![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20081107165339im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Penetration of Ambient Fine Particles into Indoor Environments
Because recent studies have indicated significant health risks associated with exposure to fine particles, there is renewed interest in fine particle concentrations indoors and their relationship to particles in the outdoors. Understanding this relationship is especially important because most of the exposure in the studies mentioned above occurred indoors while the measured particle concentrations were obtained from ambient monitoring stations. The inference has been made that the concentration of fine particles indoors that originated outdoors is comparable with the outdoor concentration. The primary objective of the study described here is to determine whether this is a reasonable inference. Understanding mechanisms of entry into buildings will be critical to understanding the controllability of indoor exposures to fine PM.
This study has two components: 1) laboratory measurements and 2) field/test-house measurements. The initial emphasis is placed on laboratory studies of entry mechanisms that provide the degree of control needed to validate mathematical models. The resulting mathematical model will be used to understand simultaneous measurements of fine particles indoors and outdoors at IEMB's indoor air quality (IAQ) test house. The laboratory facility consists of a 38-m3 chamber divided into two equal compartments. The entire facility is located indoors in a high bay space providing a conditioned environment. One compartment is used to simulate the outdoors, while the second compartment is used to simulate the indoors. Challenge aerosols are introduced into the first compartment and transported by convective driving forces into the second compartment through simulated leakage paths. Particle losses in the simulated leakage paths are determined from simultaneous measurements of concentration and size in the two compartments.
A leaktight circulation system is used to develop a specified pressure difference between the two compartments to produce the simulated driving forces for air flow. Aerosols are generated artificially in one compartment, and known convective type driving forces are applied between the two compartments. Particle transport from one compartment to the other occurs through constructed openings designed to simulate entry routes through building shells while retaining simple geometries (tubes and cracks) that are amenable to mathematical modeling. As particle-laden air flows through the simulated entry routes, particles are filtered out through their interactions with the surfaces of the entry routes. By measuring the ratios of the simultaneous particle concentrations in the two compartments as a function of time, the penetration factor associated with the filtration process can be measured. Mathematical models are under development that describe the deposition processes to predict the overall penetration as a function of particle size and air velocity.
Measurements in the test house are primarily an extension of the laboratory measurements to study real ambient aerosols penetrating into real indoor environments. However, attempts will be made to tag the outdoor air with tracer gases and particles in order to demonstrate our ability to associate indoor measurements with outdoor particles. While artificially generated aerosols are monosized for simplicity in interpreting the results, the test-house measurement will generally have to deal with the full particle size distributions of indoor and ambient aerosols. Because it is difficult to perform controlled experiments in the test house, it is important to obtain validation of modeled mechanisms in the laboratory chamber where monosized aerosols can be used. It is believed that many of the complications of indoor sources can be minimized by using an unoccupied and unfurnished (test) house for the study. Consequently, most of the usual internal sources (cooking, vacuuming, and resuspension) can be controlled.
Measurements in the test house have several unique objectives. First of all, we need to demonstrate that we can distinguish between particles generated indoors and particles that come in from the outdoors. This is by no means a trivial matter. One method will be to compare x-ray-fluorescence (XRF) analysis of samples collected on filters indoors and outdoors. While these analyses may not be definitive, they certainly can illustrate differences in particles collected indoors and outdoors. Even if particles cannot be uniquely identified as having indoor or outdoor origins, we hope to demonstrate that indoor sources are minimal in this unoccupied test house. One way to demonstrate this is to operate the house under positive pressure while filtering the intake air. Operation under this mode eliminates the entry of outdoor particles, allowing the indoor sources to be identified and perhaps removed. At least, the particles originating indoors can be identified. The intent is to reduce the background of indoor particles to a low level under the positive pressure mode of operation. The house will then be switched to operation under normal conditions (no intake ventilation air) and the buildup of particles from outdoors observed. Simultaneous measurements of indoor and outdoor aerosols during this buildup period will allow a reasonable estimate of the penetration factor for the house.
A second objective is to compare the size distribution and concentration level for the house operated with doors and windows closed and with windows open. The differences in particle size distributions and concentrations for these two operational modes can be attributed to the filtration action of the house shell, provided the outdoor aerosol remains relatively constant during the process. A third objective of the test house studies is to match model predictions of particle entry with observed values. After validating modeled mechanisms in the laboratory for entry routes with different geometries, the models will be compared to the measurements in the test house. Since most of the entry route geometries will be sensitive to the air flow velocities, the test house will be operated over a range of imposed negative pressures, giving rise to a variety of infiltration rates. Knowing the measured infiltration rate as a function of pressure, it will be determined which modeled geometry for entry routes (tubes or cracks) fits the data best.
Preliminary measurements for penetration of monosized oil droplets ranging in size from 0.1 to 5 micrometers through cylindrical tubes ranging in diameter from 1.3 to 3.4 mm have been performed in the chamber facility. Penetration of 0.1 to 5 micrometer oil particles have also been measured for 0.5 mm slits. Measurements of rates of deposition of particles on internal surfaces have also been performed. These measurements are part of the characterization of the measurement systems. The data have not yet been fully analyzed. Measurements are continuing to characterize the deposition mechanisms within narrow slits for a range of sizes and flow velocities. Measurements of indoor and outdoor particle concentrations and size distributions are underway at the test house.