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vacuum cleaning of hard floor surfaces. Andrew Hunt Ж David L. Johnson Ж J. Brooks Ж. Daniel A. Griffith. Published online: 20 June 2008. У Springer ...
Environ Geochem Health (2008) 30:597–611 DOI 10.1007/s10653-008-9183-8

ORIGINAL PAPER

Risk remaining from fine particle contaminants after vacuum cleaning of hard floor surfaces Andrew Hunt Æ David L. Johnson Æ J. Brooks Æ Daniel A. Griffith

Published online: 20 June 2008  Springer Science+Business Media B.V. 2008

Abstract In the indoor environment, settled surface dust often functions as a reservoir of hazardous particulate contaminants. In many circumstances, a major contributing source to the dust pool is exterior soil. Young children are particularly susceptible to exposure to both outdoor derived soil and indoor derived dust present in the indoor dust pool. This is because early in life the exploratory activities of the infant are dominated by touching and mouthing behavior. Inadvertent exposure to dust through mouth contact and hand-to-mouth activity is an inevitable consequence of infant development. Clean-up of indoor dust is, in many circumstances, critically important in efforts to minimize pediatric exposure. In this study, we examine the efficiency of vacuum cleaner removal of footwear-deposited soil on vinyl floor tiles. The study

A. Hunt (&) Department of Earth and Environmental Sciences, University of Texas at Arlington, Room 233A, Geoscience Building, 500 Yates Street, Box 19049, Arlington, TX 76019-0049, USA e-mail: [email protected] D. L. Johnson  J. Brooks Department of Chemistry, State University of New York, College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210, USA

utilized a 5 9 10 foot (c. 152.5 9 305 cm) test surface composed of 1-foot-square (c. 30.5 9 30.5 cm) vinyl floor tiles. A composite test soil with moderately elevated levels of certain elements (e.g., Pb) was repeatedly introduced onto the floor surface by footwear track-on. The deposited soil was subsequently periodically removed from randomly selected tiles using a domestic vacuum cleaner. The mass and loading of soil elements on the tiles following vacuuming were determined both by wet wipe collection and by subsequent chemical analysis. It was found that vacuum cleaner removal eliminated much of the soil mass from the floor tiles. However, a small percentage of the mass was not removed and a portion of this residual mass could be picked up by moistened hand-lifts. Furthermore, although the post-vacuuming tile soil mass was sizably reduced, for some elements (notably Pb) the concentration in the residual soil was increased. We interpret this increased metal concentration to be a particle size effect with smaller particles (with a proportionately higher metal content) remaining in situ after vacuuming. Keywords Cleaning  Contamination  Floor-dust  Soil  Vacuuming

Introduction D. A. Griffith School of Economic, Political and Policy Sciences, University of Texas at Dallas, Box 830688, Richardson, TX 75083, USA

Settled dust is present in the indoor environment as a composite of particulate matter derived from interior

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and exterior sources (Butte and Heinzow 2002). The size and content of the indoor dust pool will depend not only on the supply of material from the contributing sources, but also on the material residence time. For the most part, the dust residence time is controlled by the frequency of occupant cleaning. A dust accumulation phase will exist between cleaning events that serves to amass material that will act as an exposure source for any contaminants in the dust. A growing reservoir of dust contaminants is particularly hazardous for infants and young children whose exploratory mouthing behavior makes them especially susceptible to inadvertent ingestion of dust material with which they come in contact. Infants and young children tend to mouth their hands and objects by habit and by sucking reflex (Groot et al. 1998). So, exposure can be by direct mouthing of objects, unintended ingestion through hand-tomouth activity, or the consumption of food contaminated by hands (Stanek et al. 1998). Contaminants in indoor dust (e.g., pesticides and toxic metals) are of considerable importance (Roberts and Dickey 1995) as they can persist for long periods of time (Lewis et al. 1994), and can exist in readily bio-available forms (Rasmussen 2004; Turner and Simmonds 2006; Yu et al. 2006). In many situations, the principal exterior source of indoor dust is outdoor soil (Rasmussen et al. 2001). Estimates for the amount of outdoor soil that is likely to be present in indoor dust vary widely. Upper estimates have a range of 80–85% (Hawley 1985; Roberts et al. 1991), and the United States Environmental Protection Agency (EPA 1994, 1998) uses an estimate of 70% as a default value in the application of the Integrated Exposure Uptake Biokinetic (IEUBK) model for predicting community pediatric blood lead (Pb) levels. Mid-range estimates of 30– 45% have been suggested by some studies (Fergusson and Kim 1991; Trowbridge and Burmaster 1997), with other studies proposing a low-range estimate of 20–30% (Davies et al. 1985; Culbard et al. 1988; Rutz et al. 1997). The soil contribution to dust is important because soil and dust ingestion is common among young children. An intake of 100 mg is likely to be the best estimate of the daily mean soil ingestion for young children; the recommended 95th percentile soil ingestion rate for a child is 400 mg/ day (EPA 2006). How this intake leads to elevated body burdens has been demonstrated for many

