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Annals of Occupational Hygiene Advance Access originally published online on October 31, 2008
Annals of Occupational Hygiene 2009 53(1):19-25; doi:10.1093/annhyg/men068
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© The Author 2008. Published by Oxford University Press on behalf of the British Occupational Hygiene Society

Application of PUF Foam Inserts for Respirable Dust Measurements in the Brick-Manufacturing Industry

Frank De Vocht1,*, Adrian Hirst1 and Anthony Gardner2

1 Occupational and Environmental Health Research Group, School of Translational Medicine, Faculty of Medical and Human Sciences, The University of Manchester, Room C4.4, Ellen Wilkinson Building, Oxford Road, Manchester M13 9PL, UK
2 Gardner Baker Limited, Dronfield, Derbyshire S18 1YB, UK

* Author to whom correspondence should be addressed. Tel: +44-161-275-8500; fax: +44-161-275-5595; e-mail: Frank.deVocht{at}manchester.ac.uk


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Institute of Occupational Medicine dual-fraction samplers equipped with porous polyurethane foam inserts have been introduced as a cheaper alternative to cyclone pre-selectors for measuring respirable dust. Initial data from a variety of industries suggested that the dual-fraction sampler yielded similar results as personal cyclones and that the respirable selection of the foam was not adversely affected by particle loading. We conducted a similar study, but specifically in the brick industry to assess the validity of this dual-fraction sampler as an alternative to personal cyclones in this industry. A total of 72 side-by-side samples using Higgins–Dewell cyclones and dual-fraction samplers were taken in seven UK factories manufacturing a variety of bricks. A priori measurements were assigned to any of the three groups based on the dominant source of the particulates in the exposure matrix (clay, sand or mixed) at the location in the factories where the measurements were taken. After log transformation, Higgins–Dewell cyclone-measured concentrations were on average 1.9 times higher than the concentrations measured by the dual-fraction samplers, with a Pearson correlation of 0.78 (95% confidence interval 0.66–0.85). Stratified analysis by main source of exposure suggested that the correlation was best for silica dust-based exposures rp = 0.88 (0.63–0.96), but decreased with the relative importance of clay particulates in the exposure matrix to rp = 0.82 (0.59–0.93) in the ‘mixed-source’ group and rp = 0.74 (0.55–0.85) in the ‘clay particulates’ group. Similarly, performance of the dual-fraction sampler relative to the cyclone sampler was negatively associated with increased relative importance of clay particulates in the exposure matrix and ranged from similar measured concentration β = 0.96 (0.54–1.39) for silica to 50% under sampling β = 0.50 (0.33–0.67) for clay particulates. These results suggested that the overall performance of the dual-fraction sampler in the brick industry depends on the relative importance of clay particulates in the exposure matrix. As such, results from occupational hygiene compliance surveys close to the occupational exposure limit can lead to erroneous decisions on compliance.

Keywords: brick industry • dual-fraction sampler • foam inserts • respirable dust


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Traditionally, measurements of the respirable dust fraction (CEN, 1993; ISO, 1995) are conducted using a cyclone pre-selector (HSE, 2000; Gorner et al., 2001b). However, as cheaper alternative porous polyurethane foam (PUF) inserts are increasingly being used in a variety of settings and applications (Chen et al., 1998; Page et al., 2000; Kenny et al., 2001).

Institute of Occupational Medicine (IOM, Edinburgh, UK) developed a dual-fraction sampler (Vincent et al., 1993), which consists of the standard IOM sampling head with a teflon, a membrane or glass fibre filter and an additional 12 x 16.5 mm PUF foam insert having a cell diameter of 420–460 µm (SKC Ltd, Dorset, UK). Using this insert, particles with D50 ≤4–4.5 µm (depending on filter loading, Kenny et al., 2001) are collected on the filter while particles with D50 ≥4–4.5 µm are collected in and on the foam itself (Brouwer et al., 2006).

Kenny et al. (2001) conducted a study to assess the validity and practicability of the IOM dual-fraction sampler by comparing the measured concentrations with those measured by personal cyclones in side-by-side paired samples taken in a variety of industries. They showed that the IOM dual-fraction sampler yielded similar results as the personal cyclones (r2 = 0.8), with their association best described by a line passing through zero and with slope of 0.93 [unity within confidence interval (CI)]. Their data suggested that for the filter weights of inhalable and respirable dust encountered in their study, the respirable selection of the foam was not adversely affected by particle loading. And as such, they concluded that the IOM dual-fraction sampler provided a valid alternative method to the respirable cyclones for measuring personal exposures to respirable dust. Similar results were found in laboratory and field-testing studies (Teikari et al., 2003; Linnainmaa et al., 2008), although Linnainmaa et al. reported an adverse effect of particle loading for respirable dust in their study using mineral, metal and peat dusts, which was not found for mineral dusts only (Teikari et al., 2003).

