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Biomedical subjects

T A Hall

Publications and source records attributed to T A Hall.

At least 19 recordsLinked to original sources
PubMed2002

Inhibition in the propagation of fast electrons in plastic foams by resistive electric fields.

The propagation of relativistic electrons in foam and solid density targets has been studied by means of K-alpha spectroscopy. Experimental results point out the role of self-generated electric fields in propagation and the role of heating of matter induced by the passage of fast electrons. A simple analytical formulation has been given and Spitzer conductivity has been shown to be fairly compatible with experimental results.

Journal Article↗
PubMed2002

Interaction of soft-x-ray thermal radiation with foam-layered targets.

We have studied the interaction of soft-x-ray thermal radiation with foam-layered metal targets. X-ray radiation was produced by focusing a high-energy laser inside a small size hohlraum. An increment in shock pressure, up to a factor of approximately 4 for 50 mg/cm(3) foam density, was observed with the foam layer as compared to bare metal targets. This follows from the propagation of radiation-driven shock wave in the foam and the impedance mismatch at the foam-payload interface.

Journal Article↗
PubMed2002

A field comparison of the IOM inhalable aerosol sampler and a modified 37-mm cassette.

This research focused on comparing a modified 37-mm (Mod37) sampling cassette with an IOM inhalable dust sampler. Paired IOM and Mod37 breathing-zone air samples were collected for workers engaged in corrosion control maintenance operations on several types of aircraft at several U.S. Air Force bases in the United States. Sampled operations included hand and power sanding, blow-down and wipe-down to remove dust, and spray finishing. The cassettes' interior surfaces were swabbed and the swabs combined with the filters for chromium analysis by NIOSH Method 7300. This approach utilized total chromium as a sensitive surrogate indicator of total aspirated mass. The influences of work location, work type, sample duration, and sampler type on measured concentration were evaluated using analysis of variance techniques. Only work type (process) was found to be a statistically significant predictor of measured concentration. The relationship between IOM- and Mod37-measured values for paired samples was evaluated by work type using linear regression techniques. Linear regressions showed that the modified 37-mm cassette over-samples aerosol by 35 percent compared to the IOM when a wide range of aerosol concentrations and compositions for divergent work tasks in multiple field locations are sampled. Interpretation of these results in light of previous results involving filter-only Mod37 analyses suggests that while the Mod37 has a higher aspiration efficiency than the IOM, substantial Mod37 wall losses result in underestimation of exposure when only the 37-mm filter is analyzed rather than filters plus wall swabs.

Aerosols↗
PubMed2001

A review and meta-analysis of formaldehyde exposure and pancreatic cancer.

BACKGROUND: Most reviews on the carcinogenicity of formaldehyde have focused on cancers of the respiratory tract. Two recent studies have suggested that exposure to formaldehyde may increase the risk for pancreatic cancer. METHODS: We examine 14 epidemiology studies of workers exposed to formaldehyde where pancreatic cancer rates were reported and use meta-analytic techniques to summarize the findings. We also rank formaldehyde exposures for the industries in these studies. RESULTS: We found a small increase of pancreatic cancer risk in the studies overall (meta Relative Risk [mRR] 1.1, 95%CI 1.0-1.3); however, this increased risk was limited to embalmers (mRR 1.3, 95%CI 1.0-1.6) and pathologists and anatomists (mRR 1.3, 95%CI 1.0-1.7). There was no increased risk among industrial workers (mRR 0.9, 95%CI 0.8-1.1) who on average had the highest formaldehyde exposures. CONCLUSIONS: A small increased risk of pancreatic cancer from formaldehyde exposure cannot be ruled out from the studies examined. However, the null findings among industrial workers and the lack of biological plausibility would argue against formaldehyde as a cause. The increased risk of pancreatic cancer among embalmers, pathologists, and anatomists may be due to a diagnostic bias or to occupational exposures other than formaldehyde in these professions.

Formaldehyde↗
PubMed2001

A review of adverse pregnancy outcomes and formaldehyde exposure in human and animal studies.