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contaminants. For example, exposure to Pb following the soil-to-indoor dust pathway has been well documented (see, e.g., Clark et al. 2004). And the disruption of this pathway can lead to reductions in floor dust Pb levels and children’s blood Pb levels (e.g., Rhoads et al. 1999; Lanphear et al. 2003; von Lindern et al. 2003; Lorenzana et al. 2003). One important constraint on the uptake of surface dust contaminants will be particle size. Investigators have shown that fine particles, especially those\100 lm in diameter, adhere more strongly to hands (Duggan et al. 1985; Duggan and Inskip 1985; Driver et al. 1989; Sheppard and Evenden 1994; Kissel et al. 1996a), and that as particle size increases, adherence to skin decreases; notwithstanding, according to the EPA (2000), the upper bound of the size fraction adhering to skin is 250 lm, based on a review of several studies dealing with dermal contact with soil. The so-called ‘‘fine’’ fraction of a dust/soil (defined as particles \250 lm) is also likely to be enriched in Pb compared with Pb in a bulk soil sample. EPA’s guidance for the sampling and analysis of Pb-contaminated soils recommends that the maximum sieve size for such soil is 250 lm (a No. 60 sieve) (EPA 2000). Clearly, intrinsic factors that will affect pediatric exposure to indoor floor dust include the quantity and composition of the dust on the surface and the size range of the constituent particulate matter. Exposure reduction through vacuum cleaner elimination of surface dust requires the removal of a heterogeneous mixture of fine particles potentially in a wide range of sizes. In this pilot study, we tested the effect of vacuum cleaner removal of a Pb-enriched soil from an impermeable floor surface after it had been deposited by repeated footwear tracking. The principal objective was to determine the soil element loadings (quantity per unit area) and concentrations (quantity per unit mass of dust) on the floor before and after vacuum clean-up following footwear transfer. Evaluating this situation involved quantifying the surface loadings and concentrations after a brief initial tracking event and then after a longer tracking episode interspersed with repeat vacuum cleanings. The goal here was to assess the effect on soil element levels of vacuum cleaner removal after both an initial contamination event, and after repeated deposition and cleaning, the aim of the latter being to simulate conditions of multiple soil incursions interspersed with periodic removal and to test whether there is surface accumulation despite

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repeat cleaning efforts. In addition, a hand pick-up test was also utilized in order to assess exposure availability of any surface soil not removed by vacuuming. • Materials and methods A composite test soil was prepared from several samples of yard soil from Herculaneum, Missouri, U.S.A. These soils (from our laboratory archive) were chosen because of an elevated Pb content. The soils were air dried for several weeks, ground using a pestle and mortar, and screened through a 250-lm mesh. The soils were then composited and mixed (by continuous rolling) in a large glass jar. The grinding and sieving process likely produced a size distribution somewhat representative of surface soil. A test floor surface, consisting of 50 1-foot-square (c. 30.5 9 30.5 cm) vinyl tiles, was arranged in the form of a 5 9 10 foot (c. 152.5 9 305 cm) rectangle. Each tile was assigned a number and tiles were selected for post deposition sampling from a list of randomly generated numbers. A plastic tray containing the test soil was placed at each end of the rectangle to simulate an exterior source of dry soil. At the start of the test, prior to any tracking, both right and left shoes of the tester were pressed (under the weight of the tester) into one tray of soil to acquire a coating of dry soil on the sole and heel of each shoe. While other methods of distributing particulate matter onto flooring surfaces have been developed (Reynolds et al. 1997; Lewis et al. 1999), this method of introducing soil onto a floor surface was chosen to more closely approximate actual footwear track-on conditions. In the first phase of the experiment, the tester walked over the tile area in a random (as possible) manner for a timed period of 15 min, with a return to an alternate soil tray every 3 min. Thus, the test soil was collected on the soles of the shoes five times, and then walked across the tiles for 15 min. After this initial 15-min tracking episode, the deposited soil was removed/sampled from 20 randomly selected tiles. Extreme care was taken not to remove deposited dust from adjacent tiles. Surface sampling proceeded as follows. •

Surface material was removed from 5 randomly selected tiles by wet wiping using commercially





available wet wipes (vide infra) until the tile surface appeared clean (no surface discoloration on visual inspection). Particulate matter was hand-lifted from 5 other tiles by pressing a moistened (pre-cleaned) polypropylene gloved hand down 5 times onto a tile surface (in different places). Five tiles had the deposited surface soil removed by vacuum cleaner, and any remaining adhering soil after vacuuming was recovered by wet wiping. Five other tiles also were vacuum cleaned, and moistened glove hand-lift was used to sample any remaining adhering soil by pressing the glove down 10 times onto each tile.

In the second phase of the experiment (approximately 30 min after the conclusion of the first phase), the tester resumed walking over the test surface for additional multiple periods of 15 min (again with stops at the soil reservoirs every 3 min), during the course of which deposited soil was removed/sampled from an additional set of 20 randomly selected tiles (not part of the first phase of the experiment). Surface sampling proceeded now as follows. •







After 15 min, the deposited soil was removed from 10 tiles using the same upright vacuum cleaner. The other 10 tiles were not cleaned by the vacuum. This step (15-min walk followed by vacuum cleaner removal of surface soil) was repeated 4 more times. After each 15-min walking period, the surface deposit was removed from the same set of tiles (i.e., the same set of 10 tiles was repeatedly contaminated and cleaned). Surface material was removed from 5 of the selected tiles that were not vacuumed (but repeatedly walked on) by wet wiping until the tile surface appeared clean. Particulate matter was hand-lifted from the other 5 tiles that were not vacuumed (but repeatedly walked on) by pressing a moistened polypropylene gloved hand down 5 times onto a tile surface. Five tiles that had surface soil repeatedly removed by the vacuum cleaning (and had soil re-deposited on them between vacuum cleaner removal episodes) had any remaining adhering soil following the final vacuum removal recovered by wet wiping.

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The other 5 tiles that had surface soil repeatedly removed by the vacuum cleaning had any adhering soil remaining after the final vacuum removal sampled by pressing a separate moistened glove down ten times onto each tile.