However, while the validation study of Kenny et al. (2001) included a number of different industries and sampling sites to give a wide range of respirable dust concentrations and in which the samplers were exposed to a variety of airborne substances including silica dust, metal dust, metal fumes and man-made mineral fibre, it only presented an overall evaluation without specific assessment of the sampler's validity in the different industries. The authors realized this and advised to assess the validity of the dual-fraction sampler in specific industries under study.

It has been shown that the clays and sands used in the manufacture of bricks can include bentonite, kaolin and illite (WHO, 2005). Generally, it is a complex mixture of inorganic compounds including crystalline silica, iron oxide, lime, magnesium carbonate, alkalis, calcium carbonate, calcium sulphate, sodium chloride as well some organic materials and elemental carbon compounds (Zuskin et al., 1998). Valid personal exposure data are important since, although the health effects specifically attributed to clay particulates are still under discussion, long-term exposure to clay particulate-including aerosol mixtures has been associated with structural and functional damage to the lungs, including pneumoconiosis and deterioration of respiratory function. Furthermore, the quartz often present in the mixtures has been causally related to lung fibrosis and lung cancer and has been associated to increased risk of developing chronic bronchitis and pulmonary emphysema (Muhle and Mangelsdorf, 2003; WHO, 2005).

Therefore, an exposure measurement study was conducted in the UK brick-manufacturing industry in which exposures to these complex mixtures of aerosols and free silica-containing dust are encountered and specifically assess the validity of these foam inserts to measure respirable dust in this environment.


   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 
Measurements were taken in seven factories located in the UK manufacturing a variety of bricks. In principle, the manufacturing process involves the processing of clays with water to form a mixture of the required consistency. This mixture may then be extruded or pressed to form individual bricks. The bricks are often coated in sand at the pressing or extrusion stage in order to add texture and colour to the finished product. Subsequently, the bricks are transported to drying ovens and then kilns to be fired. These processes are normally operated on a continuous basis with bricks stacked on cars which move slowly through the ovens and kilns over a period of days. The finished bricks are then removed and packed. Each stage of the process can give rise to the release of clay and/or sand dust.

In addition to routine compliance testing dust measurement surveys, 72 parallel samples were taken using a standard Higgins–Dewell cyclone, recommended for measurement of the respirable dust fraction in occupational settings in MDHS 14/3 (HSE, 2000), as well the alternative IOM dual-fraction sampler. A standard plastic Higgins–Dewell-type cyclone supplied by JS Holdings (Stevenage, UK), which has been shown to have a comparable mass concentration bias and accuracy to other Higgins–Dewell cyclones for particle size distributions commonly encountered in workplaces (Gorner et al., 2001b), was used equipped with a 37-mm cassette with a pre-weighed 37-mm diameter GLA-5000 Low Ash PVC Membrane 5.0 mm filter. As the alternative, IOM dual-fraction samplers with a plastic cassette equipped with 25 mm diameter GLA-5000 Low Ash PVC Membrane 5.0 mm filters and 225–772 (SKC Ltd) ‘Respirable Foam’ inserts (Kenny et al., 2001) were used. A number of studies have been conducted to assess the efficiency and performance of these porous foam size selectors, mainly under experimental conditions in calm-air chambers using monodisperse or polydisperse test aerosols, and to determine the penetration curves (Gibson and Vincent, 1981; Vincent et al., 1992, 1993; Breum et al., 1998; Chen et al., 1998; Page et al., 2000; Kenny et al., 2001; Gorner et al., 2001a; Huang et al., 2005; Bogdanovic et al., 2006). In summary, these studies suggested that porous foam filter media are able to collect respirable fractions following the CEN/ISO conventions (CEN, 1993; ISO, 1995) and show similar penetration curve statistics as those from cyclones (Gorner et al., 2001b). Furthermore, data suggest that particle loss increases slightly with increasing particle size due to inertial impaction of larger particles, and the collection efficiency in porous foams is reduced for coarse particles (Huang et al., 2005). Further data suggested that influence on performance by mass loading was negligible (Breum et al., 1998; Kenny et al., 2001; Huang et al., 2005), although more serious effects have been reported for welding fumes (Stancliffe and Chung, 1997).

