Igor E. Agranovski

Development and evaluation of a new personal sampler for culturable airborne microorganisms

The objective of this study was to develop a new personal sampler for viable airborne microorganisms and to evaluate its performance under controlled laboratory conditions and in a field. In the sampler, air is bubbled through a porous medium submerged in a liquid layer, as has earlier been demonstrated to be highly efficient for air purification. The prototype had the physical collection efficiency >95% for particles >0.32 μm in aerodynamic diameter during 8 h of continuous operation. The pressure drop across the sampler was below 1700 Pa, much lower than that of most conventional bioaerosol samplers. The collection liquid losses due to evaporation and aerosolization did not exceed 18% in 8 h and the culturability of sampled microorganisms remained high: the recovery rate of stress-sensitive gram-negative P. fluorescens bacteria was 61±20%; for stress-resistant B. subtilis bacteria and A. versicolor fungal spores it was 95±9% and 97±6%, respectively. Six identical personal samplers were tested simultaneously on a simplified human manikin in an office environment. The culturable microbial concentration data obtained during 2, 4 and 8-h sampling were not affected by the sampling time. Inter-sample variation did not exceed 30%. The laboratory and field evaluations have demonstrated that the new sampler is capable of long-term personal sampling of airborne culturable microorganisms. The estimation of the detection limits has indicated that the sampler is capable of monitoring microbial exposure in the environments with the bacterial concentrations above 15 CFU/m3 and fungal concentrations above 5 CFU/m3 when using a sampling time of 8 h.

Inactivation of Viruses in Bubbling Processes Utilized for Personal Bioaerosol Monitoring

ABSTRACT A new personal bioaerosol sampler has recently been developed and evaluated for sampling of viable airborne bacteria and fungi under controlled laboratory conditions and in the field. The operational principle of the device is based on the passage of air through porous medium immersed in liquid. This process leads to the formation of bubbles within the filter as the carrier gas passes through and thus provides effective mechanisms for aerosol removal. As demonstrated in previous studies, the culturability of sampled bacterium and fungi remained high for the entire 8-h sampling period. The present study is the first step of the evaluation of the new sampler for monitoring of viable airborne viruses. It focuses on the investigation of the inactivation rate of viruses in the bubbling process during 4 h of continuous operation. Four microbes were used in this study, influenza, measles, mumps, and vaccinia viruses. It was found that the use of distilled water as the collection fluid was associated with a relatively high decay rate. A significant improvement was achieved by utilizing virus maintenance fluid prepared by using Hanks solution with appropriate additives. The survival rates of the influenza, measles, and mumps viruses were increased by 1.4 log, 0.83 log, and 0.82 log, respectively, after the first hour of operation compared to bubbling through the sterile water. The same trend was observed throughout the entire 4-h experiment. There was no significant difference observed only for the robust vaccinia virus.

Effect of fiber orientation on fiber wetting processes

The current work incorporates a microscopic study of the effect of fiber orientation on the fiber wetting process and flow of liquid droplets along filter fibers when subjected to airflow and gravity forces. Glass filter fibers in various combinations were oriented at various angles within a plane defined by the airflow direction and were supplied with distilled water in aerosol form. The behavior and flow of the liquid collected by the fibers were observed and measured using a specially developed microscope cell, detailed in the paper. The experimental results were compared to a theoretical model developed to describe the behavior. The theory and experimental results showed good agreement. The developed theory allows an optimum angle to be determined for the internal filter fiber structure in the design of wet filters. A sensitivity analysis of the model was conducted to determine the most important parameters. This will aid design of wet filtration systems such that maximal self-cleaning can be accomplished with minimal water use.