Wet wiping was used to gravimetrically determine the mass of soil on an individual tile surface. Commercially available wet wipes (Ghost Wipes) were used for this purpose. The wipes, each consisting of 15 9 14 cm squares of cross-linked polyvinyl alcohol material, met all ASTM E1792 specifications for sampling materials for Pb in surface dust and OSHA Methods ID-125G. Wet wiping removal from a tile surface was accomplished by following a modified version of the ASTM E1728-02 wiping methods (Hunt et al. 2006). The same pattern of surface wiping was used across the entire surface of a tile, but repeated until the tile surface showed no discoloration on visual inspection. In some instances (depending on the test or the loading of deposited soil), multiple wipes were used on an individual tile. To determine the mass of soil recovered, each wipe was initially dried overnight at 60C, set aside to equilibrate in the laboratory for a minimum of 3 days, and then weighed. Prior to soil recovery, each wipe was misted with distilled water and after sampling was again dried and weighed. The laboratory dry and wet bulb temperatures were measured during pre- and post-sampling weighing, and a relative humidity correction factor was applied before determining the recovered soil mass by difference (Johnson et al. 2005) To assess the potential for hand pick-up of surface deposited soil, polypropylene gloves were used to hand-lift surface material. Initially, each glove was misted lightly with de-ionized water to imitate a saliva coating that would more realistically represent a child’s hand. Hazardous substances are, of course, more likely to adhere to wet or sticky fingers than dry ones (Gurunathan et al. 1998). The hand pick-up method involved pressing the palm and fingers of a moistened glove (on an adult hand) onto the tile surface. Before vacuuming, the hand was pressed down 5 times on separate areas of a single tile, while material sampled after vacuuming was collected by pressing the hand down 10 times on a single tile. Following the hand-lift, each test glove was thoroughly irrigated with de-ionized water using a wash

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bottle and nozzle until, by visual inspection, all material was removed. The wash-off from each was captured via funnel and filter chimney onto separate, pre-weighed, 25-mm-diameter 0.4-lm pore size polycarbonate membrane filters. Gloves were discarded after collecting material from a specified tile. No gloves were further examined microscopically to verify complete soil removal. There are several potential drawbacks with this method. First, unlike a constant pressure technique, such as the EL method developed by Edwards and Lioy, that involves a spring-loaded press sampler device (1999), here the weight of each hand press was not controlled. Second, an adult gloved hand can only approximate an uncovered hand in terms of sebum, lipid and sweat content of the surface. Surface oil and sweat can have an important impact on adherence (Edwards and Lioy 2001). Third, the gloved adult hand, in terms of size, is not an accurate representation of a child’s hand. For a toddler (age 3 years), the median surface area of both hands is 350 cm2 (EPA 1996a). However, the gloved hand method has the advantage of not involving direct skin contact with the soil contaminants. Also, in follow-up mass recovery measurements, no artifact skin cells are incorporated in the sample to bias its mass determination. The vacuum cleaner pick-up of deposited soil was accomplished using a commercially available upright vacuum cleaner. Removal was undertaken on a tileby-tile basis using an approximately 4-in-wide (c. 10 cm), brushless, small area tool hose attachment. One-third of the tile area was vacuumed at a time by pressing the attachment flat to the tile surface (initially at the tile corner), and then slowly dragging it across the tile parallel to the edge, from one side to the other. This cleaning pass was repeated over the same area. This double pass procedure was replicated twice more (sequentially on the remaining two-thirds of the tile area) for complete cleaning coverage of the surface. Following gravimetric analysis, the Ghost Wipes and polycarbonate filters were subject to wet ashing and subsequent element analysis by inductively coupled plasma optical emission spectroscopy (ICPOES). The sample digest involved placing a wipe with collected soil on it in an Erlenmeyer flask with 25 ml of concentrated nitric acid, which was heated and taken to dryness. Each sample then was resuspended in 4 ml of concentrated nitric acid and

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2 ml of concentrated hydrochloric acid under heat. Samples were transferred to Teflon vials (with remaining residue rinsed from the flask with 5 ml of concentrated nitric acid). Two milliliters of hydrofluoric acid was added to each sample and the vials then placed in a programmable microwave digester. The microwave was ramped up to 220C over 10 min and held at this temperature for 15 min. Samples were made up to 64 ml in volume with the addition of 1 ml of saturated boric acid and 50 ml of distilled water. Calibration and quality control standards (10% of the analytes) were made up from commercially available primary standards. Element data were recorded as tile loadings (quantity of element obtained from each tile) following the subtraction of the (average) blank wipe value from each test wipe value. An element limit of quantitation was fixed at five times the detection limit (set at three times the standard deviation obtained from the wipe blank values). At this level of detection, several of the measured elements (e.g., Cu, Ni, and Zn) were only reportable for some of the wipe and filter samples, and have not been included in the summary results. Individual soil particles retrieved from the floor tiles were characterized by automated scanning electron microscopy (SEM) and X-ray energy spectroscopy (EDX). Such an approach has been used previously to assess the microscopic particle content of floor dust (Hunt et al. 1992, 1993, 2006; Thornberg et al. 2006). This analysis provides data about the size and elemental composition (X-ray spectral data) of a statistically significant number of microscopic particles in specific samples. This analysis was carried out using an ETEC Autoscan SEM operating in tandem with an Advanced Research Instruments AutoSEM Image Analysis System and a Kevex 7500 X-ray Spectrometer (Johnson and Hunt 1995). During the analysis, 16 elemental regions of interest and 32 background regions for net X-ray relative intensity computations were assigned within the X-ray spectrum. The fraction of individual particle mass contributed by the detected elements was defined by the X-ray relative intensity times the estimated particle volume (assumed to be a prolate ellipsoid rotated about its long axis). This was weighted by the common molecular form of occurrence for each element in the soil sample (Johnson et al. 1981).

