Mitigating airborne pathogens indoors: A systematic review of existing and next-generation air cleaning technologies

This technical paper provides a systematic review and comparison of existing and next-generation air cleaning technologies.

Abstract

The COVID-19 pandemic has increased global awareness of airborne pathogens and indoor air quality (IAQ), especially in confined spaces where the likelihood of transmission is higher. This systematic review evaluated 14 peer-reviewed studies investigating indoor air treatment technologies, including ultraviolet (UV) systems, filtration, ionisation, and hybrid approaches. UVC-mirror system demonstrated the highest pathogen removal efficacy (up to 99.99%) with minimal maintenance and without generating harmful by-products. However, these technologies do not distinguish between harmful and beneficial microorganisms, potentially disrupting microbial diversity and reducing exposures that contribute to overall health. In contrast, indirect evaporative cooling (IDEC) systems reduce airborne infection risk through passive mechanisms by enhancing outdoor air exchange and aerosol dispersion, while preserving microbial diversity. IDEC also operates in an energy-efficient manner with added benefits of humidity control. This review underscores the importance of integrating active and passive strategies to manage IAQ, supporting infection control while maintaining ecological balance in the indoor microbiome.

Introduction

Following the COVID-19 pandemic, public awareness of airborne pathogens and indoor air quality (IAQ) has surged, driving demand for residential and commercial air cleaning technologies. IAQ is increasingly recognised as a critical factor influencing occupant health, productivity, comfort, and wellbeing^{1, 2}. People spend over 90% of their time indoors or in air conditioned vehicles³. In enclosed residential or occupational settings, susceptible individuals can be repeatedly exposed to contagious bioaerosols. As a result, the type of ventilation system significantly influences exposure levels and potential adverse health effects. Systems with low air changes per hour (ACH), such as some air conditioning (AC) units, have been linked to increased disease risk due to continuous recirculation of the same air.

Researchers have identified that approximately 14% of the genes detected in AC environments are associated with neurodegenerative, cancer-related, viral, and bacterial infectious diseases. This association is attributed to the presence of pathogens including Mycobacterium tuberculosis, Acinetobacter baumannii, Staphylococcus aureus, Enterococcus faecalis, and viruses including HSV, HPV, HTLV-1, HIV, and influenza A⁴.

In response, numerous air cleaning technologies have been developed to reduce airborne transmission risks. Most UVC disinfection and filtration technologies are designed for broad-spectrum microbial inactivation. These systems rely on physical or chemical mechanisms, including ultraviolet-induced nucleic acid damage or reactive oxygen species, to eliminate both pathogenic and non-pathogenic organisms. While effective in reducing microbial load, these approaches may also disrupt the microbial ecology of indoor environments and reduce beneficial microbial exposures that support immune tolerance.

Among emerging alternatives, indirect evaporative cooling (IDEC) systems offer a passive and non-destructive method for reducing airborne infection risk. IDEC systems operate through enhanced outdoor air exchange and air mixing – mechanisms that dilute and disperse bioaerosols away from their source and reduce infectivity without directly inactivating microorganisms. This distinguishes IDEC from conventional disinfection systems: while most favour non-selective microbial elimination, IDEC supports a more balanced approach, reducing exposure while potentially preserving microbial diversity. This background is necessary to understand the relevance of maintaining microbial diversity, especially when considering the benefits of non-pathogens described below.

Historical evidence reinforces the need for air cleaning solutions that are effective yet ecologically balanced. Airborne pathogens have caused numerous infectious diseases, some leading to pandemics, as outlined in table 1. These include tuberculosis (Mycobacterium tuberculosis), measles and mumps (paramyxoviruses), whooping cough (Bordetella pertussis), and aspergillosis (Aspergillus species). COVID-19 (SARS-CoV-2) resulted in a global pandemic, contributing to approximately 25,000 deaths in Australia and over 7.1 million deaths worldwide⁵. One example is a 1992 tuberculosis outbreak in a Minnesota bar, where 41 of 97 patrons tested positive and 14 developed active infection despite a typical transmission rate of only 1–2% among close contacts⁶.

