David L. Bartley

The Ups and Downs of Journal Impact Factors

The journal impact factor (JIF) for The Annals of Occupational Hygiene rose 68% between 2005 and 2006. JIFs are widely publicized and may influence subscriptions and where authors submit papers, so they are much discussed in the publishing world. But although they tell us something about a journals citation performance, their shortcomings mean that they are poor general indicators of journal quality, and worse guides to the quality of authors and their institutions. The shortcomings include the following. (i) The two cited years may completely misrepresent the total current citation rate for the journal. (ii) The short citation period (1 year) results in many papers not contributing to the JIF, and usually two-thirds or more of the JIF depend on the most-cited 25% of papers. (iii) The JIF of the journal where a paper is published is therefore a very poor guide to the papers citation performance or the success of the author. Citation counts more specific to the author are much better. (iv) The JIF depends strongly on the subject of the journal, even within the published categories. (v) Statistical analysis shows that the relative standard deviation of year-to-year variation of a JIF for a journal with a JIF approximately 1.5 is likely to be between 10 and 20%, on top of any longer trend. Quotation of JIFs to three decimal places is therefore meaningless, and, for a journal like Annals of Occupational Hygiene, a single annual change of 70% could easily be due to a chance shift from a negative to positive fluctuation. (vi) The citations counted are not only of individual papers, so it is difficult to reproduce the JIF calculation. (vii) The selection of journals has been criticized, for example, the alleged emphasis on American- or English-language publications. This journals JIF does not noticeably influence the number of papers submitted to this journal, although it may influence some important authors. JIFs in our field seem to be increasing by approximately 5% a year, perhaps partly because of the various measures which editors can take to improve them, some of which represent genuine improvements to the journal, but some of which are unethical.

RESPIRABLE AEROSOL SAMPLER PERFORMANCE TESTING

Performance tests for evaluating respirable aerosol sampling methods were developed. The tests entail measurement of the flow-dependent collection efficiency of the aerosol size-discriminating part of the sampler using an aerodynamic particle sizer. The bias relative to an international sampler standard adopted by the American Conference of Governmental Industrial Hygienists, the International Standards Organization, and the Comite Europeen de Normalisation is mapped over aerosol size distributions of intended application. Imprecision from flow-effects, filter weighing errors, and intersampler variability is either measured or estimated. Uncertainty in the evaluation experiment itself is explained. Samplers can be rejected if, at specified confidence in the evaluation testing, accuracy is lacking in too large a fraction of samples. Alternatively, the data permit specifying a value of the inaccuracy. Two commercially available personal samplers were subjected to the performance tests suggested. The result ...

Light scattering from deformed droplets and droplets with inclusions. I. Experimental results

We provide experimental results from the scattering of light by deformed liquid droplets and droplets with inclusions. The characterization of droplet deformation could lead to improved measurement of droplet size as measured by commercial aerodynamic particle-sizing instruments. The characterization of droplets with inclusions can be of importance in some industrial, occupational, and military aerosol monitoring situations. The nozzle assembly from a TSI Aerodynamic Particle Sizer was used to provide the accelerating flow conditions in which experimental data were recorded. A helium-neon laser was employed to generate the light-scattering data, and an externally triggered, pulsed copper vapor laser provided illumination for a droplet imaging system arranged orthogonal to the He-Ne scattering axis. The observed droplet deformation correlates well over a limited acceleration range with theoretical predictions derived from an analytical solution of the Navier-Stokes equation.

Light scattering from deformed droplets and droplets with inclusions. II. Theoretical treatment

We provide theoretical results from the scattering of light by deformed liquid droplets and droplets with inclusions. With improved instrumentation and computer technologies available, researchers are able to employ two-dimensional angular optical scattering as a tool for analyzing such particle systems and which then could be applied in industrial, occupational, and military aerosol measurement. We present numerically calculated spatial light-scattering data from various droplet morphologies. We describe characteristic features of the theoretical data and compare these with the experimental results.

Use of Tracer Gas Technique for Industrial Exhaust Hood Efficiency Evaluation–Where to Sample?

A tracer gas technique using sulfur hexafluoride (SF6) was developed for the evaluation of industrial exhaust hood efficiency. In addition to other parameters, accuracy of this method depends on proper location of the sampling probe. The sampling probe should be located in the duct at a minimum distance from the investigated hood where the SF6 is dispersed uniformly across the duct cross section. To determine the minimum sampling distance, the SF6 dispersion in the duct in fully developed turbulent flow was studied at four duct configurations frequently found in industry: straight duct, straight duct-side branch, straight duct-one elbow, and straight duct-two elbows combinations. Based on the established SF6 dispersion factor, the minimum sampling distances were determined as follows: for straight duct, at least 50 duct diameters; for straight duct-side branch combination, at least 25 duct diameters; for straight duct-one elbow combination, 7 duct diameters; and for straight duct-two elbow combination, 4 duct diameters. Sampling at (or beyond) these distances minimizes the error caused by the non-homogeneous dispersion of SF6 in the duct and contributes to the accuracy of the tracer gas technique.

