Application of air ions for bacterial de-colonization in air filters contaminated by aerosolized bacteria

Air sampling and ATP bioluminescence for quantitative detection of airborne microbes

Exposure to bioaerosol contamination has detrimental effects on human health. Recent advances in ATP bioluminescence provide more opportunities for the quantitative detection of bioaerosols. Since almost all active organisms can produce ATP, the amount of airborne microbes can be easily measured by detecting ATP-driven bioluminescence. The accurate evaluation of microorganisms mainly relies on following the four key steps: sampling and enrichment of airborne microbes, lysis for ATP extraction, enzymatic reaction, and measurement of luminescence intensity. To enhance the effectiveness of ATP bioluminescence, each step requires innovative strategies and continuous improvement. In this review, we summarized the recent advances in the quantitative detection of airborne microbes based on ATP bioluminescence, which focuses on the advanced strategies for improving sampling devices combined with ATP bioluminescence. Meanwhile, the optimized and innovative strategies for the remaining three key steps of the ATP bioluminescence assay are highlighted. The aim is to reawaken the prosperity of ATP bioluminescence and promote its wider utilization for efficient, real-time, and accurate detection of airborne microbes.

Scoping review on the efficacy of filter and germicidal technologies for capture and inactivation of micro-organisms and viruses

The COVID-19 (SARS-CoV-2) pandemic increased the focus on preventing contamination with airborne pathogens (e.g. viruses, bacteria, and fungi) by reducing their concentration. Filtration, UV or ionization technologies could contribute to air purification of the indoor environment and inactivation of micro-organisms. The aim of this study was to identify the relevant literature and review the scientific evidence presented on the efficacy of filter and germicidal technologies (e.g. non-physical technologies) in air purification applications used to capture and inactivate micro-organisms and airborne viruses (e.g. SARS-CoV-2, rhinovirus, influenzavirus) in practice. A scoping review was performed to collect literature. Adopting exclusion criteria resulted in a final number of 75 studies to be included in this research. Discussion is presented on inactivation efficiencies of ultraviolet germicidal irradiation (UVGI) and ionization applications in laboratory studies and in practice. Specific attention is given to studies relating the use of UVGI and ionization to inactivation of the SARS-CoV-2 virus. Based on the consulted literature, no unambiguous conclusions can be drawn regarding the effectiveness of air purification technologies in practice. The documented and well-controlled laboratory studies do not adequately represent the practical situation in which the purifier systems are used.

Control technologies to prevent aerosol-based disease transmission in animal agriculture production settings: a review of established and emerging approaches

Transmission of infectious agents via aerosols is an ever-present concern in animal agriculture production settings, as the aerosol route to disease transmission can lead to difficult-to-control and costly diseases, such as porcine respiratory and reproductive syndrome virus and influenza A virus. It is increasingly necessary to implement control technologies to mitigate aerosol-based disease transmission. Here, we review currently utilized and prospective future aerosol control technologies to collect and potentially inactivate pathogens in aerosols, with an emphasis on technologies that can be incorporated into mechanically driven (forced air) ventilation systems to prevent aerosol-based disease spread from facility to facility. Broadly, we find that control technologies can be grouped into three categories: (1) currently implemented technologies; (2) scaled technologies used in industrial and medical settings; and (3) emerging technologies. Category (1) solely consists of fibrous filter media, which have been demonstrated to reduce the spread of PRRSV between swine production facilities. We review the mechanisms by which filters function and are rated (minimum efficiency reporting values). Category (2) consists of electrostatic precipitators (ESPs), used industrially to collect aerosol particles in higher flow rate systems, and ultraviolet C (UV-C) systems, used in medical settings to inactivate pathogens. Finally, category (3) consists of a variety of technologies, including ionization-based systems, microwaves, and those generating reactive oxygen species, often with the goal of pathogen inactivation in aerosols. As such technologies are typically first tested through varied means at the laboratory scale, we additionally review control technology testing techniques at various stages of development, from laboratory studies to field demonstration, and in doing so, suggest uniform testing and report standards are needed. Testing standards should consider the cost-benefit of implementing the technologies applicable to the livestock species of interest. Finally, we examine economic models for implementing aerosol control technologies, defining the collected infectious particles per unit energy demand.

