Reaerosolization of Fluidized Spores in Ventilation Systems

Abstract

This project examined dry, fluidized spore reaerosolization in a heating, ventilating, and air conditioning duct system. Experiments using spores of Bacillus atrophaeus, a nonpathogenic surrogate for Bacillus anthracis, were conducted to delineate the extent of spore reaerosolization behavior under normal indoor airflow conditions. Short-term (five air-volume exchanges), long-term (up to 21,000 air-volume exchanges), and cycled (on-off) reaerosolization tests were conducted using two common duct materials. Spores were released into the test apparatus in turbulent airflow (Reynolds number, 26,000). After the initial pulse of spores (approximately 10 10 to 10 11 viable spores) was released, high-efficiency particulate air filters were added to the air intake. Airflow was again used to perturb the spores that had previously deposited onto the duct. Resuspension rates on both steel and plastic duct materials were between 10 ؊3 and 10 ؊5 per second, which decreased to 10 times less than initial rates within 30 min. Pulsed flow caused an initial spike in spore resuspension concentration that rapidly decreased. The resuspension rates were greater than those predicted by resuspension models for contamination in the environment, a result attributed to surface roughness differences. There was no difference between spore reaerosolization from metal and that from plastic duct surfaces over 5 hours of constant airflow. The spores that deposited onto the duct remained a persistent source of contamination over a period of several hours.The threat from a bioweapon agent such as anthrax remains problematic. Because of the biological, technical, and pathological characteristics of Bacillus anthracis, it is attractive as a bioweapon (2, 10). When used as a weapon of mass destruction, the agent is dispersed in particles less than 5 m in diameter, a size that allows penetration into the pulmonary alveoli (18). Minimal data on human infective doses for inhalational Bacillus anthracis are available (14,24,25). The problem is further compounded by variations in individual susceptibility, B. anthracis strain virulence and spore preparation technique, and physical characteristics of the bioparticle. Thus, studying the fate and transport of fluidized spore concentrations is necessary in order to better predict the area of contamination and to more accurately model potential health risk.Ventilation systems provide a conduit to contaminate a building and the surrounding area. Such systems can become entry points or distribution systems for hazardous contaminants, including biological weapon agents (BWA) (15). Air circulation in ordinary buildings can assist the spread of airborne disease and can disperse contaminants (11,22). Reaerosolization of these hazardous bioparticles deposited onto surfaces can be a continuing source of contamination. For example, a study of reaerosolization of B. anthracis after dispersal in a postal facility found that a mail sorter remained contaminated many days after processing B. anthracis-contaminated letters ...

Aerosol and Surface Deposition Characteristics of Two Surrogates for Bacillus anthracis Spores

“…B. atrophaeus subsp. globigii has been used (39,40) to model the dispersal and deposition behavior of B. anthracis spores, but the work presented here, supported by morphological comparisons (8,9), suggests that it might not be the best surrogate, at least under some conditions. Further testing and characterization of BtcryϪ aerosols under different conditions are required; there is a need for a safe, accurate surrogate for B. anthracis spores to produce experimental data to augment the modeling methods currently available (41)(42)(43).…”

Monolayer and Multilayer Particle Deposits on Hard Surfaces: Literature Review and Implications for Particle Resuspension in the Indoor Environment

“…Resuspension from the surfaces of ventilation ducts is primarily associated with aerodynamic removal forces (Krauter and Biermann 2007;Wang et al 2012). For most residential and commercial building applications, velocities in ventilation ducts are generally below 10 m/s.…”

“…Resuspension from indoor particle deposits can occur due to airflow in ven-tilation ducts (Krauter and Biermann 2007;Wang et al 2012) and human activities indoors (Thatcher and Layton 1995;Ferro et al 2004;Qian and Ferro 2008;Tian et al 2011;Shaughnessy and Vu 2012). Additionally, resuspension can be an exposure pathway to the multitude of pollutants that are commonly found in settled indoor dust, such as: allergens (O'Meara and Tovey 2000), lead, pesticides, phthalates, and flame retardants (Roberts et al 2009).…”

Standard method for deposition of dry, aerosolized, silica-coated Bacillus spores onto inanimate surfaces

“…Further treatment of the bioagent (e.g. applying a uniform charge to the particles, adding a fluidizing agent) can enhance the ability of the powder to disperse by inhibiting particle impacted by deposition techniques (Krauter and Biermann 2007;Byers et al 2013). Aerobiological standards for applying micro-organisms to surfaces are becoming more common and, recently, ASTM International (ASTM) approved two standards that describe methods for depositing influenza-infused droplets and droplet nuclei onto surfaces (ASTM International 2010a,b).…”