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Results Bulk sample analysis identified the major elements in the test soil in order of abundance as silicon (Si) [ aluminum (Al) [ iron (Fe) [ calcium (Ca) and in similar order the heavy metals as Pb [ manganese (Mn) [ cadmium (Cd) (Table 1). The dominance of Si in the inorganic fraction of the soil was confirmed by automated SEM/EDX analysis of the bulk soil. Wet wipe soil recovery from floor tiles was used to assess the effectiveness of vacuum cleaner removal of dry soil from tile surfaces after the soil had been deposited by repeated footwear tracking. From a sampling viewpoint, wiping is considered a more efficient method of dust recovery than vacuuming (e.g., Farfel et al. 1994). Soil element loadings and concentrations after a brief initial tracking and single cleaning event differed from those determined after a longer tracking episode interspersed with repeated vacuum cleanings. Surface loadings and concentrations determined on a tile-by-tile basis after the initial, limited duration (15 min) soil deposition and single cleaning event (Table 1), demonstrated a significant reduction in the amount of soil on the tiles following removal. For, example, the average tile surface loading reduction for the major elements Si and Al, was respectively 91% [from 13,901 to 1,278 lg ft-2 (c. 15.0–1.4 lg cm-2)] and 85% [from 2,043 to 178 lg ft-2 (c. 3.6–0.2 lg cm-2)], and for the major trace element Pb was 85% [from 248 to 37 lg/ft2 ft-2 (c. 0.45–0.67 lg cm-2)]. The quantity of these elements in the dust recovered from a tile surface also was different after vacuuming. On average, the concentration of Si in the dust on the tiles was 11% less after the initial vacuuming. In marked contrast, on average, the concentration of Al and Pb in the dust on the tiles increased by 43% and 48%, respectively. The loading reductions clearly demonstrate substantial removal of soil from the surface by vacuuming, but the post-removal concentration changes indicate that the quantities of certain elements in the remaining surface soil increased following cleaning. The pattern of surface soil element loadings and concentrations after repeated tracking and multiple cleanings were somewhat similar to those determined after a brief initial tracking and single cleaning event (Table 1). Surface loading data demonstrated a

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Table 1 Surface loadings and concentrations of select elements in test soil samples collected by wet wipe from the tile surfaces after single and multiple walk-on and vacuuming events Mean element loading (lg ft-2)a and concentration (lg g-1) from 5 tiles

Sample

Bulk soil After first walk

After first vacuuming

Ca

Fe

Al

Si

Cd

Mn

Pb

Concentration (lg g-1)

11,021

26,023

48,197

380,303

29.27

1,076

4,420

Range

2,175

7,279

9,529

53,804

4.891

318.0

890.7

Tile loading (lg ft-2)

981.2

1,083

2,043

13,901

1.484

52.01

248.36

Range

362.3

1,006

1,706

15,353

1.298

35.64

208.95

Tile concentration (lg g-1) 19,162

19,473

37,263

249,866

26.77

935.7

4,480

Range

11,759

2,514

6,166

95,278

2.212

125.7

333.5

Tile loading (lg ft-2)

241.3

178.4

299.8

1,278

0.290

8.780

37.40

Range

153.8

41.50

128.1

868.5

0.113

3.223

16.22

Tile concentration (lg g-1) 42,583

32,341

53,190

222,945

51.47

1,564

6,630

Range

25,747

20,155

27,769

127,865

14.81

737.4

2,721

1,788 495.7

3,392 1,112

5,565 2,283

44,055 14,762

4.278 1.395

158.7 48.92

659.9 280.5

Tile concentration (lg g-1) 11,978

22,668

37,240

294,369

28.62

1,062

4,393

Range

1,481

2,136

8,946

19,955

2.845

99.31

290.6

249.5

563.9

945.2

3,125

0.746

34.07

130.0

85.76

89.76

104.7

469.7

0.129

6.845

26.43

Tile concentration (lg g ) 18,789

42,805

71,689

236,609

56.43

2,583

9,850

Range

10,502

15,187

38,845

11.99

621.4

2,395

-2

After multiple walks

Tile loading (lg ft ) Range

After multiple vacuuming Tile loading (lg ft-2) Range -1

a

-2

To convert to lg cm

5,496

multiply by 0.001076

significant reduction in the amount of soil on the tiles following vacuuming. The tile surface loading reduction for the major elements Si and Al was, respectively, 93% and 83%, and for the major trace element Pb was 80%. The quantity of these elements in the dust recovered from the tile surface had also changed after repeated vacuuming. On average, the concentration of Si in the dust on the tiles was 20% less after multiple vacuuming cleanings. In contrast, on average, the concentration of Al and Pb in the dust on the tiles increased by 92% and 124%, respectively, after repeated deposition and vacuuming events. Again, the loading reductions clearly demonstrate substantial removal of soil from the surface by vacuuming. Interestingly, Reynolds et al. (1997) found that vacuum removal of dust from smooth surfaces is more efficient at higher dust levels. However, while the removal of Si remained high, less Pb was removed after repeated tracking and multiple cleanings than after a single episode. The post-vacuuming concentration changes show that the quantities of certain elements in the remaining