Outdoor and indoor air pollution stresses our respiratory system and may increase susceptibility to viral infections. Exposure to particulate matter and ozone-induced oxidative stress has been linked to increased severity and mortality following respiratory viral infections^{7, 8}. As urbanisation continues to rise, managing airborne transmission in indoor environments is increasingly essential.

Indoor airborne microorganisms

Non-pathogenic microorganisms

Non-pathogenic microbes, including species found in indoor and airborne environments, play a critical role in human health²⁶. Breathing in these microbes, often aerosolised from dust or present in normal indoor settings, can modulate immune responses, as seen with Acinetobacter lwoffii’s anti-allergy effects²⁷. Probiotics like Bifidobacterium longum and Lactobacillus sakei are common indoors, and further support gut and sinus health when inhaled in low doses, potentially reducing inflammation²⁸. However, for sensitive, vulnerable, immunocompromised, or sick individuals, these typically beneficial microbes can pose risks. Inhaled in high concentrations, even non-pathogenic species may overwhelm weakened immune systems, potentially causing opportunistic infections or exacerbating conditions like asthma, particularly in hospital settings where Bacillus spores or Enterococcus from dust could proliferate²⁹.

Air microbes: Acinetobacter lwoffii, Bacillus subtilis, and Mycobacterium vaccae are commonly detected in ambient air and have been linked to immune regulation, gut health, stress resilience, and protection against neurodegenerative conditions³⁰.

Indoor microbes: Lactobacillus fermentum, Propionibacterium freudenreichii, and Lactobacillus helveticus are frequently found in indoor environments and are associated with cholesterol reduction, obesity management, and anti-allergy effects³⁰.

Dust-aerosolised: Propionibacterium freudenreichii and Clostridium histolyticum have been identified in indoor dust and are reported to contribute to cancer prevention, metabolic health, and treatment of Peyronie’s disease³⁰.  

Human health impacts

Health impacts were extracted from the “Health Effect” column in the Tableau database of beneficial microbes³⁰.

Indoor airborne pathogenic microorganisms

Many airborne pathogens affect multiple body systems, with their impacts varying by type and mode of exposure. Airborne transmission has been shown to be a major public health threat that caused mortality in the pre-vaccination era³¹. Some airborne pathogens – particularly in occupational settings such as agriculture or food processing – act as respiratory allergens rather than infectious agents. When inhaled, they may trigger immune responses like asthma, allergic rhinitis, or hypersensitivity pneumonitis³². Unlike infections, these conditions result from immune sensitisation rather than direct microbial invasion, leading to infectious disease³³. From a numerical point of view, bacteria-like particle and viral-like particle bioaerosol concentrations are around 4.8 × 10⁵ , 5.2  × 10⁵ particles/m³ in an indoor setting³⁴. To reduce airborne pathogens, and to render them non-infectious/non-allergenic for susceptible healthy individuals, technologies based on inactivation/removal/dilution should be focused.

Modes of transmission

Transmission of respiratory pathogens occurs via direct or indirect contact, inhalation of droplets or aerosols, or a combination of these routes. These are expelled during natural respiratory activities such as exhaling, talking, sneezing, coughing, laughing, or singing. Artificial aerosol generation occurs through activities like toilet flushing, running taps, HVAC systems, evaporative cooling, and medical procedures (e.g., intubation, and surgery). Once released, they may settle due to gravity (short-range transmission), be carried by air currents, or remain airborne (long-range transmission) before inhalation by a susceptible individual. At short range, both contact and inhalation pose risks; at longer distances, once large droplets (>100 µm) settle, inhalation predominates. Aerosols may also form via the resuspension of settled dust particles. Bioaerosols may contain viruses, bacteria, fungi, cellular debris, toxins, and electrolytes from respiratory fluids. They vary in size, with viruses ranging from 0.017–0.3µm and most fungal particles over 1µm. Transmission risk is influenced by aerosol generation rates, respiratory deposition site, and infectious doses. Environmental factors such as temperature, humidity, ventilation, UV radiation, and air exchange rates affect survival and dispersal. Viral characteristics (e.g., envelope structure, stability) and host factors (e.g., immunity, age) also shape transmission dynamics³⁵.     