These samples were taken in different areas in the factory to include a variety of size distributions and sources of respirable dust encountered in the factories. Stationary measurements were conducted to exclude within-worker variability, and parallel samples were positioned side by side assuming that they would draw air with the same concentration and size distribution. Samples were taken side by side at a height of ~1.5 m in locations where operators would spend most of their shift (e.g. close to control panels or inspection points in the factories). The samplers were positioned ~25 to 50 mm apart aiming at similar conditions at both inlets so that local air-movement effects would be minimized. Parallel measurements were done in a period of 16 months between December 2005 and March 2007, and the average sampling time was 395 min, but ranged from 255 to 478 min. Following the protocol described in MDHS 14/3 (HSE, 2000), flow rates of 2.0 and 2.2 l min–1 were used for the IOM sampler and cyclone, respectively, which was set (±0.1 l min–1) before sampling and checked before and after each measurement. After each measurement was finished, the filters were analysed gravimetrically. All samples were above the limit of detection and differences between start and end flow for all samples were <5%. Furthermore, two control filters and foams with similar temperature and relative humidity conditions as the samples were also used for each measurement. The mean of these field blanks was subtracted from the measurement filter and foam weights.

Correlations between respirable dust concentrations measured by the IOM dual-fraction sampler and the cyclone were assessed by calculating Pearson correlation coefficients (rp) for the normal, as well as for the log-transformed concentrations. Also, the association between the IOM foam sampler relative to the cyclone sampler, called a ‘performance factor’ in a study assessing different inhalable dust samplers in the European rubber-manufacturing industry (de Vocht et al., 2006), was estimated. The rationale for doing so was to assess the implications of using this cheaper alternative sampling devices for compliance testing in the brick industry, where a workplace exposure limit (WEL) of 0.1 mg m–3 8-h time-weighted average (TWA) (HSE, 2005) for free silica is used.

In addition, a priori all parallel measurements were assigned by an industrial hygienist (A.H.) to any of the three groups based on the dominant exposure (i.e. clay particulates, silica dust or mixed) at the location in the factories where the measurements were taken. More specifically, measurements taken during clay preparation, cutting fired clay, extruding and moulding were assigned to clay particulates as the dominant exposure in the aerosol mixture, while measurements taken during sand drying and staining, sanding of bricks, packing finished bricks and in the sand plant were assigned to silica dust. The dominant exposure source near brick-moulding operations, kilns, custom-made production, stacking prior to firing and in control panels was of a mixed nature, and these were assigned to an intermediate ‘without a dominant source’ group. This a priori grouping, however, was not confirmed by external determination of the size distribution at the different locations. Subsequently, correlations and associations between both sampling devices were assessed within these groups.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION AND CONCLUSIONS
 REFERENCES
 
The distribution of the parallel samples has been shown in Table 1. As shown, the respirable dust concentrations measured by the Higgins–Dewell cyclones ranged from 0.07 to 26.85 mg m–3 and are on average about a factor 2.5 higher than those measured by the IOM dual-fraction sampler (range 0.07–7.69 mg m–3). However, this can to some extent be ascribed to several outliers and the measurement data are better described by a log-normal distribution as shown by higher Shapiro–Wilk test statistics (Shapiro and Wilk, 1965) of 0.94 and 0.96, compared to 0.38 and 0.49 without log transformation. After log transformation, Higgins–Dewell cyclone-measured concentrations are on average 1.9 times as high as IOM dual-fraction-measured concentrations.


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  		Table 1. Characteristics of data set

 
The measured average respirable dust levels measured by the Higgins–Dewell cyclone [geometric mean (GM) range 0.73–0.75 mg m–3] were similar regardless whether the dominant exposure in the aerosol matrix was clay or sand particulates (Table 2), while larger differences were found when the IOM dual-fraction sampler was used (GM range 0.36–0.46 mg m–3).


