Evaluation of human exposure to indoor airborne pollutants : transport and fate of particulate and gaseous pollutants
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Building environmental conditions such as ventilation and contaminant concentrations are important factors that influence occupant health and comfort. The objective of the present work is to investigate how personal exposure to gaseous and particulate pollutants depends on indoor airflow, source characteristics, and occupant activity in commercial and residential environments. The study examines airflow and pollutant transport using experimental measurements in conjunction with computational fluid dynamics (CFD). The results demonstrate that breathing has a measurable influence on the airflow in an occupant breathing zone, but it has very small impacts on the occupant thermal plume. The results also show that breathing can significantly affect inhaled particle concentrations, even though the influence varies with source position and particle size. Also, localized hand motions of a sitting manikin do not significantly disrupt the upward thermal plume. In typical US residences, forced convection driven mixing airflow or buoyancy driven stratified airflow occurs depending on the HVAC fan operation (fan on or fan off, respectively). The measured transition period between mixing flow (fan on) and stratified flow (fan off) is approximately one minute, implying that most airflow in the residence is either dominated by mixing or stratification. A high level of exposure to short-term pollutant sources, such as resuspension of particles from floor surfaces due to human activity, more likely occurs with stratified flow than with highly mixed airflow. This is due to the strong influence of the occupant thermal plume that transports the pollutants into the breathing zone. Furthermore, by transporting air containing ozone across the reactive occupant surface, the occupant thermal plume has a large effect on exposure to ozone reaction products. Due to the reaction of ozone with the skin oils and clothing surfaces, the occupant surface boundary layer becomes depleted of ozone and conversely enriched with ozone reaction products. The parameter ventilation effectiveness quantifies the effectiveness of airflow distribution and can be used for assessment of exposure to gaseous pollutants. Based on the study results, the usefulness of ventilation effectiveness as an indicator of exposure to particulate pollutants depends on the particle size. For small particles (~1 [mu]m), an increase of ventilation effectives caused a decrease in occupant exposure, while for large particles (~7 [mu]m), source location and airflow around the pollutant source are significant factors for the exposure, and the ventilation effectiveness has very little to no effect.