Synergistic disinfection of aerosolized bacteria and bacteriophage by far-UVC (222-nm) and negative air ions

Air ionizers and 222-nm krypton-chlorine (KrCl) excilamp have proven to be effective disinfection apparatus for bacteria and viruses with limited health risks. We determined inactivation efficiencies by operating them individually and in combined modules. Gram-positive and gram-negative bacteria, non-enveloped dsDNA virus, and enveloped dsRNA virus were examined in a designed air disinfection system. Our results showed that the bioaerosols were inactivated efficiently by negative ionizers and far-UVC (222-nm), either used individually or in combination. Among which the combined modules of negative ionizers and KrCl excilamp had the best disinfection performance for the bacteria. The aerosolized virus P22 and Phi 6 were more susceptible to 222-nm emitted by KrCl excilamp than negative air ions. Significant greater inactivation of bacterial bioaerosols were identified after treated by combined treatment of negative air ion and far-UVC for 2 minutes (Escherichia coli, 6.25 natural log (ln) reduction; Staphylococcus epidermidis, 3.66 ln reduction), as compared to the mean sum value of inactivation results by respective individual treatment of negative ionizers and KrCl excilamp (Escherichia coli, 4.34 ln; Staphylococcus epidermidis, 1.75 ln), indicating a synergistic inactivation effect. The findings provide important baseline data to support the design and development of safe and high-efficient disinfection systems.

Aerosol Transmission of the Pandemic SARS-CoV-2 and Influenza A Virus Was Blocked by Negative Ions

The pandemic of respiratory diseases, such as coronavirus disease 2019 (COVID-19) and influenza, has imposed significant public health and economic burdens on the world. Wearing masks is an effective way to cut off the spread of the respiratory virus. However, due to cultural differences and uncomfortable wearing experiences, not everyone is willing to wear masks; there is an urgent need to find alternatives to masks. In this study, we tested the disinfection effect of a portable ionizer on pandemic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (strain V34) and influenza A virus (strain CA04). Negative ions significantly reduced the concentration of particulate matter in the air above and effectively disinfected viruses stuck to the solid plate at the level of both nucleic acid and virus titer. The disinfection efficiency was >99.8% after 1-h exposure. Moreover, negative ions effectively disinfected aerosolized viruses; the disinfection efficiency was more than 87.77% after purification for 10 min. Furthermore, negative ions had a significant protective effect on susceptible animals exposed to viral aerosols. When the negative ionizer was switched from off to on, the inhalation 50% infective dose (ID₅₀) for golden hamsters challenged with SARS-CoV-2 rose from 9.878 median tissue culture infective dose (TCID₅₀) [95% confidence interval (CI), 6.727–14.013 TCID₅₀] to 43.891 TCID₅₀ (95% CI, 29.31–76.983 TCID₅₀), and the inhalation ID₅₀ for guinea pigs challenged with influenza A virus rose from 6.696 TCID₅₀ (95% CI, 3.251–9.601 TCID₅₀) to 28.284 TCID₅₀ (95% CI, 19.705–40.599 TCID₅₀). In the experiment of transmission between susceptible animals, negative ions 100% inhibited the aerosol transmission of SARS-CoV-2 and influenza A virus. Finally, we tested the safety of negative ion exposure. Balb/c mice exposed to negative ions for 4 weeks showed no abnormalities in body weight, blood routine analysis, and lung pathology. Our study demonstrates that air ions can be used as a safe and effective means of blocking respiratory virus transmission and contribute to pandemic prevention and control.

