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EVALUATION OF ULTRAFINE PARTICLES AS PART OF A HEALTH EFFECTS EXPOSURE STUDY
Citation:
CASUCCIO, G. S., T. L. LERSCH, R. B. DEVLIN, AND B. RAY. EVALUATION OF ULTRAFINE PARTICLES AS PART OF A HEALTH EFFECTS EXPOSURE STUDY. Presented at Microscopy & Microanalysis 2006 Annual Meeting, Chicago, IL, July 30 - August 03, 2006.
Description:
Ambient particulate matter (PM) is a complex mixture that includes bioactive and toxic compounds of natural and anthropogenic origin. Numerous epidemiological studies have reported associations between exposure to ambient levels of PM and various indices of cardiopulmonary morbidity and mortality. However, much about the relationship between PM and health effects is still not understood. One leading hypothesis ascribes the toxicity of PM to particle size, proposing that smaller particles are relatively more potent than larger ones in producing health effects.
Sampling and analysis methods have been developed for the characterization of fine (nominal diameters <2.5 ¿m) and coarse (nominal diameters between 2.5 and 10 ¿¿m) particles. Ultrafine (UF, nominal diameters <0. 2 ¿m) particles, however, are unique in that they present a special problem in the regulation of ambient PM. Whereas current PM regulations are based on mass, UF particles contribute relatively little to PM mass due to their very small size. However, they dominate by number in ambient air, and given the high surface area of UF particles relative to their size, they have the potential to have a significant effect on health. For example, UF particles have been associated with a worsening of pre-existing pulmonary diseases and have been shown to have a higher acute inflammatory potency than larger particles in animal inhalation studies.
To provide additional insight on this issue, the U.S. Environmental Protection Agency (EPA) has initiated an experimental assessment on the health effects of exposure to UF particles at their human exposure facility (HEF) located in Chapel Hill, NC. Healthy adults are exposed to the UF particles in a specially designed aerosol chamber. A photograph of the chamber is provided in Figure 1. Ambient air is introduced to the chamber from an inlet located on the roof of the HEF. The inlet removes coarse particles using a series of virtual impactors. The UF particles are then conditioned (temperature and humidity) and concentrated, and passed through a particle inlet port into the exposure chamber at a flow rate of approximately 200 l/min.
Real time size distribution measurements are made using two scanning mobility particle sizer (SMPS) instruments. One SMPS measures particle size distribution of the supply (inlet) air of the UF particle concentrator while a second SMPS, downstream of the concentrator's outlet and immediately upstream of the chamber's inlet, measures the particle size distribution of the concentrated UF particles delivered to the chamber. The concentrator is adjusted for optimal operation by comparing the outlet SMPS reading with the inlet SMPS reading. The SMPS is programmed to report counts as 2 minute averages of particle concentration/cm3.
Field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) are also being employed in the evaluation of the UF particles. UF particles are being collected on polycarbonate (PC) filters using active sampling techniques, and on Formvar® coated TEM grids using passive samplers. UF particle measurements obtained using FE-SEM and TEM are being compared to the particle size distributions obtained using the SMPS measurements. An example of an FE-SEM image from an UF particle sample is provided in Figure 2. In this figure, the image on the left is a duplicate of the right hand image illustrating the measurements of UF particles.
The results from this study will provide important data about the characteristics of UF particles to assist the EPA in determining whether the UF fraction should be considered in the promulgation of future PM standards.