# Combating Pathogens Using Carbon-Fiber Ionizers (CFIs) for Air Purification: A Narrative Review
## 4 Discussion
### The Other Side of the Coin: How to Avoid Harmful Byproducts of Air Ionization? Despite its potential benefits, air ion generation, especially associated with corona discharge or electrostatic methods, often produces ozone as a byproduct, posing risks to human health [73; 76; 83]. [PERSON] et al. [18] highlighted the effectiveness of ionizers equipped to minimize ozone production, focusing on the generation of ions as the main method of achieving bactericidal effects. The health and safety issues stemming from ozone generation by air ionizers degrading indoor air quality are often emphasized [84]. Ozone, as a strong oxidant, can cause respiratory problems and worsen conditions such as asthma, potentially negating the benefits of removing airborne pollutants. Additionally, ozone can react with volatile organic compounds (VOCs) in indoor air to form harmful secondary pollutants, such as formaldehyde and ultrafine particles, which can cause more harm than the original pollutants [83; 84; 85]. This creates a complex situation in which devices intended to purify the air can paradoxically release harmful ozone, negating the benefits of removing airborne pollutants. Therefore, it is essential to evaluate air purifiers not only on their ability to remove pollutants but also on their ability to emit ozone by meeting stringent regulatory standards. The World Health Organization's (WHO) ozone guidance emphasizes managing ozone concentrations to minimize respiratory health risks caused by high ozone concentrations. These guidelines serve as a benchmark for national air quality standards, calling on states to monitor and manage ozone, especially from industrial and automotive sources, to achieve cleaner air and minimize health impacts [86]. Ozone exposure can have significant effects on human health, depending on the dosage delivered to the respiratory tract and the duration of exposure. Short-term exposure to ozone can cause inflammation and damage to the airways, followed by respiratory symptoms such as coughing and throat irritation, thus exacerbating preexisting respiratory conditions. Having this in mind, individual responses to ozone exposure can vary widely. Short-term ozone exposure is linked to respiratory issues, leading to hospital admissions for conditions like asthma and chronic obstructive pulmonary disease (COPD). Ozone levels between 60 and 80 ppb over an 8-h period are used to assess potential health risks for both healthy individuals and vulnerable groups, such as children and the elderly [86; 87]. Epidemiological studies on short-term exposure thresholds show inconsistent evidence, and health impact calculations assume linear concentration-response relationships with appropriate cut-off points: one at 10 ppb for daily maximum 8-h ozone and one at 35 ppb [87]. Long-term exposure to ozone is associated with chronic respiratory diseases and increased respiratory mortality, particularly in individuals with predisposing conditions, even at lower threshold levels around 40-60 ppb. It is also linked with deficits in lung function growth and increased asthma incidence, especially among children [86; 87; 88]. The number of electrons produced and the ion emission rate, as well as the ozone output, are correlated with the input power [89]. However, it is possible to effectively generate air ions while minimizing ozone generation, for example, through the use of carbon fiber brushes. Deployment of carbon fiber brushes has the potential to increase ion emission rates while reducing or even eliminating ozone production [11; 90]. The use of carbon fiber brushes has been shown to optimize the performance of ionizing air filtration systems by effectively charging and capturing particles, potentially reducing particle aggregation and improving dispersion and persistence of ions without the high ozone emissions typical of traditional ionizers [71; 73; 91].