Positive and Negative Ions Potently Inhibit the Viability of Airborne Gram-Positive and Gram-Negative Bacteria

Positive and negative ions (PAIs and NAIs, respectively) generated by air ionizers curb indoor spread of airborne pathogens through cellular oxidative damage. Thus, here, we asked whether ion exposure of Staphylococcus aureus and Escherichia coli bacteria—either plated on agar or trapped in air filters—would affect their viability and whether this effect would be influenced by variations in bacterial type and load, action area, distance from the ion generator, exposure time, or filter type. We selected these two vegetative bacterium species because, besides being representative of Gram-positive and Gram-negative strains, respectively, they are widely recognized as the two most common airborne pathogens. We observed a robust ion inhibitory effect on the viability of free bacteria regardless of the experimental condition employed. Specifically, 12-h ion exposure of plated S. aureus and E. coli, at either 5 cm or 10 cm from the ion source, reduced bacterial viability by ∼95% and 70%, respectively. Furthermore, 3-h ion exposure was sufficient to reduce the viability of both bacterial species trapped in filters. Our results showing a strong antibacterial activity of PAI and NAI under all experimental conditions tested further support the use of air ionizers for preventing and/or containing airborne infection in domestic and nondomestic settings.

IMPORTANCE Indoor air is a well-established vehicle for direct and indirect spread of a wide variety of human pathogens—as bioaerosols are composed of bacteria, viruses, fungi, and other types of organisms—that may trigger some pathologies. Plasmacluster ionizers are known for their ability to generate positively or negatively charged air ions (PAIs and NAIs, respectively) that can kill/inactivate indoor airborne pathogens, through oxidative stress-induced damage, in various environments. Given these premises, the aim of this study was to evaluate the viability of Gram-positive and Gram-negative bacteria exposed to PAI and NAI under different experimental variables such as bacterial type and load, action area, distance from the ion generator, ion exposure time, and filter type. Altogether, our findings, demonstrating a remarkable PAI and NAI antibacterial activity, stress the importance of using air ionizers to prevent indoor airborne infection.

Indoor PM2.5 removal efficiency of two different non-thermal plasma systems

The use of non-thermal plasma (NTP) generators in air processing systems and their duct networks to improve indoor air quality (IAQ) has grown considerably in recent years. This paper reviews the advantages and disadvantages of NTP generators for IAQ improvement in biological, chemical and particulate pollutant terms. Also, it assesses and compares the ability of a multipin corona discharge (MPCD) and a dielectric barrier discharge (DBD) generator to reduce the concentration of fine particulate matter (PM2.5) in recycled, unfiltered air in a refrigeration chamber. The MPCD generator was found to have a higher PM2.5 removal efficiency; also, it was faster in removing pollutants, used less energy, and produced much less ozone. The fact that the MPCD generator performed better was seemingly the result of its increased ion production mainly. NTP generators, however, cannot match air filtration media purifiers in this respect as the latter are much more effective in removing particles. Besides, NTP-based air purifying technology continues to be subject to a major drawback, namely: the formation of ozone as a by-product. In any case, the ozone generation was uncorrelated to ion emission when using different technologies.

Washable antimicrobial polyester/aluminum air filter with a high capture efficiency and low pressure drop

Here, we introduce a reusable bifunctional polyester/aluminum (PET/Al) air filter for the high efficiency simultaneous capture and inactivation of airborne microorganisms. Both bacteria of Escherichia coli and Staphylococcus epidermidis were collected on the PET/Al filter with a high efficiency rate (∼99.99%) via the electrostatic interactions between the charged bacteria and fibers without sacrificing pressure drop. The PET/Al filter experienced a pressure drop approximately 10 times lower per thickness compared with a commercial high-efficiency particulate air filter. As the Al nanograins grew on the fibers, the antimicrobial activity against airborne E. coli and S. epidermidis improved to ∼94.8% and ∼96.9%, respectively, due to the reinforced hydrophobicity and surface roughness of the filter. Moreover, the capture and antimicrobial performances were stably maintained during a cyclic washing test of the PET/Al filter, indicative of its reusability. The PET/Al filter shows great potential for use in energy-efficient bioaerosol control systems suitable for indoor environments.