We examine the potential for reproductive and developmental effects from formaldehyde exposure. Formaldehyde is unlikely to reach the reproductive system in humans in concentrations sufficient to cause damage since it is rapidly metabolized and detoxified upon contact with the respiratory tract. While there are effects seen in in vitro studies or after injection, there is little evidence of reproductive or developmental toxicity in animal studies under exposure levels and routes relevant to humans. Most of the epidemiology studies examined spontaneous abortion and showed some evidence of increased risk (meta-relative risk=1.4, 95% CI 0.9-2.1). We found evidence of reporting biases and publication biases among the epidemiology studies and when these biases were taken into account, we found no evidence of increased risk of spontaneous abortion among workers exposed to formaldehyde (meta-relative risk=0.7, 95% CI 0.5-1.0). The small number of studies on birth defects, low birth weight, and infertility among formaldehyde workers; the limitations in the design of these studies; and the inconsistent findings across these studies make it difficult to draw conclusions from the epidemiology data alone. However, information from experimental studies and studies of metabolism indicate reproductive impacts are unlikely at formaldehyde exposures levels observed in the epidemiology studies.

Abnormalities, Drug-Induced↗
PubMed2001

Posterior disk displacement: morphologic assessment and measurement reliability-lumbar spine.

BACKGROUND: Magnetic resonance imaging is often used to assess for disk displacement after manipulation, but limited information about the true incidence of iatrogenic herniations exists. To design a study that evaluates for a causal relationship, preliminary data must be obtained relating to the size of different types of disk displacement. The reliability of chiropractic radiologists in assessing disks and a comparison of different measuring devices should also be evaluated. OBJECTIVE: To identify average measurements for normal and displaced disks and to assess the reliability of measurements by chiropractic radiologists. STUDY DESIGN: Intraobserver and interobserver reliability study assessing disk displacement on magnetic resonance scans. METHODS: Three evaluators assessed the disks on 122 magnetic resonance scans from two imaging centers. Six categories were graded, and digitizer and ruler measurements were compared. Forty-four scans were reassessed for intraobserver agreement. Intraobserver and interobserver variations were measured with intraclass correlation coefficient and kappa statistical analysis. Measurement device correlation was assessed with Pearson's r. RESULTS: Clear size differences between different types of disk displacement were noted. Interexaminer measurement reliability was 0.78 to 0.84. Agreement concerning the presence of disk displacement was 85% (kappa = 0.68), and the classification of disk displacements was 76% (kappa = 0.60). Intraexaminer measurement reliability was 0.40 to 0.49. Intraexaminer agreement concerning the presence of disk displacement was 76% (kappa = 0.52), and the classification of disk displacements was 62% to 69% (kappa = 0.38 to 0.46). Normal versus bulged disk distinctions demonstrated the most disagreement. The ruler and digitizer correlation coefficient was 0.968. CONCLUSIONS: Different disk types demonstrated distinct size averages. Interexaminer agreement was good concerning disk assessment and measurements. Intraexaminer agreement was lower than expected. A millimetric ruler is an acceptable alternative to digital measurement devices.

Adult↗
PubMed2001

Characterization of RNase P holoenzymes from Methanococcus jannaschii and Methanothermobacter thermoautotrophicus.

The partial purification and basic biochemical characterization of the RNase P holoenzymes of two species of methanogenic Archaea, Methanothermobacter thermoautotrophicus (previously Methanobacterium thermoautotrophicum strain deltaH) and Methanococcus jannaschii, are described. The properties of these enzymes, particularly buoyant density in Cs2SO4 and recent information about the subunit composition of the archaeal enzymes, suggest that RNase P enzymes in Archaea are much more alike than earlier studies in Sulfolobus acidocaldarius and Haloferax volcanii suggested.

Centrifugation, Density Gradient↗
PubMed2000

The continuum normalization method for quantification of X-ray spectra in biological microanalysis. 1. Generalized bremsstrahlung production cross-sections and analysis using standards.

