Simultaneous sampling of three dust fractions (inhalable, thoracic, respirable) has been achieved using porous polyurethane foams, which serve both as selecting and sampling media. The particle penetration was measured in laboratory tests. Foam geometries were predicted using a semi-empirical model. Prototype samplers were constructed based on the IOM and GSP inhalable personal samplers. Weighing and chemical analysis procedures were checked for the foam

Aitken, R. J.

Görner, P.

Kenny, L. C.

Möhlmann, C.

Vuduc, T.

Zambelli, G.

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386 Ann. occup. Hyg., Vol. 46, Supplement 1, pp. 386–389, 2002© 2002 British Occupational Hygiene SocietyPublished by Oxford University PressDOI: 10.1093/annhyg/mef691Size-selective Personal Air Sampling: a New Approach Using Porous FoamsC. MÖHLMANN1*, R. J. AITKEN2, L. C. KENNY3, P. GÖRNER4, T. VUDUC5 and G. ZAMBELLI6*Author to whom correspondence should be addressed.e-mail: moehlman@hvbg.de1Berufsgenossenschaftliches Institut für Arbeitssicherheit, D-53754 Sankt Augustin, Germany;2Institute of Occupational Medicine, 8 Roxburgh Place, Edinburgh EH8 9SU, UK; 3Health and Safety Laboratory, Broad Lane, Sheffield S3 7HQ, UK; 4Institut National de Recherche et de Sécurité, Av. de Bourgogne, F-54501 Vandœuvre les Nancy, France; 5Institut Universitaire Romand de Santé au Travail, 19, rue du Bugnon, CH-1005 Lausanne, Switzerland; 6Lavoro e Ambiente s.r.l., Via Cartesio 30, I-47100 Forli, ItalySimultaneous sampling of three dust fractions (inhalable, thoracic, respirable) has beenachieved using porous polyurethane foams, which serve both as selecting and sampling media.The particle penetration was measured in laboratory tests. Foam geometries were predictedusing a semi-empirical model. Prototype samplers were constructed based on the IOM and GSPinhalable personal samplers. Weighing and chemical analysis procedures were checked for thefoams.Keywords: foam; size-selective samplingINTRODUCTIONOccupational diseases of the respiratory tract relatedto exposure to airborne substances are a major factorin the compensation by occupational insurances in allmember states of the European Union. In order to getreliable data of worker’s exposure, a measurementsystem ideally needs to be able to determine the threeparticle fractions in accordance with the standard EN481 (CEN, 1993).Within a European project (SMT4-CT96-2137) anew reference method for the simultaneous measure-ment of hazardous particulate matter in accordancewith EN 481 is being developed. This developmentincludes the design and testing of two three-stagepersonal air samplers based on the existing IOM andGSP samplers for the inhalable fraction incorporatingadditional porous plastic foams. Suitable analyticalprocedures are also being assessed.Polyurethane foam has been found to serve well asa sampling and selection medium for particles(Aitken et al., 1993; Vincent et al., 1993; Chung etal., 1997; Chen et al., 1998; Kenny et al., 1998, 1999;Page et al., 2000).MATERIALS AND METHODSThe physical and chemical behaviour of porousfoams was investigated using optical and scanningelectron microscopy, pressure drop measurements,and elemental and organic chemical analyses (atomicabsorption spectrometry, inductively coupled plasmamass spectrometry, extraction of organics and massspectrometry).A semi-empirical model was developed and usedto predict foam plug geometries, suitable as goodstarting points for particle penetration tests usingpolydisperse dusts (glass microspheres) in windtunnel (velocity: 0.15 m/s) and calm air chamber tests(Görner et al., 2001). The foams were mounted in aholder or prototype sampler, and the penetration wascompared with an empty reference holder or thesurrounding total airborne dust, respectively. Particledistributions were measured with aerosol particlesizers (TSI, St Paul, MN). The resulting 50% penetra-tion particle diameters (D50) were compared withthose required by EN 481.The analysis of dust-loaded foams was checked byweighing experiments and suitable chemical analysisSize-selective personal air sampling 387procedures. Additional field tests with