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surface soil dramatically increased following cleaning, while that of Si was substantially reduced. Automated SEM/EDX analysis of particulate matter collected from the tile surface after the initial tracking effort (with no vacuuming), and after repeated tracking and multiple cleanings, provide additional evidence for Si depletion after vacuuming. Automated particle search in the SEM was conducted using a low backscatter electron image threshold in order to permit all inorganic particles to be recognized in the search. An analysis of the sampled deposition soil particles (n = 1,822) identified a number of different particle types, 94% of which had an area equivalent diameter \10 lm (diameter based on the projected 2-D area of a particle). However, this fraction accounted for only 24% of the total volume (an approximation of mass) of the particles analyzed (individual particle volumes based on a spherical equivalent estimate). In contrast, the analysis of a sample of post-vacuuming particles (n = 2,208) revealed the presence of the same particle types, but 99% of the particles in this sample

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40

Frequency (%)

35 30 25 20 15 10 5 0 10

Area Equivalent Diameter (µm)

Fig. 1 Frequency histograms of particle diameter for Si-only particles identified by automated SEM/EDX analysis in samples of the initial deposition soil (j) and in the postvacuuming soil (h)

were \10 lm, accounting for 73% of the total volume of the particles analyzed. This suggests that the vacuuming operation effectively removed coarser particles. The size distributions by area-equivalent diameter for the particle type we defined as Si-only (particles with a Si relative X-ray intensity [90%), identified in the deposition soil [n = 431 (23.7%) of 1,821] and in the post-vacuuming soil [n = 643 (29.1%) of 2,208], are portrayed in Fig. 1. The Sionly particles represent the quartz fraction of the soil. The difference in size distributions indicates that the large quartz particles had been selectively removed by vacuum cleaning. This is evident from the almost complete absence of particles [10 lm in diameter in the post-vacuuming soil, compared to the deposition soil, where this fraction accounted for approximately 10% of the Si-only particles analyzed. The element loadings and concentrations associated with the soil collected by hand lift after the brief initial soil tracking and single cleaning, and after the repeated soil deposition and multiple vacuuming events, are reported in Table 2. The hand lift protocol determined that measurable amounts of soil could be recovered by moist hand contact from tile surfaces after vacuum cleaning. As anticipated, the amount of soil retrieved by hand lift from the vacuumed surfaces was markedly less than that collected from the un-vacuumed surfaces after the initial tracking. The amount of Si transferred to the hand (per five hand presses) was approximately 7% of that transferred from the un-vacuumed surface. Similarly, approximately 4% and 5% of the Al and Pb, respectively, that was transferred by hand lift from the un-cleaned surface could still be picked-up by

hand lift after vacuuming. The concentration of Al and Pb in the soil was approximately the same in the hand lift from the un-vacuumed tiles as from the vacuumed tiles. The outcome of the hand lift collections after repeated tracking and multiple cleanings proved similar to that after a brief initial tracking and single cleaning event. After repeated walking, approximately 3% of the amount of Si, Al, and Pb picked up by the pre-vacuuming hand presses was picked up by the post-vacuuming hand presses. However, the amount of Pb, Al, and Si hand lifted after repeated tracking and multiple cleanings, was greater (for Pb and Al, approximately 3 times greater) than hand lifted after a single cleaning event following a brief initial tracking. This finding strongly suggests that despite the repeated vacuum cleaning during the extended period of soil tracking, there was sufficient surface accumulation to increase the quantity of inorganic elements available for hand pick-up. The concentrations of the inorganic elements in the soil hand lifted after the repeated tracking, and then after the multiple cleanings, generally were similar. This result is dissimilar to the tile wipe concentrations, which were elevated for Pb and reduced for Si after vacuuming. While the Pb concentration in the soil on the vacuumed tiles was similar to that in the soil on the un-vacuumed tiles (99%), we posit that, in this experimental setup, the hand lifts of soil remaining after vacuuming (following extended tracking deposition) were selective for the coarse particles on the surface.

Discussion The bulk chemical analysis of the wet wipes demonstrated that residual quantities of Pb, and other soil elements, were present on the test tile surfaces after the vacuum cleanings. The principal reason for this was the inefficiency of the vacuum removal process. Suction entrainment is dependent on several factors, most notably the power of the vacuum cleaner fan. However, the size of the particles, their distribution across the surface, the degree of particle aggregation, and relative humidity also play a role. Detachment, which will be resisted by surface adhesion, is unlikely to apply equally to all surface deposited particles. Fine particles adhere to surfaces as a result of van der Waals (vdW) forces (i.e., the dispersion force, the

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Table 2 Quantities of select elements in test soil samples collected by hand pick-up from the tile surfaces after single and multiple walk-on deposition and vacuuming events Mean element mass picked-up (lg) and concentration (lg g-1)

Pick-up per 5 hand presses

After first walka

After first vacuumingb

After multiple walksa

Ca

Fe

Al

Si

Cd

Mn

Pb

Hand mass pick-up (lg)

81.35

47.63

134.1

1,015

0.095

2.618

12.23

Range

68.03

59.93

125.2

765.1

0.115

3.126

13.21

Concentration (lg g-1)

22,167

11,500

34,488

274,389

24.11

655.3

3,072

range

12,726

10,187

11.972

137,033

32.40

413.6

1,694

Hand mass pick-up (lg)

7.702

c

5.470

71.03

0.013

0.140

0.550

Range

11.06

c

4.461

115.8

0.009

0.107

0.526

Concentration (lg g-1)

36,168

c

28,102

365,907

66.46

711.3

2,860

Range

41,084

c

54,471

235,591

87.56

1,066

4,565

Hand mass pick-up (lg)

150.85

289.1

675.9

4,625

0.426

13.14

57.11

Range

55.65

109.7

330.7

1,843

0.270

7.296

30.70

Concentration (lg g ) Range

10,558 1,156

20,035 6,510

47,156 7,126

324,190 20,351

29.49 8.668

912.5 245.4

3,979 942.2

Hand mass pick-up (lg)

