James H. Vincent

Evaluation of single-use masks and respirators for protection of health care workers against mycobacterial aerosols.

BACKGROUND The recent increase in multidrug-resistant tuberculosis has spawned a major controversy concerning the degree of respiratory protection needed by health care workers, particularly during sputum-inducing procedures. The objective of this study was to measure the filtration efficiencies of a single-use submicron surgical mask, two disposable dust/mist respirators, a dust/mist/fume respirator, and a high-efficiency particulate air respirator against aerosolized mycobacteria. Facial fit was not addressed. METHODS In a specially designed enclosed test apparatus, an aerosol was generated with a Collison nebulizer from a known concentration of Mycobacteria chelonae, used as a surrogate for Mycobacterium tuberculosis. Aerosol concentrations were measured with Anderson samplers upstream and downstream of the test masks and respirators, which were heat sealed to a metal plate. RESULTS Mean efficiencies ranged from approximately 97% for the surgical mask and a dust/mist respirator to more than 99.99% for the high-efficiency particulate air respirator. Measurements of filter efficiency with an Aerodynamic Particle Sizer for the M. chelonae aerosol and independent challenge tests with latex spheres correlated closely with measurements of M. Chelonae collection efficiency determined with Andersen samplers. CONCLUSIONS Analysis of variance and Tukeys method for multiple comparisons indicated that the dust/mist/fume respirator and the HEPA respirator collected M. chelonae with significantly greater efficiency than did either the surgical mask or the dust/mist respirator. Even the least efficient mask tested, however, had a filter efficiency of more than 97% against particles averaging less than 1 micron in aerodynamic diameter.

On the aspiration characteristics of large-diameter, thin-walled aerosol sampling probes at yaw orientations with respect to the wind

Abstract Experiments were carried out in a large wind tunnel to investigate the aspiration efficiencies of thin-walled aerosol sampling probes of large diameter (up to 50 mm) at orientations with respect to the wind direction ranging from 0 to 180 degrees. Sampling conditions ranged from sub- to super-isokinetic. The experiments employed test dusts of close-graded fused alumina and were conducted under conditions of controlled freestream turbulence. For orientations up to and including 90 degrees, the results were qualitatively and quantitatively consistent with a new physical model which takes account of the fact that the sampled air not only diverges or converges (depending on the relationship between wind speed and sampling velocity) but also turns to pass through the plane of the sampling orifice. The previously published results of Durham and Lundgren (1980) and Davies and Subari (1982) for smaller probes were also in good agreement with the new model. The model breaks down, however, for orientations greater than 90 degrees due to the increasing effect of particle impaction onto the blunt leading edge of the probe body. For the probe facing directly away from the wind (180 degree orientation), aspiration efficiency is dominated almost entirely by this effect.

Porous plastic foam filtration media: Penetration characteristics and applications in particle size-selective sampling

Abstract Porous plastic foam media have been the subject of a number of studies with a view to their practical application as particle collectors, most notably as pre-selectors in particle size-selective aerosol sampling devices. This paper describes a semi-empirical model of the penetration of a filter based on such media, and how it has been tested against available experimental data. The model is demonstrated to be a useful predictor of penetration characteristics for plugs of foam media suitable for use as pre-selectors in personal samplers for the thoracic and respirable aerosol fractions.

A Bayesian Approach to Retrospective Exposure Assessment

A variety of health effects are caused by chronic, cumulative exposure over time to pollutants. In these cases, to establish dose-response relationships for epidemiological and risk assessment purposes, it is vital to determine the exposures of individuals or cohorts as functions of time. Most existing occupational exposure databases, however, do not contain continuous records of historical exposures to airborne contaminants. These gaps in the historical record may be filled by using the knowledge base that experts and professionals in the field possess. In this article we present a new framework, based on Bayesian probabilistic reasoning, for obtaining estimates of exposure histories for airborne particulates from limited historical measurements, using subjective expert judgment. The framework has great potential applications in instances where there is sparse information or missing data on past exposures. Expert judgment, in the form of inputs to physical models, provides additional knowledge to retrospectively estimate exposure as a function of time from discrete and incomplete measurements. The expert judgments are informed by knowledge of historical plant conditions and work practices, and models describing process-dependent aerosol generation, ventilation, and worker activity patterns. The result will be probability distributions of the exposure of task-groups of workers as a function of time, in the form of a matrix.

Occupational exposure to inhalable and total aerosol in the primary nickel production industry.

OBJECTIVES--This paper describes a study that was carried out in the primary nickel production industry to investigate the levels of personal exposure to aerosols containing nickel and the impact on exposure assessment of introducing new personal sampling techniques with performance consistent with the latest particle size-selective criteria. METHODS--Experiments were carried out at workplaces in mining, milling, smelting, and refining works to investigate the effect of changing from the current method of total aerosol (with the widely used 37 mm filter holder) to the new method of measuring inhalable aerosol (with the Institute of Occupational Medicine (IOM) inhalable aerosol sampler). RESULTS--The results show that inhalable aerosol exposure concentrations--for both overall aerosol and for total nickel--were consistently and significantly higher than the corresponding total aerosol concentrations. Weighted least squares linear regression yielded IOM/37 mm factors ranging from about 1.2 to 4.0. The exposure data for each company process were found to be log-normally distributed. CONCLUSIONS--The results suggest the possibility of generating a single pragmatic factor for each company process for converting current total aerosol exposures to new exposures based on the inhalability concept contained in the latest particle size-selective criteria for aerosol exposure assessment. Such data may be important in determining new occupational exposure limits for nickel.