Long-Term Sampling of Viable Airborne Viruses

A novel bioaerosol sampling technique, which utilizes the bubbling process in the collection fluid, has recently been developed and found feasible for a long-term personal sampling of airborne bacteria and fungal spores as it maintained high physical collection efficiency and high microbial recovery rate for robust and stress-sensitive microorganisms. Further tests have shown that the new technique also has potential to collect viable airborne viruses, particularly when utilized for a short-term sampling of robust strains. As the short-term sampling has a limited application for assessing personal exposure in bioaerosol-contaminated environments, the present study was undertaken to investigate the feasibility of the “bubbler” for a long-term monitoring of viable airborne viruses. Liquid droplets containing Vaccinia virions (that simulate Variola, a causative agent of smallpox) were aerosolized with a Collison nebulizer into a 400-liter test chamber, from which the droplets were collected by three identical prototype personal samplers in the liquid medium during different time periods ranging from 1 to 6 hours. The viral content was measured in the collection fluid of the sampler and in the initial suspension of the nebulizer using the fluorescence-based method and by enumerating plaque-forming units per milliliter of the fluids. The relative recovery of viruses after the sampling act was determined. The results show that the “bubbling” technique has consistent collection efficiency over time and is capable of maintaining the viability of Vaccinia, for at least 6 hours, with a loss in recovery rate of about 10%. The data demonstrate a good potential of the new technique for measuring personal exposure to robust airborne viruses over a long period.

Removal of viable bioaerosol particles with a low-efficiency HVAC filter enhanced by continuous emission of unipolar air ions

UNLABELLED Continuous emission of unipolar ions has been shown to improve the performance of respirators and stationary filters challenged with non-biological particles. In this study, we investigated the ion-induced enhancement effect while challenging a low-efficiency heating, ventilation and air-conditioning (HVAC) filter with viable bacterial cells, bacterial and fungal spores, and viruses. The aerosol concentration was measured in real time. Samples were also collected with a bioaerosol sampler for viable microbial analysis. The removal efficiency of the filter was determined, respectively, with and without an ion emitter. The ionization was found to significantly enhance the filter efficiency in removing viable biological particles from the airflow. For example, when challenged with viable bacteria, the filter efficiency increased as much as four- to fivefold. For viable fungal spores, the ion-induced enhancement improved the efficiency by a factor of approximately 2. When testing with virus-carrying liquid droplets, the original removal efficiency provided by the filter was rather low: 9.09 +/- 4.84%. While the ion emission increased collection about fourfold, the efficiency did not reach 75-100% observed with bacteria and fungi. These findings, together with our previously published results for non-biological particles, demonstrate the feasibility of a new approach for reducing aerosol particles in HVAC systems used for indoor air quality control. PRACTICAL IMPLICATIONS Recirculated air in HVAC systems used for indoor air quality control in buildings often contains considerable number of viable bioaerosol particles because of limited efficiency of the filters installed in these systems. In the present study, we investigated - using aerosolized bacterial cells, bacterial and fungal spores, and virus-carrying particles - a novel idea of enhancing the performance of a low-efficiency HVAC filter utilizing continuous emission of unipolar ions in the filter vicinity. The findings described in this paper, together with our previously published results for non-biological particles, demonstrate the feasibility of the newly developed approach.

Collection of Airborne Microorganisms into Liquid by Bubbling Through Porous Medium.

A new method for the removal of airborne particles by air bubbling through fibrous filters immersed into a liquid has recently been developed (Agranovski et al. 1999) and shown to be very efficient for cleaning air environments with ultra-fine aerosol particles. The principal objective of the present study was to evaluate the new bubbling technique for the collection of airborne bacteria into a liquid for subsequent physical and microbiological analysis. It was found that the technique is capable of achieving a physical collection efficiency of 98.5% or higher for particles larger than 0.3 w m in aerodynamic diameter. The physical collection efficiency of the prototype bubbler remained at that high level for 8 h of continuous operation with negligible variation of the pressure drop across the device. Evaporation of the collection fluid did not exceed 20% during 8 h, and the reaerosolization effect on the physical collection efficiency of the bubbler prototype was <8%. The recovery rate of gram-negative Pseudomonas fluorescens bacteria collected for 20 min was shown to be as high as 74% - 10%. Its decrease with time was not statistically significant: the recovery rate reached 63% - 15% and 58% - 16% after 4 and 8 h of continuous operation, respectively. Thus the bubbling technique was demonstrated to be suitable for collecting viable airborne bacteria even if they are sensitive to the stress.