10.84

3.678

17.27

132.6

0.016

0.445

1.700

Range

12.18

2.882

15.92

84.98

0.014

0.333

1.160

Concentration (lg g-1)

24,227

7,720

38,706

288,910

37.62

1,032

3,941

Range

17,382

9,463

27,439

175,613

25.99

989.2

3,286

-1

After multiple vacuumingb

a b c

Based on 5 hand-presses Estimated 5 hand-press values (from 10 actual hand-presses) \Detection limit

Debye-induction force, and the Keesom-orientation force (Israelachvili 1992)), electrostatic forces, and capillary forces. Compared to vdW forces, capillary and electrostatic forces are smaller in magnitude (Visser 1995). Electrostatic force will hold a particle [100 nm to a surface because it will carry some small net charge (inducing surface charges when contact is made with the surface). The electrostatic force is directly proportional to particle diameter and typically will only be dominant for particles[50 lm. Capillary forces arise when water vapor condenses on particles and surfaces, and at high humidities ([60%), such forces will be the dominant factor in adhesion, particularly for particles [50 lm in diameter (Zimon 1982). A thin physisorbed surface film can flow into the region near the contact point, and with enough condensed water a liquid bridge will form (Morgan 1961). The surface tension associated with such a liquid bridge provides an attractive force. On hydrophilic soil particle surfaces (e.g., silica or mica), adhesion commonly increases monotonically with relative humidity. With hydrophobic surfaces, as might be expected, capillary forces do not develop

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(Podczeck et al. 1997). In terms of particle size, given consistent particle shape and surface roughness, with increasing particle size there is an increase in the adhesion force (Corn 1961). In the size range 10–30 lm, however, the adhesion force appears to be independent of particle size (Zimon 1982). But, as detachment forces are dependent on mass, smaller particles exhibit greater resistance to removal (Corn 1966). Particle shape will control the effective separation between particle and surface. For particle shapes that maximize surface contact (e.g., flat, platy forms), the overall vdW forces will be increased. Particle and contact surface roughness are also important controls on adhesion as roughness affects separation between surfaces, which in turn determines the influence of vdW forces that are effective ˚ (Corn 1961). The exisonly at separations \10 A tence of asperities on surfaces may decrease contact area and increase separation between a particle and a surface. Of most importance in relation to particle size is the size of the asperities, and the frequency of recurrence of the asperities. A contacting surface can be visualized as either ideally smooth, having micro-

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roughness, or having macro-roughness (Zimon 1982). In the case of micro-roughness, the surface asperities are large in relation to particle diameter, but the asperity separations are not greater than the particle diameter. So, with micro-roughness, particle contact is limited to the tops of the asperities, and this leads to significantly decreased vdW forces. In contrast, in the case of macro-roughness, where the particle diameter is smaller than the asperity separation, particles located between asperities have increased contact area and greater vdW adhesion. Macroroughness, under appropriate conditions, can also lead to particle trapping. Particle roughness will have the same effect as surface roughness on the separation between particles and between particles and contact surface. Similar outcomes can be expected when there is a mixed distribution of fine particles and coarser particles. In effect, the fine particles will act like asperities that reduce contact area and reduce the adhesion forces. Depending on particle composition, wetting of surfaces (and contact through liquid bridges) can lead to surficial dissolution. Upon subsequent drying, the re-crystallization of dissolved material can lead to the formation of crystal bridges (Rumpf 1977) which will increase adhesion. Prolonged contact between a particle and a surface can produce an ageing effect that increases adhesion. This effect results from the formation of liquid bridges, chemical reactions between the materials of the particle and the surface, and contact surface deformation. Deformation will be determined by the viscoelastic properties of the contacting surfaces: when the hardness of a particle is less than that of the surface, the particle will flatten; when the particle is harder the surface will indent; and, when the particle and the surface are equally hard, the particle will flatten and the surface will indent (Dahneke 1972). Particle and/or surface deformation will decrease the separation between the particle and the contact surface, and hence increase the attractive vdW forces. Surface adhesion will also be affected by any surface layering of particles. Detachment of aggregated particles, either individually or en masse [as superficial layer(s)], requires inter-particle cohesion to be overcome, although inter-particle adhesion may be better referred to as autohesion (Zimon 1982), because the term cohesion more correctly applies to particles separated by an atomic distance.

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To overcome the adhesion forces experienced by deposited soil particles, an appropriate detachment force is required. The factors that determine whether particles are being pulled into a vacuum cleaner or driven airborne are the static lift and the airflow characteristics of the machine. Static lift is the ability of the vacuum’s airflow to lift dirt, and airflow is the volume of air displaced by the vacuum. In terms of vacuum cleaner performance, static lift is measured in ‘‘inches of lift’’ determined by how many inches of water or mercury the vacuum cleaner’s airflow can pull. The vacuum cleaner’s airflow is measured in the cubic feet per minute (CFM) of air moved through its system. The vacuum cleaner achieves static lift by pulling air (through its fan operation) into the system, creating an area of low pressure above the floor surface. Aerodynamic detachment of small particles is difficult as they are immersed in the viscous sublayer next to the surface where aerodynamic forces are small. The flow of air around small surface particles is typically laminar, and viscous drag is the predominant drag force. When the flow is not laminar (i.e., turbulent), pressure drag forces will operate. Particle detachment is traditionally viewed as the result of an external force exceeding the surface adhesion force. Detachment is considered either in terms of force balance (where detachment occurs once the threshold balance between instantaneous aerodynamic lift and particle surface adhesive forces is exceeded), or in terms of energy accumulation (a particle, when exposed to turbulent flow, can be detached when it accumulates enough vibrational energy from the turbulent flow to allow removal from the adhesive potential well) (Ziskind et al. 1995). However, suspension as a result of the accumulation of vibrational energy (Reeks et al. 1988) will not apply to small particles because the adhesion forces will be too great (Ziskind et al. 1995). Large particles, which extend above the viscous sublayer into the turbulent boundary layer, are affected by large-scale turbulent structures. Small particles, according to Ziskind et al. (1995), are entrained as a result of a flow-field in which vortices and sloping shear layers (caused by interaction of vortices with the viscous near-wall flow) interact in the near-wall region. In this study, we used a commonly available upright vacuum cleaner to entrain the particulate soil deposited on the tile surfaces. A nozzle attachment,