Numerical and experimental study on airborne disinfection by negative ions in air duct flow

In this paper, we develop a mathematical model that aims (1) to predict the distribution of negative ions generated by an air ionizer installed in a ventilation duct and (2) to predict the efficiency with which it inactivates bacteria. The transportation equation for the negative ions was resolved combined with the bulk air velocity and the electric field. The bacteria distribution was solved numerically by integrating the susceptibility constant, which was acquired from the experiments. Two types of bacteria (Serratia marcescens, Staphylococcus epidermidis) were aerosolized and released into a 9-m ventilation duct system. Inactivation efficiencies were calculated for inlet velocities from 2 to 6.5 m/s and for various ion intensities. The efficiencies for S. marcescens and S. epidermidis were 31.53% (SD, 11.4%) and 12.17% (SD, 0.43%), respectively, with susceptibility constants of 8.67 × 10-11 Colony-Forming Units (CFU)/ions and 2.72 × 10-11 CFU/ions, respectively. The modeling results matched those of the experiments well. The pressure penalty at the maximum velocity (6.5 m/s) was only 9 Pa. The results show that the use of negative ions has great potential to enhance indoor air quality by reducing airborne microorganisms in ventilation systems.

Application of corona discharge-generated air ions for filtration of aerosolized virus and inactivation of filtered virus

The effect of corona discharge-generated air ions on the filtration of aerosolized bacteriophage MS2 was studied. A carbon-fiber ionizer was installed upstream of a medium-efficiency air filter to generate air ions, which were used to charge the virus aerosols and increase their filtration efficiency. After the virus aerosols were captured by the filter for a certain time interval, they were exposed to a newly incoming air ion flow. Captured virus particles were detached from the filter by sonication, and their antiviral efficiency due to air ions was calculated by counting the plaque-forming units. The antiviral efficiency increased with ion exposure time and ion concentration. When the concentration of positive air ions was 10⁷ ions/cm³, the antiviral efficiencies were 46.1, 78.8, and 83.7% with exposure times of 15, 30, and 45 min, respectively. When the ionizer was operated in a bipolar mode, the number concentrations of positive and negative ions were 6.6×10⁶ and 3.4×10⁶ ions/cm³, respectively, and the antiviral efficiencies were 64.3, 89.1, and 97.4% with exposure times of 15, 30, and 45 min, respectively. As a quantitative parameter for the performance evaluation of air ions, the susceptibility constant of bacteriophage MS2 to positive, negative, bipolar air ions was calculated as 5.5×10^-3, 5.4×10^-3 and 9.5×10^-3, respectively. These susceptibility constants showed bipolar ion treatment was more effective about 1.7 times than unipolar ion treatment.

Review of bioaerosols in indoor environment with special reference to sampling, analysis and control mechanisms

Several tiny organisms of various size ranges present in air are called airborne particles or bioaerosol which mainly includes live or dead fungi and bacteria, their secondary metabolites, viruses, pollens, etc. which have been related to health issues of human beings and other life stocks. Bio-terror attacks in 2001 as well as pandemic outbreak of flue due to influenza A H1N1 virus in 2009 have alarmed us about the importance of bioaerosol research. Hence characterization i.e. identification and quantification of different airborne microorganisms in various indoor environments is necessary to identify the associated risks and to establish exposure threshold. Along with the bioaerosol sampling and their analytical techniques, various literatures revealing the concentration levels of bioaerosol have been mentioned in this review thereby contributing to the knowledge of identification and quantification of bioaerosols and their different constituents in various indoor environments (both occupational and non-occupational sections). Apart from recognition of bioaerosol, developments of their control mechanisms also play an important role. Hence several control methods have also been briefly reviewed. However, several individual levels of efforts such as periodic cleaning operations, maintenance activities and proper ventilation system also serve in their best way to improve indoor air quality.