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  		Table 2. Summary of respirable dust concentrations in milligrams per cubic metre measured by the Higgins–Dewell cyclone and IOM dual-fraction sampler, stratified by dominant source of particulates in the exposure matrix

 
The paired samples have been plotted graphically before (Fig. 1) and after logarithmic transformation (Fig. 2). The influence of these outliers on the correlation coefficient was marginal; whereas the Pearson correlation between the un-transformed respirable dust concentrations measured by the cyclone or IOM foam sampler had a Pearson correlation coefficient (rp) of 0.76 (95% CI 0.64–0.84) (P < 0.01), the correlation was similar after log transformation, with rp = 0.78 (0.66–0.85) (P < 0.01) (Table 3). Stratified analysis in the three groups with different relative contributions of clay particulates and silica dust suggested that the correlation was best when main exposure was sand/silica dust rp = 0.88 (0.63–0.96), but reduced when the relative importance of clay particulates increased from the ‘mixed-source’ group rp = 0.82 (0.59–0.93) to the ‘clay particulates’ group rp = 0.74 (0.55–0.85), although the 95% CIs largely overlap.


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  		Table 3. Pearson correlations (rp) between respirable dust concentrations measured by the IOM dual-fraction sampler and by the Higgins–Dewell cyclone

 


Figure 1
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  		Fig. 1. Parallel respirable dust measurements measured by Higgins–Dewell cyclones and dual-fraction IOM samplers. Dotted line = least-squares linear regression: Yi = concentration (mg m–3) measured with IOM dual-fraction sampler = 0.26 x concentration (mg m–3) measured with cyclone + error.

 


Figure 2
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  		Fig. 2. Log-transformed parallel respirable dust measurements measured by Higgins–Dewell cyclones and dual-fraction IOM samplers. Dotted line = least-squares linear regression: Yi = ln concentration (mg m–3) measured with IOM dual-fraction sampler = 0.86 x lnmeasured with cyclone + error.

 
The performance ratio of the IOM dual-fraction sampler relative to the Higgins–Dewell cyclone (Table 4) also showed that the IOM foam sampler undersamples the respirable dust fraction relative to the cyclone, with 60% after loge conversion, respectively, but that the different samplers only explained 51% of the variability of the measurements. Additional analysis by dominant particulates in the exposure mixture showed that performance of the dual-fraction sampler relative to the cyclone increased with increased relative importance of silica dust as main exposure in the matrix; from a relative performance ratio of 0.50 of the dual-fraction sampler (0.33–0.67) when measuring primarily clay particulates to similar performance [ratio = 0.96 (0.54–1.39)] when silica dust was the main exposure source. The explained variability also increased from 44 to 59% with silica dust compared to clay particulates as the dominant source.


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  		Table 4. Performance ratio of IOM dual-fraction sampler relative to Higgins–Dewell cyclone-measured concentrations estimated by a univariate least-squares regression model [lncyclone(mg m–3)= β x lnIOM(mg m–3)+ error]

 

   DISCUSSION AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
REFERENCES
 
This survey was aimed at assessing the IOM sampler with PUF foam inserts as an alternative measurement method to quantify exposure to respirable dust specifically in the brick-manufacturing industry. The rationale for performing this study was that in addition to respirable dust exposure with a silica origin, additional exposure in this industry occurs originating from the clays used to manufacture the bricks.

Other studies assessing levels of respirable dust exposure in the brick-manufacturing industry in the previous 20 years have been carried out in South Africa (Myers et al., 1989), Scotland (Love et al., 1999) and Croatia (Zuskin et al., 1998). Although the range of measured concentrations largely overlap with respirable dust measurements collected in the Croatian (Zuskin et al., 1998) (range 0.9–2.9 mg m–3) and Scottish (Love et al., 1999) (range 0.3–4.8 mg m–3) brick-manufacturing industry, the GM exposure level in this study [0.75 mg m–3 (cyclone) or 0.39 mg m–3 (dual-fraction sampler)] was approximately a factor 1.3–5.7 lower than in the other studies. Presumably, this can be ascribed to newer technology and/or better exposure control measures installed in the factories where this study was conducted, if only because this study was conducted about a decade after the Croatian and Scottish measurements were collected.

It was shown in the original validation study (Kenny et al., 2001) in several industries that the IOM dual-fraction sampler yielded similar results as the personal cyclones with an explained variance (r2) of 0.8 and an association (β) of 0.93, not different from unity. That is somewhat better than found in this study in the brick-manufacturing industry where the explained variance (r2 = 0.51) as well as the association [slope = 0.67 (95% CI 0.46–0.73)] were lower than in the original study. Moreover, depending on the dominant source of respirable dust in the aerosol matrix, this association decreased from 0.96 (0.54–1.39) for silica dust particles (r2 = 0.59), being comparable to that of the original validation study, to 0.61 (0.39–0.83) for mixed-sources exposure (r2 = 0.60) and to 0.50 (0.33–0.69) for clay particulates (r2 = 0.44). This stratification suggests that when using the IOM dual-fraction sampler as an alternative to the Higgins–Dewell cyclone to measure respirable dust, the source of respirable dust should be taken into account. More specifically, measuring respirable dust exposure originating from clay in the brick-manufacturing industry using the IOM dual-fraction sampler should be avoided since these data suggest significant undersampling when the IOM dual-fraction sampler is used in this environment.

