Resuspension of Allergen-containing Particles under Mechanical and Aerodynamic Disturbances from Human Walking - Introduction to an Experimental Controlled Methodology

Epidemiological evidence indicates that common environmental allergens found in building reservoirs are strongly associated with the development of bronchial hyper-reactivity (BHR) or asthma, affecting up to half the population in North America and Europe. Although they are rarely life threatening, these diseases cause much distress and lost time from school and work. These diseases are believed inhalation sensitized and developed, suggesting an aerobiological pathway of allergen-containing carrier particles from reservoir to occupant respiration. This study presents and develops a controlled and characterized method to explore the influence of human walking on the aerosolization of allergen-containing particles. Time resolved particle size distribution and allergen content are measured for particles resuspended from representative samples of flooring materials and for different sets of floor disturbances in an environmentally controlled experimental chamber. Initial results, when placed in the context of previous investigations, indicate the method can be utilized to develop a database for particle resuspension rates. INDEX TERMS Particle resuspension, Resuspension rates & factors, Allergens, Bio-aerosols, Asthma INTRODUCTION Epidemiological studies indicate continuing significant increases in health care and hospitalization of patients for respiratory system related diseases such as asthma. According to the Center for Disease Control and Prevention (Anon), approximately 6% of all Americans suffer from asthma and approximately 5,000 people die each year of asthma or related complications. The economic burden of this illness in the United States is estimated at $12.7 billion dollars per year (Anon A). Symptoms of asthma may be triggered in genetically predisposed individuals and developed in non-atopic individuals by exposure to allergens. Common indoor allergens are found in cat and dog fur or saliva, cockroach and dust mite droplets and body parts. After disintegrating, these materials adhere to inert dust in carpets, upholstery, and other reservoir surfaces making allergens available for secondary aerosolization when disturbed by human activity (walking, vacuum cleaning, etc.). Specific allergens associate with different ranges of carrier particle sizes (NAS 2000) and, after reentrainment, can stay airborne for relatively long periods of time as respirable particles. • Corresponding author email: [email protected], [email protected] The first studies examining the impact of human activity on particle resuspension were conducted in the 1960’s with radioactive material from the floor of nuclear facilities. The resuspension factors from these studies and from more recent ones were summarized by the US Nuclear Regulatory Commission (NRC 2002) and ranges from 6x10 to 7x10 m. Sehmel (1980) presented a summary of particle resuspension factors caused by mechanical disturbance from indoor human activities (walking and sweeping) and outdoor activities (pedestrian walking and vehicular traffic), ranging from 1x10 to 3x10 m. Until the 1990’s, occupancy-related particle resuspension in residential and offices buildings was seldom explored. Extrapolated resuspension values from the previously nuclear material studies had very limited application. More recently, due to the increase of particulate-related respiratory diseases (in particular asthma) and the emergence of CBW attacks on civilian populations, several studies on indoor particle resuspension have been performed (Table 1, Weis et al. 2002, Matsumoto 2003). Table 1 summarizes relevant findings. Table 1. Summary of Particle Resuspension Studies Floor Ambient Resusp. Source Location Type Dust Floor Load Conditions Activity Resuspension Part. Size Hambraeus Hospital Vinyl Bacteria-carrying 1.5x10 T=NR 4 persons walking, et al. Operation particles 1.6x10 RH=NR 30 min 2.5x10 m 3-6 μm 1978 Room dp=3-6 μm 3.4x10 4 Ventl=off Mopping, 10 min 2.0x10 m 3-6 μm V=112 m, A=35 m ρ=NR cfu/m No air leakge Hair dry jet, 10 min 1.2x10 m 3-6 μm Thatcher Residential Bldg Carpet Inert dust Average T=NR 4 persons 1.65x10 min 0-0.5 μm et al. 1 floor Wood ρ=1 g/cm (assm) 39 μg/cm RH=NR walking and 7.33x10 min 0.5-1.0 μm 1995 V=360 m Vinyl Sitting 3.00x10 min 1-5 μm A=150 m 1.38x10 min 5-10 μm 6.33x10 min 10-25 μm 5.67x10 min >25 μm Karlsson Experimental NR Grass Pollen 2.6x10 #/m T=NR 1 person walking 8.0x10 m 6.3 μm et al. Room 6x4 μm RH=NR at 0.7 m table 7.8x10 min 6.3 μm 1996 A=20 m dm=6.3 μm 1 hour ρ=1 g/cm (assm) Karlsson Experimental PVC Freeze dried spores 1x10 #/m T=NR 1 person walking 1.8x10 min 12 μm et al. Room Bacillus Subtilus or RH=NR 4 persons walking 2.45x10 min 12 μm 1999 V=45 m 1.8x0.9 μm 120 mg/m 75 steps/min A=15 m davg,aglom=12 μm ρ=1.3 g/cm Buttner Experimental Vinyl Penicillium 1x10 cfu/m HVAC, HEPA Walking in Vinyl, Comm Crpt et al. Room Com. carpet Chrysogenum 1x10 cfu/m 5 Pa pressz prescribed pattern 10-10 m 2-3 μm 2002 Resemb. residentl Resid carpet Spores T=NR 