The thin self-supporting biological specimens used for quantitative X-ray microanalysis are problematical because the sections are most unlikely to be uniform in thickness or density, so the intensities of the characteristic lines alone are not a good measure of composition. The method developed to overcome these problems was introduced by T. A. Hall in 1971 and uses the bremsstrahlung or continuum intensity recorded in the X-ray spectrum to normalize each characteristic line, and hence is frequently referred to as the continuum normalization (CN) procedure. Reformulating the CN method of quantification in terms of generalized cross-sections and calculating more accurate values of bremsstrahlung production using a formula allows us a better understanding of the options open to the analyst of biological thin sections by which the errors in the measurement may be reduced. If one chooses to use the original Hall (1971) method using Kramers cross-sections, the window measuring the continuum for normalization should be set in the 4-7 keV region for typical scanning electron microscope and microprobe beam energies, 20-40 kV, and above 10 keV for transmission electron microscope energies of 80 kV and above. Although it is clear that peak counts must not contribute to the white count, the window should be as wide as possible to reduce statistical errors.

Journal Article↗
PubMed2000

Theoretical investigation of the interrelationships between stationary and personal sampling in exposure estimation.

In exposure estimation, personal sampling is the method of choice as it is a nearby representative of the contaminant concentration in the breathing zone. Due to the versatility of the stationary sampling in obtaining much higher sensitivity, in its adaptability to telemetering observations, it may also be an attractive sampling method for many circumstances. However, the two sampling methods differ in many theoretically important ways that go beyond the obvious differences. The theoretical investigation of the stationary and personal sampling methods vis-à-vis sampling for exposure estimation shows that the area sampling can be used to represent personal sampling under restricted conditions. Under the restricted conditions, an area of concentration within specified bounds may be determined in relation to a reasonably well-defined source. The extension of the theory to multiple or ill-defined sources pose potential complications that may be intractable through a theoretical analysis. These limitations and restrictions are inherent to the underlying premises of the two methods; therefore they are not amenable to easy correction. Even though these restrictions may suggest only a limited role for area sampling in exposure assessment, the theory shown also suggests areas of further applied and theoretical research to extend the proper use of area sampling in exposure assessment.

Environmental Exposure↗
PubMed1999

RNase P RNAs from some Archaea are catalytically active.

The RNA subunits of RNase Ps of Archaea and eukaryotes have been thought to depend fundamentally on protein for activity, unlike those of Bacteria that are capable of efficient catalysis in the absence of protein. Although the eukaryotic RNase P RNAs are quite different than those of Bacteria in both sequence and structure, the archaeal RNAs generally contain the sequences and structures of the bacterial, phylogenetically conserved catalytic core. A spectrum of archaeal RNase P RNAs were therefore tested for activity in a wide range of conditions. Many remain inactive in ionically extreme conditions, but catalytic activity could be detected from those of the methanobacteria, thermococci, and halobacteria. Chimeric holoenzymes, reconstituted from the Methanobacterium RNase P RNA and the Bacillus subtilis RNase P protein subunits, were functional at low ionic strength. The properties of the archaeal RNase P RNAs (high ionic-strength requirement, low affinity for substrate, and catalytic reconstitution by bacterial RNase P protein) are similar to synthetic RNase P RNAs that contain all of the catalytic core of the bacterial RNA but lack phylogenetically variable, stabilizing elements.

Archaea↗
PubMed1999

Sample size-based indication of normality in lognormally distributed populations.

Occupational and environmental hygiene sampling strategies are usually dictated by factors that limit sample sizes to relatively small numbers. Often, parameters estimated from small sample sizes are then used to make further estimates of the occurrence of extreme events, which are governed by the underlying exposure distribution. We investigated the limitations superimposed by the number of samples in distinguishing an asymmetric (Lognormal) distribution through the rejection of a hypothesized symmetric (Normal) distribution. Sets of 5 to 250 synthetic samples from underlying Lognormal distributions with unit median were generated for 24 separate geometric standard deviations (GSDs), ranging from 1.25 to 7.00. Each simulated combination was repeated in blocks of 200 and each block was repeated tenfold. The synthetic samples were then tested for goodness of fit for Normality by using the Shapiro and Wilk's W Test. Results indicated that the number of samples required to distinguish between Normal and Lognormal distributions was inversely related to GSD. When GSD = 1.25, 169 samples were required for 90 percent distinction at alpha = 0.05. The criteria for success for GSD of 2.00 and 4.00 were 25 and 15 samples, respectively. These results led to the conclusion that the general inability to distinguish an underlying distribution may impose serious difficulties in the estimation of extreme events associated with occupational and environmental hygiene-related sampling.