Particle Bounce During Filtration of Particles on Wet and Dry Filters

This paper experimentally examines the bounce and immediate re-entrainment of liquid and solid monodisperse aerosols under a stable filtration regime (precake formation) by wet and dry fibrous filters. PSL and DEHS were the solid and liquid aerosols, respectively, used in four monodisperse sizes of 0.52, 0.83, 1.50, and 3.00 w m. Three different fibrous filters were used to filter the aerosol streams, and the efficiency of the filtration process for each aerosol type under dry and wet regimes was measured. It was found that the solid particles generally exhibited a lower fractional filtration efficiency than liquid particles, although this difference decreased in the smaller size fractions. The difference between solid and liquid efficiencies was found to be greatest in the 1.5 w m size range. As particle sizes of liquid/solid aerosols and filtration parameters were similar, this difference is most likely to be due to the effect of particle bounce and or immediate re-entrainment occurring inside the filter, with the greater efficiency of filtration of the liquid particles being due to their greater capacity to plastically/elastically deform in order to absorb the impact forces. However, for the wet filtration regime (each fibre of the filter was coated by a film of water), no significant difference in filtration efficiency was detectable between solid and liquid aerosols. Therefore, the conclusion can be drawn that the either the bounce effect of the particles is inhibited by the liquid film, or the filtration conditions in the wet filter are so different that the aerosol properties are less significant with respect to capture.

Monitoring of viable airborne SARS virus in ambient air

Abstract Due to recent SARS related issues (Science 300 (5624) 1394; Nature 423 (2003) 240; Science 300 (5627) 1966), the development of reliable airborne virus monitoring procedures has become galvanized by an exceptional sense of urgency and is presently in a high demand (In: Cox, C.S., Wathers, C.M. (Eds.), Bioaerosols Handbook, Lewis Publishers, Boca Raton, FL, 1995, pp. 247–267). Based on engineering control method (Aerosol Science and Technology 31 (1999) 249; 35 (2001) 852), which was previously applied to the removal of particles from gas carriers, a new personal bioaerosol sampler has been developed. Contaminated air is bubbled through porous medium submerged into liquid and subsequently split into multitude of very small bubbles. The particulates are scavenged by these bubbles, and, thus, effectively removed. The current study explores its feasibility for monitoring of viable airborne SARS virus. It was found that the natural decay of such virus in the collection fluid was around 0.75 and 1.76lg during 2 and 4h of continuous operation, respectively. Theoretical microbial recovery rates of higher than 55 and 19% were calculated for 1 and 2h of operation, respectively. Thus, the new sampling method of direct non-violent collection of viable airborne SARS virus into the appropriate liquid environment was found suitable for monitoring of such stress sensitive virus.

Removal of aerosols by bubbling through porous media

Aerosols can be filtered on fibrous filters with or without the pres ence of circulating water. The wet filtration leads to the formation of bubbles within the filter as the carrier gas passes through. This provides alternate mechanisms for the removal of aerosols. Experiments are described to investigate the effects of the irrigating fluid and to compare the efficiency of wet and dry filtration. The results indicate a marked increase in efficiency of filtration of the wet filter as compared with the dry filter.

Mechanism of nanoparticle agglomeration during the combustion synthesis

The mechanism of agglomeration of nanoparticles generated during combustion synthesis is discussed. This is based on the analysis of the transmission electron microscope images of probes collected at different heights. Although direct temperature measurements were not available, the qualitative temperature dependence of the particle formation streamlines is taken into account. It is demonstrated that agglomeration of the MgO nanoparticles, which are formed during a Mg particle combustion, occurs as the result of bonding the mature nanoparticles by the supercritical clusters existing in the system. Accumulation of these supercritical clusters in the flame has been revealed and their nature has been explained in our recent paper [I.S. Altman, I.E. Agranovski, and M. Choi, Phys. Rev E70, 062603 (2004)]. Also, some inspection of the previously published experimental data on the nanoparticle generation shows that the similar supercritical clusters may exist in another flame reactor generating titania nanopaprticles. If this is the case, the cluster-based process of nanoparticle bonding we suggest can be considered to be general.