three-stageprototype samplers were carried out.RESULTS AND DISCUSSIONCharacterization of foamsThe first steps were to assess the quality of poly-urethane foams with open cell surfaces between thefilaments and to determine their porosity. The struc-ture of porous polyurethane foam is shown in Fig. 1and is characterized by recurrent elements.Manufacturers describe the porosity of foams interms of pores per inch (ppi). The range generallydeliverable is ~10–100 ppi. A foam cell is ideallycharacterized by a regular dodecahedron (Fig. 2), thatis, 12 pentagons in a football-like configuration. Thecell diameter in this ideal shape is used to charac-terize the porosity. It allows the more precise deter-mination of the foam material rather than theempirical value p.p.i., and is the parameter on whichthe manufacturers own quality systems are based.The cell diameter can easily be measured by opticalmicroscopy with a SD of ~5%.A theoretical model to predict foam pluggeometries for desired aerodynamic functions sup-ports the developments. With the help of this semi-empirical model, originally proposed by Vincent etal. (1993) and reformulated by Kenny et al. (2001),we can determine the penetration P of particlespassing through such a foam material.The model uses impaction and sedimentationcomponents, represented by the Stokes number,St = Dae2 γU/(18ηdf), and the gravitational param-eter, Ng = Dae2 γg/(18ηU). t is the thickness of thefoam the particles have to pass through and df is thefilament diameter, namely the solid parts or the edgesof the cell. The parameters are given in SI units. Thefilament diameter has an empirical relationship to thecell diameter (Kenny et al., 2001).Using this model, we can determine the penetrationstarting with a selected foam geometry. Alterna-tively, if we fix the penetration as 0.5 for a certainparticle diameter (e.g. 4.3 or 11.7 µm) according tothe respirable and thoracic dust fraction (CEN, 1993),we can deduce the foam geometry. In this way, wehave a good starting point to construct prototypesamplers.Several tests for particle penetration through foamplugs with nominal porosities of between 45 and 170p.p.i. (equal to 1.5 and ~0.25 mm cell diameter) werecarried out (Kenny et al., 2001). The results indicatedthat selected foams with predicted geometry andporosity comply with the required penetration curvessufficiently well.The data in Fig. 3 represent the particle penetrationthrough two different foam modules in a prototypesampler based on the GSP inhalable sampler (similarto Fig. 4, upper row). Further results are given else-where (Görner et al., 2001). Typical cell diameters ofthe foams used are 480 and 1600 µm.Analysis of foamsBesides the development of a sampler, specialattention is paid to the development of suitableanalytical procedures to determine the mass andchemical composition of the sampled dust collectedin each stage. For gravimetric analysis, foams areweighed before and after sampling, as they serve bothas selecting and dust-retaining media. Tests showedthat the dependency on humidity is similar tomembrane filters, and the SD of weighing is <0.1 mg.Using better controlled conditions, such as in a gloveFig. 1. Polyurethane foam of 90 p.p.i.Fig. 2. Shape of an idealised foam cell, combining multiple pores: dodecahedron.P( )ln tdf---- 5.486St2.382 3.891Ng0.880+{ } 4×10=388 C. Möhlmann et al.box, even one order of magnitude less can bereached. A limit of quantification is typically 0.6 mg.As a comparison, the typical limit of quantification ofa membrane filter used in a GSP inhalable sampler bygravimetic analysis is 1 mg.The chemical analysis of elements in the sampleddust can be performed using atomic absorption spec-trometry or inductively coupled plasma with massspectrometry with an acid digestion (HCl 25%,HNO3 65%, 1:2) of the foam matrix, similar tostandard procedures for membrane filters (BIAFolder, 2001). The foam material is sufficientlydestroyed to reach the inner dust deposits. Becauseof high impurities in the polyurethane material, theanalysis of calcium and tin is not recommended. Inaddition to the analysis of elements in dust loadings,the ability of