\  & & exposure & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of major types of air purification technologies. is the installation of UVGI in the upper room zone, but with proper installation to avoid interaction with human skin and eyes [107, 108]. This installation should be combined with a vertical air flow mechanism in order to adequately clean the air in the entire volume of indoor space [109].