Methods

The Scopus database was searched using the terms “indoor air” OR “indoor environment” OR “indoor air quality” OR “IAQ” combined with “pathogen removal” OR “pathogen inactivation”, resulting in 14 identified articles. Screening was conducted as outlined in Figure 1. Abstracts of the selected articles were included in the final analysis.

Results

A total of 14 peer-reviewed studies were evaluated to determine the primary objective of pathogen control technologies used for indoor air disinfection. As shown in Figure 4, inactivation emerged as the dominant strategy, reported in 50% of studies (n=7). Removal-based technologies were applied in 35% of studies (n=5) while systems combining both filtration and inactivation accounted for 15% (n=2). Only a small fraction of studies (7%) specifically focused on virus inactivation or virus removal, respectively. These results indicate that most mitigation strategies target broad-spectrum microbial inactivation over virus-specific approaches. Figure 5 presents the proportional distribution of the different pathogen control strategies among the included indoor air treatment studies.

Inactivation

All seven studies employed ultraviolet-based systems, including ultraviolet C radiation (UV-C 254nm), far-UVC (222nm), and vacuum UV (185nm), targeting airborne pathogens via DNA/RNA damage or oxidative stress^{37, 40, 41, 44-46}. UVC systems disrupted genetic material, preventing microbial replication^{37, 41, 44-46}, while VUV-induced reactive oxygen species (ROS) and ozone further contributed to pathogen inactivation⁴⁹. Most systems achieved rapid inactivation (<1–10 min), with log reductions ranging from 3–6-log^{41, 44, 49}. Only two systems reported co-benefits for particle or volatile organic compound (VOC) oxidation^{40, 49}. By-product generation was noted in systems using 222nm and 185nm, primarily ozone and secondary organic aerosol (SOA)^{40, 49}, whereas UVC systems (254nm) did not report harmful residues^{37, 41, 44-46}. Advantages included high inactivation efficacy, compact design, and integration potential with HVAC^44-46. Disadvantages involved ozone toxicity risks^{40, 49} and reduced performance against UV-resistant pathogens⁴⁶. Ideal application settings included hospitals, offices, ventilation ducts, and small enclosed rooms. Systems functioned effectively under controlled humidity and moderate airflow, with optimal performance at 18–25 °C and RH ~50%^{36, 41, 44, 46, 49}.

Removal

The reviewed studies focused on removal-based indoor air disinfection strategies, including surface hygiene^{38, 39}, ventilation and filtration⁴³, antimicrobial HVAC filters with UV⁴⁷, and ion-enhanced deposition⁴⁸. These technologies removed pathogens through physical mechanisms such as disinfection of fomites, dilution and capture of aerosols, or enhanced particle deposition via electrostatic charge. Surface cleaning and hand hygiene showed significant reductions in surface contamination within 70 minutes, though were ineffective when applied in isolation. Ventilation and HEPA filtration effectively removed airborne pathogens and also addressed other co-occurring indoor pollutants such as CO₂ and particulate matter (PM), achieving up to 99% filtration efficiency. Antimicrobial filters combined with UV offered rapid disinfection (<2 minutes) but raised concerns about potential microbial resistance. Ionisation increased deposition rates by up to 650% for virus-sized particles, with effectiveness dependent on airflow direction and ion concentration. Importantly, none of these methods produced harmful by-products, with the exception of biocidal surface treatments, which may contribute to the development of resistant organisms. These systems were most effective under controlled indoor conditions (specifically temperatures between 20–25 °C and relative humidity levels of 40–60%) and were applicable to diverse settings such as hospitals, offices, schools, and transport. Overall, removal-based technologies provided complementary protection to inactivation systems, with their success contingent on consistent use, strategic placement, and environmental HVAC control.

