Influence of geocoding quality on environmental exposure assessment of children living near high traffic roads

BACKGROUND: The widespread availability of powerful geocoding tools in commercial GIS software and the interest in spatial analysis at the individual level have made address geocoding a widely employed technique in epidemiological studies. This study determined the effect of the positional error in street geocoding on the analysis of traffic-related air pollution on children. METHODS: For a case-study of a large sample of school children in Orange County, Florida (n = 104,865) the positional error of street geocoding was determined through comparison with a parcel database. The effect of this error was evaluated by analyzing the proximity of street and parcel geocoded locations to road segments with high traffic volume and determining the accuracy of the classification using the results of street geocoding. Of the original sample of 163,886 addresses 36% were not used in the final analysis because they could not be reliably geocoded using either street or parcel geocoding. The estimates of positional error can therefore be considered conservative underestimates. RESULTS: Street geocoding was found to have a median error of 41 meters, a 90(th )percentile of 100 meters, a 95(th )percentile of 137 meters and a 99(th )percentile of 273 meters. These positional errors were found to be non-random in nature and introduced substantial bias and error in the estimates of potential exposure to traffic-related air pollution. Street geocoding was found to consistently over-estimate the number of potentially exposed children at small distances up to 250 meters. False positives and negatives were also found to be very common at these small distances. CONCLUSION: Results of the case-study presented here strongly suggest that typical street geocoding is insufficient for fine-scale analysis and more accurate alternatives need to be considered

Improving environmental exposure analysis using cumulative distribution functions and individual geocoding

BACKGROUND: Assessments of environmental exposure and health risks that utilize Geographic Information Systems (GIS) often make simplifying assumptions when using: (a) one or more discrete buffer distances to define the spatial extent of impacted regions, and (b) aggregated demographic data at the level of census enumeration units to derive the characteristics of the potentially exposed population. A case-study of school children in Orange County, Florida, is used to demonstrate how these limitations can be overcome by the application of cumulative distribution functions (CDFs) and individual geocoded locations. Exposure potential for 159,923 school children was determined at the childrens' home residences and at school locations by determining the distance to the nearest gasoline station, stationary air pollution source, and industrial facility listed in the Toxic Release Inventory (TRI). Errors and biases introduced by the use of discrete buffer distances and data aggregation were examined. RESULTS: The use of discrete buffers distances in proximity-based exposure analysis introduced substantial bias in terms of determining the potentially exposed population, and the results are strongly dependent on the choice of buffer distance(s). Comparisons of exposure potential between home and school locations indicated that different buffer distances yield different results and contradictory conclusions. The use of a CDF provided a much more meaningful representation and is not based on the a-priori assumption that any particular distance is more relevant than another. The use of individual geocoded locations also provided a more accurate characterization of the exposed population and allowed for more reliable comparisons among sub-groups. In the comparison of children's home residences and school locations, the use of data aggregated at the census block group and tract level introduced variability as well as bias, leading to incorrect conclusions as to whether exposure potential was higher at school or at home. CONCLUSION: The use of CDFs in distance-based environmental exposure assessment provides more robust results than the use of discrete buffer distances. Unless specific circumstances warrant the use of discrete buffer distances, their applcation should be discouraged in favor of CDFs. The use of aggregated data at the census tract or block group level introduces substantial bias in environmental exposure assessment, which can be reduced through individual geocoding. The use of aggregation should be minimized when individual-level data are available. Existing GIS analysis techniques are well suited to determine CDFs as well as reliably geocode large datasets, and computational issues do not present a barrier for their more widespread use in environmental exposure and risk assessment

The tick-borne rickettsia Cowdria ruminantium has a Chlamydia-like developmental cycle

The development of the tick-borne rickettsial pathogen Cowdria ruminantium (S stock) was studied in bovine umbilical endothelial (BUE) cell cultures and in goat choroid plexus, by light- and electron microscopy. Cowdria divided by binary fission within intracytoplasmic vacuoles resulting in large colonies of reticulate bodies. After three to four days in culture, reticulate bodies developed into smaller intermediate bodies characterized by an electron-dense core. Shortly before disruption of the host cells, intermediate bodies condensed further into electron-dense elementary bodies, which were released into the culture medium. Elementary bodies invade other endothelial cells thus initiating a new infectious cycle which lasts between 5 and 6 days. In the infected goat choroid plexus similar reticulate and intermediate bodies were identified within vacuoles of capillary endothelial cells. However, extracellular elementary bodies were not detected. Another stock of Cowdria (W) showed an identical developmental cycle as that of the S stock. The W isolate was also pathogenic for mice, making it possible to test the infectivity of reticulate and elementary bodies in these animals. Reticulate bodies appeared to be less infective than elementary bodies. The developmental cycle of Cowdria resembles the cycle known to occur in Chlamydia. Moreover, Cowdria has other similarities with Chlamydia. It has a Gram-negative envelope, it does not store iodine-stainable carbohydrates and may lack peptidoglycan as does Chlamydia. It is concluded, that Cowdria and Chlamydia are to a certain extent related, confirming a recent report that both organisms have certain antigenic determinants in common. Since Cowdria is also related to Ehrlichia it may well be that Cowdria takes an intermediate position between Chlamydia and Ehrlichia. The phylogenetic relationship between Cowdria and Chlamydia and also with Ehrlichia should be further elucidated by molecular analysis using 16S ribosomal DNA sequences.The articles have been scanned in colour with a HP Scanjet 5590; 600dpi. Adobe Acrobat XI Pro was used to OCR the text and also for the merging and conversion to the final presentation PDF-format.European Community (Directorate General XII).mn201

Dispersion of finite-size particles probing inhomogeneous and anisotropic turbulence

A series of 8 laboratory experiments was used to investigate the dynamics of a few almost neutrally-buoyant finite-size particles in the entire volume of a rectangular tank open to air and filled with water. Stirring was achieved by a cylinder executing a two-dimensional periodic Lissajoux figure. The rate and direction of stirring by the cylinder was varied. The particle motions were analyzed using a tracking method developed for the experimental design. The Reynolds number associated with the large-scale stirring motion was in a turbulent range of [5,693-11,649] across all experiments. The absence of stirring in the direction of the cylinder axis, the constant interference of the cylinder with the eddies and the presence of walls and the free-surface resulted in a flow that was both inhomogeneous and anisotropic as recorded by the particle motion. Despite these unusual conditions, the single-particle dispersion across all experiments could be seen to follow a ballistic regime until about two-fifths of the particle Lagrangian velocity auto-correlation time T_L. It was followed by a brief diffusive regime between T_L and 2.5 T_L, after which the presence of the boundaries prevented further dispersion. Such evolution is consistent with classic predictions for fluid tracer dispersion in homogeneous and isotropic turbulence. Particle-pair dispersion was more complex. Both the fixed time-averaged and length-scale-dependent particle-pair dispersion rates averaged across pairs showed the ballistic dispersion regime, whereas the subsequent diffusive regime was better borne out by the length-scale-dependent particle-pair dispersion. A super-diffusive Richardson regime was not unmistakably detected. Substantial variability was however found across the different pairs of particles, which was linked to differences in the decorrelation time of the velocity difference as a result of the inhomogeneity of the turbulence. For short initial separations, some particle pairs had a better separation of the time scales delimiting the ballistic and diffusive regimes and showed hints of a brief Richardson regime