Worker exposures to inhalable and total aerosol during nickel alloy production

This paper describes a study that was carried out at a North American nickel alloy production facility to compare the levels of personal exposures to inhalable and total nickel-containing aerosols. It is part of a large body of work aimed at assessing the impact of introducing new personal sampling instrumentation with performance consistent with the latest criteria proposed by the International Standards Organization (ISO), the Comité Européen Normalisation (CEN) and the American Conference of Governmental Industrial Hygienists (ACGIH). Side-by-side sampling using the 37-mm filter holder (for total aerosol) and the so-called IOM inhalable aerosol sampler was conducted for the personal exposures of workers in a range of workplaces throughout the facility. The results showed that inhalable aerosol exposure levels-for both overall aerosol and for total nickel-were consistently and significantly higher than the corresponding total aerosol levels. Weighted least-squares linear regression yielded factors ranging from about 1.3 to 2.4 for overall dust and from about 1.5 to 3.5 for nickel. Inspection of the statistical distribution of the exposures for the whole plant suggested that it was log-normal.

Worker Exposure to Nickel-Containing Aerosol in Two Electroplating Shops: Comparison Between Inhalable and Total Aerosol

Abstract This article describes a study that was carried out at two electroplating shops to investigate the levels of personal exposure to inhalable nickel-containing aerosols and the impact of introducing new personal sampling instrumentation with performance consistent with the latest criteria proposed by the International Standards Organisation, the Comite Europeen Normalisation, and the American Conference of Governmental Industrial Hygienists (ACGIH). Experiments were carried out to investigate the effect of changing from the current widely used method for total aerosol (using the closed-face, 37-mm filter holder) to a new method for inhalable aerosol (using the IOM inhalable aerosol sampler). The results showed that measured inhalable aerosol exposures—for both overall aerosol and total nickel—were consistently and significantly higher than the corresponding measured total aerosol exposures. Weighted least-squares linear regression yielded biases for one company ranging from about 1.3 to 2.5 and for...

Impaction model for the aspiration efficiencies of aerosol samplers at large angles with respect to the wind

Abstract This paper starts off by recognising that the development of a full theoretical description of the aspiration efficiencies of samplers like those used in real-life situations is still some way off. As an interim contribution to solving the problem, it describes how the simple impaction model approach, which has previously been widely applied to thin-walled sampling probes and to blunt samplers under very idealized conditions, can be extended semi-empirically to describe the performances of contrasting types of sampler—thin-walled probes and the human head—oriented at 90° and 180° to the wind, respectively. Physically-based models for aspiration efficiency at these orientations are suggested and their coefficients found by non-linear regression against the limited amount of relevant data available in the literature. Agreement between such models and the experimental data is found to be quite good. Such models have the potential to provide useful insight into the performances of practical sampling devices. Furthermore they can provide valuable guidelines for the development of more rigorous approaches to blunt sampler theory.

Inversion techniques for personal cascade impactor data

Abstract We examined two inversion procedures for solving the Fredholm integral equation of the first kind to obtain aerosol particle size distributions from a set of measured masses collected on the various stages of a personal cascade impactor. The problem is essentially ill-conditioned, in that many solutions satisfy exactly an integral equation slightly perturbed from the original due to measurement error. The two methods, although derived from different families of inversion techniques, fit into the general framework of Tikhonov regularization. Both try to optimize the a posteriori degree of matching of the solution to the measured data and the a priori judgments about the likelihood of a solution in terms of its smoothness. The first method uses a weighted least squares optimization and zeroth-order regularization to fit a priori bi-modal log-normal distribution functions, using an intermediate step to define an appropriate starting point for the optimization routine. The second involved “blind” inversion of the impactor data to express the second derivative of the particle size distribution function as a linear combination of orthogonal basis functions, chosen so that the resulting solution is smooth and positive. The orthogonal functions are constructed from the eigenvectors and eigenvalues of a kernel covariance matrix. The personal inhalable dust spectrometer (PIDS), used to illustrate the application of these methods, is an eight-stage cascade impactor which selects the inhalable fraction of the aerosol by means of a specially designed inlet. Both inversion methods explicitly include consideration of the aerosol that is collected in the sampler entry between the inlet and the first impactor stage, something that applies to all cascade impactors but which has not usually been taken into account in the past. An important parameter in inversions, the expected value of measurement error for each stage, was estimated from a series of wind-tunnel experiments. Both methods work well for simulated PIDS data as well as for experimental wind-tunnel data for a wide range of sets of aerosol size distribution parameters.

Application of Porous Foams as Size Selectors for Biologically Relevant Samplers

Abstract In recent years it has become accepted that dust sampling instrumentation used to assess airborne exposure to airborne contaminants should sample according to biologically relevant criteria. This article describes progress toward the development of a personal sampling instrument which uses porous foams to select the thoracic and respirable subfractions of the inhalable fraction of total airborne particulate. The new instrument is based on the Institute of Occupational Medicine personal inhalable aerosol sampler and comprises an inhalable entry and two selection foams in series between the entry and the collection filter. Measurements of the foam penetration characteristics and the reproducibility of these characteristics have been carried out. In most cases different samples of the same grade of foam have been shown to be reproducible in their selection characteristics. Where this is not the case a simple quality control method, based on measurements of pressure drop, has been identified. Aitken,...