The Estimation of Resuspension Rates of Fine Particulate Matter in an Environmental Chamber

Valerie Bauza[1] and Andrea Ferro2

Department of Civil and Environmental Engineering

A great deal of research has been conducted to study outdoor air quality, resulting in state and federal regulations that are enforced to protect the health and welfare of the general public. Indoor air quality, on the other hand, is not regulated, with the exception of the workplace. Indoor environments account for a significant portion of human exposure to hazardous particles, as Americans spend an average of 87% of their time indoors (Ott et al. 2007, Thatcher 2002). As a result of both infiltration and track-in, many toxic chemicals are brought into homes and become existent in common household dust, some at high concentrations. Pollutants of concern commonly found in household dust include allergens, heavy metals, pesticides, polycyclic aromatic hydrocarbons (PAHs), plus several endocrine disruptors and carcinogenic compounds (Tong 2000, Rudel 2003, Ott et al. 2007). Normal human activities, preformed throughout the house, cause dust to become resuspended in the air, resulting in exposure to elevated levels of particulate matter. Small children, who spend a lot of time close to the floor, are especially at risk for health problems due to indoor dust and resuspension (Ott et al. 2007). Short term exposure to elevated levels of particulate matter has been associated with asthma symptoms, chronic obstructive pulmonary disease (COPD), and decreased heart rate variability (Delfino et al. 1998, Morgan et al. 1998, Gold et al. 2000).

Since walking is a common activity responsible for dust resuspension, it has been used to model resuspension rates of dust and human exposure due to resuspension. In 2005, Jing Qian conducted 54 experiments within a controlled environmental chamber to investigate the resuspension rate of particulate matter. For these experiments, flooring samples were seeded with ultrafine Arizona Test Dust (ATD)that has a distribution of particles in the size range of 0.1-10 μm. Participants then performed walking and sitting activities for a specified amount of time in the chamber, while the particle concentration and size distribution was measured using optical

particle counters (OPCs). Over the course of these 54 experiments, particle concentrations were

measured in relation to several factors including flooring type, ventilation type, particle loading, and person-to-person variability. Experiments were preformed using hard floor, new carpet, old carpet, mixing ventilation, displacement ventilation, and different walking styles to see how these factors affected the resuspension rates of particles. Following the experiments, resuspension rates were calculated for particles in the size ranges 0.8-1.0 μm, 1.0-2.0 μm, 2.0-5.0 μm, and 5.0-10.0 μm. The estimated particle resuspension rates from the experiments varied from 10-5 to 10-2 hr-1. These experiments showed that person-to-person variability in walking style accounted for the largestvariance in resuspension rates among experiments with a “heavy and fast” walking style resuspending more particles being than walking styles that involve less activity. Furthermore, it was observed that for the size range 0.8-10 μm, larger particles have larger resuspension rates (Qian 2007).
Although the experiments performed in the environmental chamber used particle monitors that recorded airborne particle concentration in the range of 0.4-0.8 μm, these data points were not included in the overall analysis. This range was not included because the distribution of particles within this size range was not specified in the manufacturer’s specifications of the ATD test dust, and this distribution is needed in order to calculate floor particle loading. This range of particles is part of the accumulation mode of fine particles (0.1-1.0 μm) which are less affected by removal mechanisms and thus stay airborne longer than other sizes of particles. This size range of particles can also penetrate further into the lungs than larger particles, as they are less likely to be removed my interception or gravitational settling (Ott et al. 2007). My contribution to this study was to investigate the distribution of ATD within the size range 0.4-0.8 μm, and use this data to calculate resuspension rates within this size range and compare them to resuspension rates for larger size ranges. The size distribution was obtained using two methods: running a particle pulsing experiment with OPCs measuring the count distribution in the chamber, and using a previously published paper that had measured the size distribution using OPCs. After the size distribution was estimated, continuous floor loading was calculated using Eqn.1 and resuspension rates were calculated using Eqn.2andthe concentration time series measured during previously preformed experiments.

Where L(t) represents the floor concentration at time t, L(0) is the initial floor loading, V is the indoor air volume, A is the floor area, Ci(t) is the indoor concentration at time t, Ci(0) is the initial indoor concentration, and a is the air exchange rate.

Where r is the resuspension rate, Ar is the seeded floor area, k is the deposition rate, and all other variables are defined above. Equations 1 and 2 are derived from a two-compartment materials balance model which assumes instantaneously well-mixed compartments.

Preliminary results show that resuspension rates in the size range of 0.4-0.8 μm are lower than larger size ranges, following the overall trend of resuspension rates increasing with increasing diameter. However, contradictory to previous work (e.g., Thatcher and Layton 1995), this study demonstrates that particles in the 0.4-0.8 μm size range are resuspendable by normal human activity, and walking across residential flooring results in substantial submicron airborne particle concentrations.

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[1] Class of 2008, Civil and Environmental Engineering, University of Wisconsin-Madison, REU

2 Assistant Professor, Department of Civil and Environmental Engineering, ClarksonUniversity