The simple, involuntary act of breathing constantly exposes the human respiratory system to a host of biological and nonbiological particles capable of causing diseases that range from microbial respiratory infections to allergic reactions that may exacerbate underlying asthmatic conditions. Aerosol transmission has been implicated to various degrees in a number of viral and bacterial infections, including smallpox (1
), influenza (2
), tuberculosis (3
), and anthrax (4
). Asthma, a chronic respiratory disease that can be triggered by inhalation exposure to diesel soot and allergens, such as fungal spores, pollen, and pet dander, ranked as the fifth most costly health care expenditure in the United States at $51.3 billion in 2006 (5
). Considering that citizens of developed nations normally spend 87% of their time indoors (6
), properly maintaining indoor air quality is clearly an absolute necessity for the protection of public health.
The capture of aerosol particles by filtration is the most common method for air cleaning. High-efficiency particulate air (HEPA) filters, which remove airborne particles of >0.3 μm with 99.97% efficiency, are generally employed for applications requiring the highest level of particle removal. For example, HEPA filters have been incorporated into hospital air handling systems that service operating rooms where bone marrow transplant (8
) and medical device implant surgeries (10
) are performed. Over time, however, as particle loading increases, pores can become clogged, resulting in a high-pressure drop across the filter material. This pressure drop, in turn, requires additional energy consumption in order to maintain a consistent airflow rate. Regular filter replacement can alleviate pressure drop concerns, but at a price.
Because of lower power requirements and reduced maintenance costs, electrostatic precipitation (ESP) has established itself as a feasible particulate control alternative. ESPs act by imparting charge on airborne particles which, in the presence of an electric field, are then directed to and deposited on a metallic collection plate. Corona inception is an operational condition in which a dense cloud of free electrons and positive ions is first established around a discharge electrode (11
). In the presence of this corona, noncharged or weakly charged particles that otherwise are inefficiently collected by ESPs become charged as they collide with ambient unipolar ions. An electric field then drives these particles toward an oppositely charged collection electrode, with physical capture occurring upon contact with the inner wall of the ESP. With airflow occurring tangential to the collected material, these devices are characterized by low-pressure drop and, subsequently, have lower power requirement than HEPA filtration. The usefulness of ESP technology in mitigating biological aerosols of >1 μm in diameter has been demonstrated using bacterial endospores and various bacterial species (12
). Other research involving electrostatic precipitation of bacterial cells and spores has focused on survival rates (i.e., bioavailability) after electrical charging as well as sampling efficiency for exposure assessment (14
ESPs have size-dependent collection efficiencies, and while overall mass-based collection efficiencies may be high (e.g., 99%), the collection efficiencies of particles in the submicrometer and nanometer particle size range, the size range associated with viruses and some allergens, are typically low (19
). In this ESP penetration window (0.1- to 1-μm size range), electrical mobility, which is influenced by both particle charge and mechanical mobility, is at a minimum, and consequently, collection efficiency is also at a minimum. At diameters below this size range, diffusion charging becomes more efficient, and particle collection efficiency increases. Similarly, at particle sizes above this size range, field charging becomes more efficient, leading to an increase in particle collection efficiency. The collection efficiency of particles in this ESP penetration window can be enhanced through incorporation of in situ
soft X-ray irradiation, which increases bipolar ion concentration and direct particle photoionization (19
). Soft X-rays generated by the emitter have wavelengths of 0.12 to 0.41 nm, which slightly overlap the range of extreme UV light. At these wavelengths, the soft X-rays have energies of 3.5 to 9.5 keV, which makes them less energetic than hard X-rays (medical diagnostics) but more energetic than the majority of UV light. Hogan et al. (23
) demonstrated both enhanced charging and improved collection efficiency of aerosolized bacteriophage MS2. In a follow-up study, molecular microbiological techniques were used to quantify the level of in-flight inactivation of bacteriophages T3 and MS2 passing through a soft-X-ray-enhanced ESP system (24
This study aimed to apply soft-X-ray-enhanced electrostatic precipitation in a fashion relevant to (i) infectious disease control for a range of microorganisms and (ii) attenuation of airborne allergen concentration. With handling ease and worker safety concerns in mind, agents were chosen to represent a very broad spectrum of particle sizes and morphologies, including surrogates for potential bioterror agents as well as more typically encountered biological particles. Specifically, surrogates for smallpox, tuberculosis, anthrax, and airborne allergens were aerosolized, challenged by the ESP system, and then delivered to murine models in a nose-only exposure chamber. While filter testing or biological air samplers could quantify particle penetration, the use of animal exposures allows for a truer assessment of system protectiveness. Biological indicators of infection, i.e., seroconversion, lung infectivity burden, and mortality, were monitored postexposure to evaluate system effectiveness for prevention of respiratory infections. Airway infiltrations of inflammatory cells such as granulocytes (eosinophils and neutrophils) as well as the presence of cytokines in pulmonary secretions, which mimic outcomes in human asthma (25
), were used to assess allergic responses.