Filtration-inactivation hybrid

The plasma air filtration systems (PAFS) and copper coordination polymer particle-decorated non-woven fibre filters (Cu-CPP/NWF) evaluated integrated with pathogen inactivation mechanisms, achieving dual control of bioaerosols and chemical pollutants^{36, 42}. The PAFS used corona discharge and Fe-Ni metal foam to trap and inactivate aerosols, employing reactive oxygen and nitrogen species (RONS) such as OH, HOONO, and H₂O₂ for cell lysis³⁶. Similarly, the Cu-CPP/NWF system inactivated bacteria via Cu²⁺-induced membrane disruption and ROS production⁴². Both systems were highly effective, with reported pathogen reductions of ≥99.3% and 100% for E. coli respectively^{36, 42}. These technologies also demonstrated strong particulate matter removal (≥90.9% PM³⁶ and 99.99% PM2.5⁴². VOC (e.g., p-xylene) and CO₂ adsorption were an added benefit of the copper-based filter that achieved full disinfection within 45 minutes. In comparison, one hour is required for effective inactivation due to the additional filters used to minimise ozone and NOx by-products from plasma discharge^{36, 42}. Advantages included reusable modular components, high removal efficiency, and integration potential with HVAC systems. However, PAFS risk low-level ozone generation, while the Cu-CPP/NWF’s performance declines under high humidity. Both systems operated optimally under standard indoor conditions (20–25°C, 40–55% RH) and are suitable for enclosed public environments, hospitals, and offices^{36, 42}.

Highest removal rate

Among the evaluated technologies, ultraviolet-based systems demonstrated the highest pathogen inactivation, reporting up to 99.99% bacterial and 90% viral reduction using a UVC-mirror system⁴⁴. Similarly, the ultraviolet-C LED system and UVC air treatment system (254nm) achieved over 99% inactivation with UVC filters and up to 6.48-log₁₀ reduction for MS2 coliphage, respectively^{38, 42}. The VUV system showed ≥6-log₁₀ reductions in E. coli and 5-log₁₀ for Mycobacterium tuberculosis using vacuum UV, while Cu-CPP/NWF reported 100% E. coli inactivation with copper-based filters⁴². Moderate efficacy was observed in hybrid systems such as HEPA filtration and UV irradiation, which recorded 3-log₁₀ reductions in under 11 minutes⁴⁵. Ion-assisted deposition improved removal by up to 654%, and manual hygiene strategies showed strong surface decontamination^{37, 39}. Lower-performing studies, including GUV 222 system UVGI, lacked quantitative metrics or raised concerns around by-product formation^{40, 44}. Overall, UVC-based interventions, particularly duct-integrated systems, offer the most reliable and practical solution for airborne pathogen control in indoor environments.

Long-term feasibility

Among the technologies reviewed, the UVC-mirror system stands out as the most suitable for long-term, low-maintenance use. It operates passively once installed, requiring only UV lamp replacement every 8,000–10,000 hours and no regular cleaning. Its reflective cavity design enhances inactivation efficiency while protective UV-transparent glass minimises fouling⁴⁶. In contrast, other UV systems demand regular maintenance, careful lamp monitoring, or system calibration to avoid ozone generation or exposure risks^{37, 40, 41, 44}. Filtration-based and plasma systems require regular cleaning, component monitoring, or replacement, which increases long-term upkeep. Behavioural strategies, while important during outbreaks, rely heavily on compliance and do not offer consistent automated control^{37-39, 42, 43}. Therefore, the UVC-mirror’s minimal intervention requirement, combined with effective airborne pathogen inactivation, makes it the most practical choice for sustained application.

Evaluating trade-offs: Efficacy, risks, suitability, and ventilation integration

Among the technologies reviewed, the UVC-mirror system offers the most practical and effective long-term solution for indoor air disinfection. It achieves high pathogen inactivation (up to 99.99 %) and does not produce harmful by-products⁴⁶. Designed for integration within HVAC ducts, it operates efficiently under standard temperature and humidity conditions without disrupting airflow. In contrast, while plasma filtration and VUV systems are effective, they generate ozone, posing potential health risks^{36, 49}. Manual hygiene interventions are reliant on user compliance and not suited to airborne control. Although single-pass UVC is highly effective, it demands warm-up time and close environmental/operational control⁴¹. Copper-based filters show promise but perform poorly in humid conditions⁴². Ion-based systems enhance deposition but lack pathogen inactivation⁴⁸. Overall, the UVC-mirror system provides the best balance of efficacy, safety, and ease of operation for sustained indoor air quality management.