1 minute Resitl carpet 4.0x4.0x2.2 m 1.8x3.5 μm RH=NR 10-10 m 2-3 μm Ferro Single family Wood NR NR T≈20°C el al. home Rug RH=NR PM2.5 PM5 2004 Walking on wood 1.27x10 4.93x10 Vacuum on wood 3.05x10 5.45x10 Dance on wood 2.00x10 1.00x10 NR Not reported; assm assumed mg/(min.pr) Allergen concentrations on home floors as well as allergen concentration in the air for quiescent and disturbed conditions have also been measured and their ranges are represented in Table 2 (Blay et al. 1997, Custovic et al. 1997, Custovic et al. 1999, Lidia and Salthammer 2003). Table 2. Reservoir and air allergen concentration However, the absence of controlled parameters in the experiments of previous studies limits their application in the health risk assessment. Environmental conditions, such as humidity, were rarely considered or controlled to isolate their importance in particle resuspension. Systematic parametric variation, such as floor type, dust type and load, contaminant concentration in dust load, have not been performed, leading to difficulties in interpreting available data. Although it is generally accepted that floor disturbance mechanisms are mechanical, aerodynamic and electrostatic in nature, there is no consensus, explanatory theory for particle resuspension that can predict the effects of floor surface disturbances on particle re-entrainment. This report describes the development of an experimental and analytical methodology to examine particle surface-to-air aerosolization when reservoirs are subjected to human-related disturbances. The purpose is to establish a data bank detailing particle resuspension as a function of floor vibrations and transient near surface air flows, characteristic of human walking. This methodology was tested by conducting a set of resuspension experiments using carpet and linoleum flooring loaded with reference quartz and German roach dusts. RESUSPENSION CHAMBER METHOD Experiments are conducted in a constructed experimental chamber (400x200x200 mm) with temperature and relative humidity control. Dust samples are prepared containing various types of allergen (Bla g 1, Bla g 2, Der p 1, Der p 2, Can f 1 and Fel d 1) separated into precise particle size (8 bins from 0.4 to >9.0 μm) and allergen concentration ranges. The prepared dust is uniformly deposited on typical building floor samples (size: 90x90 mm) using designed particle disperser equipment. The selected surface samples are subjected to computer controlled levels of aerodynamic and mechanical disturbances, simulating the disturbance conditions associated with human walking. Resuspended particles are carried by a particle-free cross air flow and sampled by both optical particle counters (Sensors Inc., Semtech PM-300) and a Cascade Impactor (Andersen Mark II, Series 20-800). This air flow sample provides time resolved particle size distribution and size resolved samples to be tested for allergen concentration using ELISA assay techniques (Chapman et al. 2000). The aerodynamic disturbance was simulated with the impingement of six small air jets over the flooring sample. To understand the walking-related airflow motion nearby the floor, experiments were developed in a close environmental chamber using CO2 vapor released over the floor (Gomes 2004). These experiments provided the range of horizontal air velocity and the visualization of large scale air turbulence resulting from a human walking. The mechanical disturbance was simulated with a system that replicates field collected floor vibration data caused by human walking. Floor vibration acceleration generally falls between 0 and 5% of g (gravitational acceleration) with frequencies ranging from 4 to 20 Hz (Hurst and Lezotte 1970, Chui and Smith 1988, Hanagan et al. 1996, Hanagan et al. 2003). However, it is not uncommon to find accelerations higher than 70% of g (Hu et al. 1994). Electrostatic built-up voltage caused by the shoes/floor interaction can reach values higher than 10,000 volts (Robinson-Hahn 1995) and can potentially interfere with surface-to-air particle aerosolization, in particular organic or organic-containing material. This phenomenon is not presently incorporated on this research, but will be include in the near future. RESULTS Calibrated quartz particles (Particle Technology Limited, Crushed Quartz #10, United Kingdom) of known density, size distribution and composition are used to establish a basis of comparison for resuspension behavior with laboratory-produced, German roach dust particles. Two types of flooring, plastic carpet (100% olefin) and linoleum, were utilized. The floor samples were uniformly loaded with ≈50 mg of the two types of dust. The temperature was kept in between 26°C and 28°C and the relative humidity kept constant at 45%. Three sets of floor disturbance were implemented: (1) floor vibration, (2) air puff and (3) combination of both. For comparison, clean flooring samples were tested to the same set of disturbance and revealed no particle resuspension. Figure 2 shows the vibration and aerodynamic floor disturbance signal used. Floor Disturbance Signals -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0