Measuring Aerosol Particle Behaviour due to Human Activity Indoors for Re-Exposure Evaluation
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The transport and fate of hazardous aerosols, including those of a radiological and biological nature, have been extensively studied both experimentally and theoretically. However, less attention has been given to the subsequent behaviour of the deposited aerosol particles. Deposited aerosol particles can be spread from one surface to another through the transport processes of resuspension and contact transfer; these processes typically involve human activity and can have an important impact on airborne contamination levels and hence, on a person's exposure potential. An experimental study was conducted to quantify the transport process of resuspension of hazardous aerosol particles from clothing surfaces for which limited data currently exist. Four different clothing samples were 'contaminated' with silica particles (3, 5 and 10 [micro]m) labelled with rare earth metals, which were then worn by volunteers who engaged in one of two pre-defined levels of physical activity, within a purpose-built room-sized chamber. Neutron Activation Analysis (NAA), Aerodynamic Particle Sizing (APS) and Scanning Electron Microscopy (SEM) were used to analyse the proportion of the original deposit which was resuspended, as this proportion has a potential for causing re-exposure by the inhalation route. The results show that physical activity can cause up to 67 % of contamination deposited on clothing to be resuspended back into the air. Larger particles are found to be more likely to resuspend; a difference in the general size distribution between deposited and resuspended particles, as well as a shift towards a larger Mass Median Diameter (MMD) within the individual size distributions of resuspended particles (3, 5 and 10 [micro]m) was observed. The NAA data showed an average Resuspended Fraction (RF) of 28 ± 8 %, for initially deposited particles, from the clothing of a person engaged in low physical activity; a corresponding value of 30 ± 7 % was found during high physical activity. The APS data indicated a tenfold increase in the cumulative mass of airborne particles during high physical activity in comparison to that during low physical activity. It was observed that the contaminated clothing's fibre type had no influence on the levels of particle resuspension from their surface, but that the material's weave pattern (and hence the material's surface texture) significantly influenced the levels of particle resuspension. In addition, following analysis of the airborne mass concentration variation with time during resuspension, the data were found to be separable into two regimes: the first regime showed a high, positive rate of change of airborne particle concentration relative to the second regime, and occurred within the first 1.5 minutes of the beginning of the resuspension event. The second regime revealed a slower rate of variation in particle concentration. As a complementary study to the resuspension investigations, experiments were conducted with the aim of quantifying the mass transfer efficiency of deposited particles when various soft and hard surfaces come in contact. The surfaces used were 100 % cotton, synthetic fleece, plastic laminate and brass. Contact transfer efficiencies ranging from 2 to 45 % were observed. Other observations included an increase in the particle mass transferred between surfaces with increased surface roughness. An increase in the applied pressure (from 130 to 9400 Pa) between the two surfaces in contact was seen to lead to contact transfer efficiencies that varied according to pressure, in two distinct pressure regions, with the transition pressure depending upon the surface types in question. The duration of contact between surfaces and the contaminant loading upon them had little effect on the mass transfer efficiencies that were calculated. The transport of aerosols via the above processes can increase a person's whole body dose following accidental or deliberate airborne releases of hazardous aerosols. The data generated in this work can be used to refine models for radiological exposure assessment. In addition, the results are applicable to biological aerosol transport and infectious disease transmission.
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