Negative ions detection in air using nano field-effect-transistor (nanoFET)

We firstly demonstrated the detection of anions in air using a nano field-effect transistor (nanoFET) device. Negative ions in air charged the top surface of the silicon nanoFET channel affecting the fieldeffect and making a conductance change of the channel proportional to anion concentration around the nano channel sensing surface. The real-time detection of anions in air with the nanoFET was performed for various anion concentrations which were differentiated by the distance of the anion generator to the nanoFET sensor. The air anions detection characteristics of the nanoFET device were evaluated with sensitivity and conductance change rates analysis.

Filtration and inactivation of aerosolized bacteriophage MS2 by a CNT air filter fabricated using electro-aerodynamic deposition

Carbon nanotubes (CNTs) were coated on a sample of glass fiber air filter medium at atmospheric pressure and room temperature using electro-aerodynamic deposition (EAD). In the EAD method, CNTs (diameter: 50 nm, length: 2-3 μm) were aerosolized, electrically charged, and injected through a nozzle. A voltage was applied externally between the ground nozzle and a planar electrode on which the sample was located. The charged CNTs were deposited on the sample in a vertically standing posture even at a low flow velocity. Before the deposition experiment, a calculation was performed to determine the applied voltage by simulating the electric field, flow field, and particle trajectory. Using CNT-coated filter samples, virus aerosol filtration and anti-viral tests were carried out using the aerosol number counting method and the plaque counting method, respectively. For this purpose, bacteriophage MS2 was aerosolized with an atomizer. The particle filtration efficiency was increased to 33.3% in the most penetration particle size zone (100 nm) and the antiviral efficiency of the CNT filter was 92% when the coating areal density was 1.5 × 10⁹ #/cm². The susceptibility constant of virus to CNTs was 0.2 cm²/μg.

Real-time monitoring of bioaerosols via cell-lysis by air ion and ATP bioluminescence detection

In this study, we introduce a methodology for disrupting cell membranes with air ions coupled with ATP bioluminescence detection for real-time monitoring of bioaerosol concentrations. A carbon fiber ionizer was used to extract ATP from bacterial cells for generating ATP bioluminescence. Our methodology was tested using Staphylococcus epidermidis and Escherichia coli, which were aerosolized with an atomizer, and then indoor bioaerosols were also used for testing the methodology. Bioaerosol concentrations were estimated without culturing which requires several days for colony formation. Correlation equations were obtained for results acquired using our methodology (Relative Luminescent Unit (RLU)/m(3)) and a culture-based (Colony Forming Unit (CFU)/m(3)) method; CFU/m(3)=1.8 × measured RLU/m(3) for S. epidermidis and E. coli, and CFU/m(3)=1.1 × measured RLU/m(3) for indoor bioaerosols under the experimental conditions. Our methodology is an affordable solution for rapidly monitoring bioaerosols due to rapid detection time (cell-lysis time: 3 min; bioluminescence detection time: <1 min) and easy operation.

Susceptibility constants of airborne bacteria to dielectric barrier discharge for antibacterial performance evaluation

Dielectric barrier discharge (DBD) is a promising method to remove contaminant bioaerosols. The collection efficiency of a DBD reactor is an important factor for determining a reactor's removal efficiency. Without considering collection, simply defining the inactivation efficiency based on colony counting numbers for DBD as on and off may lead to overestimation of the inactivation efficiency of the DBD reactor. One-pass removal tests of bioaerosols were carried out to deduce the inactivation efficiency of the DBD reactor using both aerosol- and colony-counting methods. Our DBD reactor showed good performance for removing test bioaerosols for an applied voltage of 7.5 kV and a residence time of 0.24s, with η(CFU), η(Number), and η(Inactivation) values of 94%, 64%, and 83%, respectively. Additionally, we introduce the susceptibility constant of bioaerosols to DBD as a quantitative parameter for the performance evaluation of a DBD reactor. The modified susceptibility constant, which is the ratio of the susceptibility constant to the volume of the plasma reactor, has been successfully demonstrated for the performance evaluation of different sized DBD reactors under different DBD operating conditions. Our methodology will be used for design optimization, performance evaluation, and prediction of power consumption of DBD for industrial applications.