sampling organic compounds is alsoinvestigated.Fig. 3. Penetration of particles through foam plugs with selected geometry and porosity in a prototype sampler.Fig. 4. Prototypes for three-stage dust sampling on the basis of the IOM and GSP inhalable samplers.Size-selective personal air sampling 389Prototype samplersOn the basis of the semi-empirical model, variousplug geometries for the two size-selective foam plugswere chosen to fit into the modified IOM and GSPinhalable samplers. Penetration tests were conductedin the laboratory to show the applicability (Görner etal., 2001) and match with the sampling conventions.Prototype samplers have been constructed and furtherfield tests will be undertaken to provide informationon the handling.The three-stage samplers consist of two foammodules and one membrane filter in series (Fig. 4).By the appropriate choice of foam selection charac-teristics, a sampler of this type can collect the respir-able fraction on the filter, the thoracic fraction on thefilter plus the adjacent foam plug, and the inhalablefraction on the filter plus both plugs. The designdetails are to be discussed with potential manufac-turers.CONCLUSIONSPorous polyurethane foam was found to be a suit-able particle-selecting and sampling medium. Themost important characteristic of a foam is its celldiameter, which can be measured sufficiently well.Appropriate geometries of foam plugs can be foundwith the help of penetration tests, so that the thoracicand respirable dust fractions can be separated inpersonal air samplers based on the IOM and GSPinhalable samplers. Weighing procedures can beapplied to determine the mass loading of a foam.Standard elemental analyses are seen to be applicableafter adaptation to the special needs of the poly-urethane matrix.At the end of the project, recommendations forsubsequent use in different national air samplingsystems shall be given in order to reach a higherdegree of European harmonization in measuringoccupational exposures.Acknowledgements—The work was supported by the EuropeanCommission, Directorate General Research, under contractSMT4-CT96–2137. Further funding for part of the work wasprovided by the Nickel Producers Environmental ResearchAssociation (NiPERA). The authors wish to acknowledge theexperimental contribution of Geoffrey Beaumont, IOM,Richard Wrobel, INRS, and the support from Recticel Inter-national Development Center, Wetteren, Belgium.REFERENCESAitken RJ, Vincent JH, Mark D. (1993) Application of porousfoams as size selectors for biologically relevant samplers.Appl Occup Environ Hyg; 8: 363–9.BIA Folder. (2001) Measurement of hazardous substances.Berlin: Erich Schmidt Verlag. ISBN 3 503 02085 3.CEN. (1993) EN 481: Workplace atmospheres: size fractiondefinitions for measurement of airborne particles. Brussels:Comité Européen de Normalisation.Chen C-C, Lai C-Y, Sih T-S, Yeh W-Y. (1998) Developmentof respirable aerosol sampler using porous foams. Am IndHyg Assoc J; 59: 766–73.Chung KYK, Aitken RJ, Bradley DR. (1997) Development andtesting of a new sampler for welding fume. Ann Occup Hyg;41: 335–72.Görner P, Wrobel R, Fabriès J-F, Aitken RJ, Kenny LC,Moehlmann C. (2001) Measurement of sampling efficiencyof porous foam aerosol sampler prototypes. J Aerosol Sci; 32(suppl. 1): 1063–4.Kenny LC, Stancliffe JD, Crook B, Stagg S, Griffiths WD,Stewart IW, Futter SJ. (1998) The adaptation of existingpersonal inhalable aerosol samplers for bioaerosol sampling.AIHA J; 59: 831–41.Kenny LC, Bowry A, Crook B, Stancliffe JD. (1999) Fieldtesting of a personal size-selective bioaerosol sampler. AnnOccup Hyg; 43: 393–404.Kenny LC, Aitken RJ, Görner P. (2001) Investigation andapplication of a model for porous foam aerosol penetration.J Aerosol Sci; 32: 271–85.Page SJ, Volkwein JC, Baron PA, Deye GJ. (2000) Particulatepenetration of porous foam used as a low flow rate respirabledust size classifier. Appl Occup Environ Hyg; 15: 561–8.Vincent JH, Aitken RJ, Mark D. (1993) Porous plastic foamfiltration media: penetration characteristics and applicationsin particle size-selective sampling. J Aerosol Sci; 24:929–44.

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