Diffusive Monitoring of Fluctuating Concentrations

The fluctuation-induced error in estimates of time-weighted-averages (TWA) of fluctuating concentrations is analyzed theoretically for a specific type of diffusive monitor in which the concentration is close to zero at the sorbent. Sampler calibration which results in unbiased TWA estimates is discussed. The error is found to have a lower, though no upper, bound. The variance of the error averaged over a variety of sampling start times (with fixed sampling period) is presented in terms of a sampler response function and the autocorrelation function of the concentration. A logarithmic dependence on pulse width is found for the variance of the error in the special case of concentration pulse sampling and indicates that significant error can be present in estimates of averages of rapidly varying concentrations. Similar conclusions are reached from numerical computation of the error and its variance using time-dependent field concentration data. The results indicate that excessive errors in exposure estimates...

Analysis and optimization of the performance of the 10 mm cyclone

Values were determined for the flow rate Q and calibration factor k which optimize the performance of the 10 mm cyclone in estimating respirable mass as defined by the British Medical Research Council (BMRC). To this end, flow dependent monodisperse cyclone collection efficiency data were analyzed as to reliability in predicting the collection of distributed dusts and were modelled mathematically. Furthermore, a simple criterion for evaluating cyclone performance was adopted. Performance maximization resulted in suggested values of 1.2 L/min and k=0.91 for BMRC dust concentration estimation. In order to achieve independence of the mathematical model, penetration measurements of the MSA sampling unit operated at 1.2 L/min (as well as 2.0 L/min) were carried out. Using results of these experiments, the bias between respirable and sampled dust concentrations was calculated in terms of geometric standard deviation and mass median diameter for a range of log-normally distributed dusts and is presented in graph...

Critique of 1985 ACGIH Report on Particle Size-Selective Sampling in the Workplace

The criteria recently proposed by ACGIH for judging the acceptability of respirable and other dust fraction samplers are analyzed. Implications on the sampling of workplace aerosol are determined. With the consideration of both bias and imprecision, the overall accuracy limited by the criteria is estimated for the sampling of coal mine dust as characterized by various researchers. The accuracy limits thus found appear to be excessively broad. As an example with actual workplace dust distributions in the sampling of a single aerosol (mass median diameter = 18.6 micrometers and geometric standard deviation = 2.3) with respirable dust concentration near 2 mg/m3, two samplers acceptable according to the proposed criteria could be found giving respirable dust measurements equal to 0.71 mg/m3 and 4.3 mg/m3 (even after excluding 5% of the low and high measurements from each sampler, respectively). Large variation in samplers acceptable according to the criteria is found for many other distributions as well; this indicates that tighter requirements are necessary. Seldom attained are both the single-sample +/- 25% accuracy at the 95% confidence level required of sampling/analytical methods endorsed by the National Institute for Occupational Safety and Health (NIOSH) and the tighter ISO dust fraction measurement requirement that 67% of the measurements of a sampled dust fall within 10% of a true value. Suggestions are given for sharpening the criteria without eliminating all samplers from acceptability.

DROPLET DISTORTION IN ACCELERATING FLOW

Several commercial instruments size particles based on their acceleration in a high-velocity flow field. Previous work suggested that droplet distortion in these instruments resulted in inaccurate sizing. Liquid aerosol droplet shape distortion produced in an accelerating flow field was therefore computed through analytical solution of the Navier–Stokes equation for comparison to experiment. A high-Reynolds-number empirical approximation to the pressure external to the droplet was used in these calculations. Within the droplet, the longest-lived excitations correspond to a quadrupolar distortion of shape. Droplet excitations were obtained in terms of aerosol diameter, viscosity, surface tension and density. At the largest viscosities considered (as in many oils), only a damped relaxation was found, whereas at lower viscosities and high surface tension (as in water) damped capillary oscillations were predicted as possible, given rapid shifts in the surrounding air flow. In order to compute the effect of airflow varying in time, an approximate Greens function was used. The Greens function in the frequency domain was approximated using only a pair of poles, thereby accounting for only the longest-lived excitations. In application of the theory to compute aerosol distortion on passage through an aerodynamic particle sizer (APS) acceleration nozzle, the change in air velocity was found to be so gradual that no oscillations were induced for droplets as small as 20-μm diameter. Measurements of droplet undersizing in the APS compared favorably with the theoretical predictions. The theoretical results were also consistent with photographs of distorted oleic acid and only slightly distorted water droplets emerging from a nozzle.

Pump Fluctuations and Their Effect on Cyclone Performance

Research has been carried out to characterize personal sampling pump pulsations and their effect on the performance of the 10 mm nylon cyclone. The fluctuation about mean flows equal to 1.2 L/min and 2.0 L/min of four each of three commercially available pumps has been determined. Measurements were made both near the sampling head as well as at the sampling pump without load, since resonance of the air column between pump and sampler was found to alter the fluctuations significantly. The cyclone itself was characterized by measuring the aerodynamic diameter dependent penetration under purely harmonic flow conditions in terms of frequency and amplitude. This information is combined with Fourier analysed pump data in determining the expected effect of given pump characteristics on cyclone sampling of dust of specific size distribution.