Environmental Monitoring↗
PubMed1999

Chirped pulse reflectivity and frequency domain interferometry in laser driven shock experiments.

We show the simultaneous applicability of the frequency domain interferometry and the chirped pulse reflectometry techniques to measure shock parameters. The experiment has been realized with the laser at the Laboratoire pour l'Utilisation des Lasers Intenses (LULI) with a 550-ps pulse duration and an intensity on target approximately 5 x 10(13) W/cm(2) to produce a shock in a layered aluminum-fused silica target. A second low energy, partially compressed chirped probe beam was used to irradiate the target rear side and the reflected light has been analyzed with a spectrometer, achieving a temporal resolution of the order of 1 ps.

Journal Article↗
PubMed1999

Assessment of medical personnel exposure to nitrogen oxides during inhaled nitric oxide treatment of neonatal and pediatric patients.

OBJECTIVE: This study was an assessment of potential exposures of medical personnel to nitrogen oxides during simulated and actual inhaled nitric oxide treatment of newborn and pediatric patients. DESIGN: Breathing zone exposures to nitric oxide (NO) and nitrogen dioxide (NO(2)) were monitored using data-logging personal dosimeters during simulated and actual administration of NO gas to patients in an intensive care setting. Sample. A total of 28 bedside nurses and 18 respiratory therapists were monitored during 6 different patient treatments. ANALYSIS: The highest measured concentrations of NO and NO(2) in the personal breathing zones of the nurses and respiratory therapists were peak readings (<1 minute in duration) of 6.7 parts per million (ppm) NO and 3.1 ppm NO(2). Exposures averaged throughout 15 minutes and throughout the work shift were below the limit of detection (0.8-ppm NO and 0.5-ppm NO(2)). CONCLUSION: Detectable exposures to NO and NO(2) were brief, infrequent, and well below Occupational Safety and Health Administration permissible exposure limits or any other exposure guideline, eg, American Conference of Governmental Hygienists Threshold Limit Values.

Administration, Inhalation↗
PubMed1995

Air pollution and daily mortality in Philadelphia.

Many recent analyses have reported associations between air pollution and mortality in U.S. cities. In this paper, we present the results of regression analyses of daily mortality in Philadelphia during the period 1973-1988. Pollution variables included in the analyses were total suspended particulates (TSP), sulfur dioxide, and ozone. We controlled for the effects of weather on mortality by analyzing mortality separately for each season and explicityly including quintiles of temperature in the regression models. In regression models that consider weather and pollution variables simultaneously, daily mortality is associated with hot days in summer [relative risk (RR) for highest quintile of temperature = 1.07; 95% confidence interval (CI) = 1.04-1.10], and with cold days in spring (RR for lowest quintile of temperature = 1.07; 95% CI = 1.04-1.10), fall (RR for lowest quintile of temperature = 1.05; 95% CI = 1.02-1.08), and winter (RR for lowest quintile of temperature = 1.04; 95% CI = 1.01-1.07). When all three pollution covariates and weather are considered simultaneously in the regression model, ozone is associated with mortality in summer (RR = 1.15; 95% CI = 1.07-1.24) and sulfur dioxide is associated with mortality in spring (RR = 1.19; 95% CI = 1.06-1.33), fall (RR = 1.14; 95% CI = 1.00-1.29), and winter (RR = 1.21; 95% CI = 1.09-1.35), where the relative risks are for incremental changes of 100 parts per billion in ozone and sulfur dioxide on the previous day.(ABSTRACT TRUNCATED AT 250 WORDS)

Air Pollutants↗