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with a smaller intake port, was also used, which will have moved the air faster than a typical brush attachment. In all likelihood, after deposition, the surface soil was present on parts of the tile surfaces as a extended continuous layer (of multiple layers of soil particles if deposition was considerable), as aggregated clumps of soil, or in a dispersed form with individual soil particles not in contact with other soil particles. Our data suggest that despite the airflow velocities in the operational set-up employed here, it was not possible to overcome the surface attraction forces holding down all of the soil particles. Unsurprisingly, other studies of vacuum cleaner removal of contaminated dust from hard floor surfaces have also shown incomplete recovery. Different vacuum cleaners remove and retain different amounts of dust from the same surface (Ewers et al. 1994; Hegarty et al. 1995). Ewers et al. (1994) found that roughly similar quantities of dust (approximately 50%) could be removed from hard wood and linoleum by an initial (HEPA equipped) vacuum cleaning (1 min m-2). But that with follow-up repeated cleanings of the same duration, hard wood flooring (totaling 5 vacuuming efforts) yielded over 95% of the deposited dust, while a maximum of 80% of the dust could be removed from the linoleum after a total of 2 vacuuming efforts (with no dust recovered from subsequent efforts). The study found that approximately 20% of the linoleum surface dust was recoverable with a final wet washing using tap water and a sponge. Despite differing levels of performance by vacuum cleaners, vacuum cleaner removal of dirt from flooring (notably from carpeted floors) has frequently been advocated (see, e.g., Roberts et al. 1999; Yiin et al. 2002). The type of incomplete surface removal of surface material identified here has been seen elsewhere. Rich et al. (2004), in an effort to reduced the loading of Pb dust from hard flooring by vacuum cleaning operating in tandem with detergent and water cleaning, found that the removal of the surface Pb could not be fully accomplished. Similarly, when Farfel et al. (1994) compared vacuum and wipe sampling methods, they encountered sample loss that can likely be attributed, to some degree, to incomplete surface detachment. The proportionately greater quantities of some soil elements (notably Pb) reported by the analysis of the wet wipes demonstrated that residual amounts of these elements were concentrated on the surface following vacuuming, and we attribute this to the physico-

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chemical speciation of these elements in soil. Lead is a common contaminant in residential soils. It is estimated that, in the U.S., 18 million privately owned homes built before 1980 have soil Pb levels that exceed 400 lg g-1, 6 million of such homes have Pb levels in excess of 2,000 lg g-1, and 2.5 million have Pb levels in excess of 5,000 lg g-1(EPA 1996b). Typically, Pb is partitioned among many soil compartments. It is associated with the adsorption surfaces of the clay– humus complex, Fe and Mn oxides, alkaline earth carbonates, precipitated forms, and silicate lattices (Alloway 1993; Chaney et al. 1984; Johnson and Hunt 1995). Kabata-Pendias (1980) found that metals (including Pb) are sorbed on minerals in soils in the decreasing order: Mn-oxides [ montmorillonite [ kaolinite [ Fe-oxides [ illite; and that the amorphous hydrous oxides of Mn and Fe had the highest affinity for Pb. The amorphous oxides have larger sorption capacities than crystalline oxides (Trivedi and Axe 2001). Iron and Mn-oxides, because of their reactivity and generally high surface area, facilitate sorption of Pb (Rickard and Nriagu 1978). Lead specifically sorbs as inner sphere complexes to Fe and Mn-oxides, but Mn-oxides [e.g., birnessite (a-MnO1.7)] sorbs more Pb than Fe-oxides, Al-oxides, organics or clays (Aualiitia and Pickering 1987; McKenzie 1980). In the presence of available phosphorus, secondary Pb phosphate minerals [of the pyromorphite family ((Pb,Ca)5(PO4)3(OH,Cl,F))], can be a common precipitated form in soil (Davis et al. 1993; Cotter-Howells et al. 1994; Ruby et al. 1994). These associations in soil mean that Pb is typically present in the finest size fractions. Similarly, Al, which is typically associated with the finely divided clay fraction of the soil, is also present in the finest soil particle sizes. The enrichment of Pb and other metals in finely divided soil particles has been repeatedly documented (e.g., Spittler and Feder 1979; Dong et al. 1984; Qian et al. 1996). The bioavailability of Pb following ingestion will be dependent upon its form in the soil. Soils contaminated with Pb inputs from mining activities have less bioavailable Pb than urban soils (Steele 1990; Ruby et al. 1992; Davis et al. 1992). The bioavailability of Pb will be controlled by particle size, the speciation of the Pb and the presence of any protective matrix binding the Pb (EPA 1999). However, it has long been recognized that, irrespective of the biosolubility of the Pb, there is an increase in bioaccessibility with decreasing particle size (Barltrop and Meek 1979).