Timeframe for inactivation

When prioritising rapid microbial inactivation, the ultraviolet-C LED system demonstrates the fastest response, achieving over 99% pathogen reduction within approximately 24ms³⁷. This far outpaces other systems, including the UVC-mirror system which, although highly effective and low-maintenance, requires under one second exposure⁴⁶. Similarly, single-pass UVC systems reach complete inactivation in less than one second, but need 30 minutes of warm-up before effective operation, reducing their practical responsiveness⁴¹. Systems like plasma filtration and copper-based filters achieve strong microbial reduction but require up to 45–60 minutes for full effect^{36, 42}. Manual hygiene strategies and ion-enhanced deposition take tens of minutes to reach steady control, making them less suitable when immediate air disinfection is required^{37, 39, 38}. While several technologies are effective, UV-C clearly leads in speed, making it best suited for high-risk, time-critical environments such as hospitals or rapid-response containment areas.

Discussion

Targeted mitigation vs IDEC-based passive mechanism

Among the technologies reviewed, none explicitly aim to safeguard non-pathogenic airborne microorganisms. Most systems – such as UVC disinfection, plasma air filtration, and copper-based antimicrobial filters – are designed for broad-spectrum microbial inactivation. These methods rely on direct physical or chemical mechanisms, including ultraviolet-induced nucleic acid damage or reactive oxygen species, which effectively eliminate both pathogenic and non-pathogenic organisms. While these methods enhance disinfection efficacy, they may inadvertently disrupt the microbial ecology of indoor environments and reduce the beneficial microbial exposures that support immune tolerance.

In contrast, indirect evaporative cooling (IDEC) systems function through passive mechanisms, including the introduction of large volumes of outdoor air, enhanced air mixing, and promoting droplet evaporation. The processes by which evaporative cooling dilutes bioaerosols include high air exchange rates (20–30 ACH), creation of positive indoor pressure, filtration through media pads, shortened residence time of indoor pollutants, and the absence of air recirculation – all of which contribute to the physical removal and dispersion of airborne microorganisms^50-53. These processes dilute bioaerosols, disperse particles away from their source, and reduce particle size, thereby lowering the infectivity of viruses and reducing occupant exposure risk. Notably, IDEC does not directly inactivate microorganisms, thereby avoiding collateral impacts on beneficial microbiota. This mode of action may support healthier indoor environments by preserving microbial diversity while still reducing infection risk through physical removal pathways. Therefore, IDEC represents a distinct approach among air treatment strategies by mitigating pathogen exposure without compromising the microbial balance of the built environment.

Conclusion

This systematic review highlights the technologies available for mitigating airborne pathogens in indoor environments, with ultraviolet-based systems. While most technologies prioritise broad-spectrum microbial inactivation through direct physical or chemical mechanisms, none consider the ecological implications of indiscriminately eliminating both harmful and beneficial microorganisms. In contrast, IDEC presents a passive yet effective alternative by promoting dilution, dispersion, and evaporation, thereby lowering pathogen exposure without disrupting microbial diversity.

The UVC-mirror system demonstrated the most balanced performance across efficacy, operational simplicity, safety, and HVAC compatibility, positioning it as the most practical long-term solution. However, technology selection must be context- and parameter-specific, considering the pathogen load, building design, occupant vulnerability, and maintenance capacity. Future strategies should incorporate an understanding of the indoor microbiome, recognising the potential health benefits of preserving non-pathogenic organisms. Integrating passive and active approaches – rather than relying solely on pathogen-targeted systems – may offer a more sustainable and holistic pathway for managing IAQ and protecting public health.

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