Presumably, this can be explained by different size distributions and/or morphologic characteristics of clay and silica dust particulates. This is in agreement with a previous validation study which suggested slight increased particle loss with increasing particle size and reduced efficiency of porous foams for coarse particles (Huang et al., 2005). Furthermore, whereas silica dust particulates are relatively inert, clay particulates generally consist of stacked, weakly bonded negative charged layers, which allow water and other polar molecules to enter between layers and induce an expansion of the mineral structure (WHO, 2005). As such, clay particulates might stick to the foam inserts and/or stick together and form particles with increased aerodynamic diameters that will subsequently be captured by the foam insert. The influence of particle moisture absorption was also noted in another field study assessing the IOM dual-fraction sampler (Linnainmaa et al., 2008), although only problems with the inhalable fraction and not the respirable fraction were noted by the authors. However, in agreement with conclusions in others publications (Breum et al., 1998; Kenny et al., 2001; Huang et al., 2005), but not all (Stancliffe and Chung, 1997), no adverse effects by particle loading were found at the concentrations described in this study, with similar correlations at lower and higher concentrations regardless of the main source of exposure (data not shown). In contrast, the study by Linnainmaa et al. (2008) suggested not to use the foam inserts for samples exceeding 4 mg since samples at high dust loads would underestimate the respirable fraction because of increased efficiency in the filtering of the foam.

Another potential sources of error in this study are differences in porosities between different foam batches, which can affect D50 cut-off values (Kenny et al., 2001; Bogdanovic et al., 2006). These theoretically could have been investigated in this study by utilizing multiple cyclones or IOM dual-fraction samplers in the same sampling rig. However, these measurements were not conducted in this study because this source of error is expected to be small (Bogdanovic et al., 2006) and because this study focussed on the IOM dual-fraction sampler as an alternative to the Higgins–Dewell cyclone in terms of collected respirable dust mass specifically in the brick industry.

The main concern in this industry is exposure to free respirable silica from the clays and dusts, and consequently the WEL (8-h TWA) of 0.1 mg m–3 (HSE, 2005) is applied. The percentage of free silica in different clays or pottery bodies varies widely and can be between 1 and 100%, although is for most sources 30–40% (HSE, 2007). Since the focus of this study was on the validity of using the IOM dual-fraction sampler compared to the ‘traditional’ cyclone for respirable dust sampling in the brick-manufacturing industry, the measurements were not analysed for free silica content using infrared spectroscopy or X-ray diffraction analyses of the respirable dust containing filters (HSE, 2005).

However, these data show that specifically for the brick-manufacturing industry results from compliance testing (Rappaport, 1991) surveys can lead to significant measurement error when the IOM foam inserts are used, depending on the relative contribution of clay particulates to the exposure matrix. Nonetheless, additional data should be collected to investigate whether this is associated with differences in the size distribution, morphology or chemical composition of the different particles. Because of the practical implications for compliance testing specifically in the brick-manufacturing industry, these data should ideally be collected under field conditions, instead of under experimental conditions in calm-air chambers, using standard techniques such as impactors to assess the size distribution simultaneously to parallel sampling and subsequent laboratory assessment of morphology and chemical composition using a variety of techniques (Dasch and D'Arcy, 2008). Furthermore, from a practical viewpoint additional data on the maximum dust load specifically for these exposure matrices should be collected for future measurement surveys.

In conclusion, these data suggest that although the overall Pearson correlation coefficients for respirable dust measurements in the UK brick industry measured by personal cyclones and the IOM dual-fraction sampler are comparable to those found by Kenny et al. (2001), performance depended on the relative importance of clay particulates in the exposure matrix. As such, using this sampler as an alternative to the standard cyclone pre-selectors for occupational hygiene surveys in the brick industry can lead to significant measurement error which, especially at exposure levels close to the free respirable silica WEL, can lead to erroneous decisions on compliance.

Received March 19, 2008; in final form September 10, 2008


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION AND CONCLUSIONS
 REFERENCES
 

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