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The degree of hand pick-up of soil deposited on the test tiles is not surprising for two reasons. First, large amounts of surface dust can be collected on the hand. Kissel et al. (1996a) found that up to approximately 15 mg cm-2 of moist soil could be picked up by placing a hand palm down on a container of soil followed by gentle agitation for 30 s. Under worst case conditions, loading values in excess of 1 mg cm-2 have been recorded for children playing in mud (Kissel et al. 1996b). The transfer of surface contaminants to hands can also increase with the number of hand-to-surface contacts due to increased palm surface exposure (Brouwer et al. 1999). The risk for repeated pick-up of indoor floor dust by children is substantial by virtue of almost continuous exposure. For example, children less than 3 years old spend between 19 and 21 h a day indoors (EPA 2006). In terms of transfer, the amount of dust that will adhere to the hand of a child per unit surface area (the adherence factor) will depend upon the properties of the dust and the child’s activities (EPA 2004). This will be facilitated by the palm size of the child and any surface coating. A natural surface coating on the hand will consist of sebum, lipids and sweat. This fine layer (0.4–4 lm in thickness) will be irregularly distributed across the hand (Tregear 1996), and its effect on particle pick-up and retention will be variable. Initial contact with this layer may lead to increased, if uneven, adherence of dust. Repeated contact with a dusty surface may lead to the sebum and sweat layer becoming so loaded that adherence is curtailed, or the layer may be removed to some degree. Clearly, the rate of sweat and sebum secretion will be important, and the relative production of each may affect adherence. For example, it has been shown by Edwards and Lioy (2001) that the retention of specific pesticides when added to housedust then hand-lifted from a deposition surfaces depends on the proportions of sweat and sebum on the hand. For the young child, more important may be the saliva coating that is often present on the hand. Once on the child’s hand, contaminant ingestion will, of course, depend on the rate of transfer to the child’s mouth. Mean hand-tomouth contact ranges from 4 contacts/h (for 6 to \11 year olds) to 20 contacts/h (for 6 to \12 month olds). Furthermore, the total mean mouthing time ranges from 7 min/day (for 2 to 6 year olds) to 65 min/day (for 6 to \12 month olds), and the total mean mouthing frequency ranges from 5 contacts/h

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(for 6 to \11 year olds) to 54 contacts/h (for 1 to \2 year olds) (EPA 2006). The degree of hand pickup in this study was most likely facilitated by the water present on the polypropylene gloves (applied to represent a saliva coating). The hydrogen bonds between water molecules and soil particles created sufficient adhesion to assist in the hand lift of those particles that could not be picked up by the vacuum cleaner. For a young child, in the situation where there may be sticky candy residue on the hand, the potential for adherence is most likely further enhanced.

Conclusions Many types of commercial vacuum cleaner designs are used for cleaning floors in residential and commercial environments. They range from small robotic vacuums, and broom vacuums, to powerful upright vacuums. The efficiency of these devices varies with the power of their vacuum systems. The potential exists for vacuum cleaners to vary considerably in cleaning efficiency. In this study, we found that using a domestic vacuum cleaner (although used vigorously) to remove soil that had been footwear-tracked onto a vinyl floor surface can produce a substantial reduction in the surface dust loading. For example, when soil was repeatedly deposited and repeatedly vacuum cleanerremoved from the tiles, the average reduction (after the final vacuuming) in the quantity on a tile of Si was 93%, of Al was 83%, and of Pb was 80%. However, the elimination of deposited surface particles was not complete. Moreover, the hand-lift tests indicated that even the small amount of material remaining could, in part, be removed by hand-to-tile contact. This suggests that a potential exposure threat (particularly for the young child engaged in hand-tomouth activity), most likely remains after vacuuming. It is apparent that the size distribution of the particulate matter remaining on floor surfaces after vacuum cleaning is different from that originally deposited. Following vacuum cleaning, the uncollected surface material is composed of finer sized particles than those of the soil prior to vacuuming. We attribute this to the preferential removal of coarser particles by the vacuum cleaner, and the cleaner’s difficulty in overcoming the particle-surface adhesion forces in the finest size fractions.

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It is quite likely that the form of the track-in soil and the nature of any constituent contaminants will be important exposure factors. It is possible that if the soil size distribution is skewed toward the finest sized particles, and if any contaminants are present in the smaller particles, the potential exists for post cleaning surface residue to be enriched in potentially hazardous material. For example, some toxic trace elements (e.g., Pb) in soil tend to be elevated in the finer particle fractions, and we have found in this study that the concentration of Pb from soil on a tile surface can increase after repeated deposition and cleaning. The results presented here have implications for exposure studies that utilize indoor dust samples to assess exposure to environmental contaminants. Dust collection by vacuum cleaner sampling may under-report concentrations and surface loadings of specific elements on hard flooring. Employing a soil tracer element that is unlikely to have an indoor source (e.g., zirconium from zircon) to assess soil Pb ingress indoors may misreport the indoor Pb contribution from outdoor soil. If the tracer element used is associated with larger size particles, there may be differential recovery by vacuum sampling. Therefore, in the absence of an internal dust source of Pb, using the ratio of Pb to tracer element in the soil to estimate the indoor Pb derived from soil there may be a different ratio in the dust vacuum sampled indoors. This may erroneously suggest some differential loss (dilution) of Pb during transport. Alternatively, in the situation where there is an internal source of Pb (e.g., flaking Pb-based paint), the ratio in the indoor dust compared to the ratio in the outdoor soil may under-predict the indoor Pb contribution (depending on the size of the indoor Pb particles). In these circumstance (incomplete vacuum sampling of indoor dust), ratios of Pb to tracer elements may not be reliable for assessing outdoor or indoor Pb contributions to Pb in indoor dust. Acknowledgement Funding Sources: National Foundation (NSF) Grant # BCS-0221949.

Science

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