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Ecotoxicology and Human Environmental Health

Dynamics of lead bioavailability and speciation in indoor dust and x-ray spectroscopic investigation of the link between ingestion and inhalation pathways Farzana Kastury, Euan Smith, Enzo Lombi, Martin Donnelley, Patricia Cmielewski, David Parsons, Matt Noerpel, Kirk G. Scheckel, Andrew Kingston, Glenn Myers, David J Paterson, Martin D de Jonge, and Albert L. Juhasz Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b03249 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Dynamics of lead bioavailability and speciation in indoor dust and

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x-ray spectroscopic investigation of the link between ingestion

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and inhalation pathways.

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Farzana Kastury1*, Euan Smith1, Enzo Lombi1, Martin W. Donnelley2-4, Patricia L.

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Cmielewski2-4, David W. Parsons2-4, Matt Noerpel5, Kirk G. Scheckel6, Andrew M.

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Kingston7, Glenn R. Myers7, David Paterson8, Martin D. de Jonge8, Albert L. Juhasz1,

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1*

Future Industries Institute, University of South Australia, Australia;

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Women’s and

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Children’s Hospital, Adelaide, Australia; 3Adelaide Medical School; 4Robinson Research

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Institute, University of Adelaide, Australia, 5Oak Ridge Institute for Science and Education,

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Cincinnati, OH USA.

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USA; 7Department of Applied Mathematics, Australian National University, Australia;

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8

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United States Environmental Protection Agency, Cincinnati, OH

Australian Synchrotron, ANSTO, Australia.

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*Corresponding author: Farzana Kastury, Future Industries Institute, University of South

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Australia, Building X, Mawson Lakes Campus, Adelaide, SA 5095, Australia, Phone:

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+61433100212, Email: [email protected]

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Abstract:

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Lead (Pb) exposure from household dust is a major childhood health concern due to its

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adverse impact on cognitive development. This study investigated the absorption kinetics of

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Pb from indoor dust following a single dose instillation into C57BL/6 mice. Blood Pb

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concentration (PbB) was assessed over 24 h and the dynamics of particles in the lung and

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gastro-intestinal (GI) tract were visualized using X-ray Fluorescence (XRF) microscopy. The

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influence of mineralogy on Pb absorption and particle retention was investigated using X-ray

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Absorption Near Edge Structure spectroscopy. A rapid rise in PbB was observed between

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0.25-4 h after instillation, peaking at 8 h, and slowly declining during a period of 24 h.

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Following clearance from the lungs, Pb particles were detected in the stomach and small

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intestine at 4 and 8 h respectively. Analysis of Pb mineralogy in the residual particles in

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tissues at 8 h showed that mineral sorbed Pb and Pb-phosphates dominated the lung, while

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organic bound Pb and galena were the main phases in the small intestines. This is the first

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study to visualizing Pb dynamics in the lung and GI tract using XRF microscopy and linking

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the inhalation and ingestion pathways for metal exposure assessment from dust.

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Keywords: Dust; Lead bioavailability; XANES; XRF; EXAFS; Blood lead level



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Introduction

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Lead (Pb) is a ubiquitous heavy metal of major health concern owing to the well documented

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causal relationship between Pb exposure and childhood cognitive development1,2. Once in the

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blood stream, Pb toxicity and accumulation in tissues are independent of exposure routes and

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may affect all organs, including the neurological, cardiovascular, hepatic, renal or

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haematological systems3, Incidental ingestion of soil/surface dust in children is considered

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the major pathway for childhood Pb exposure. However, in regions with current and historic

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metal processing industries, inhalation of Pb impacted ambient particulate matter may also be

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considered a significant exposure pathway3,4. Aeolian resuspension of household dust

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impacted by Pb based paints or mining/smelting activities may contribute to indoor Pb

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exposure, with children being at higher risk compared to adults due to their higher respiratory

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rate5 , their closeness to the ground relative to adults, and their higher fractional deposition of

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particles in the lungs6.

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Inhalation studies utilizing PbCl2 and Pb(OH)2 with an average aerodynamic size of 0.25 µm

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suggest that approximately 23-26% of inhaled Pb particles may deposit in the lungs7 and

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disperse in surfactants and extra-cellular fluid lining the respiratory system (pH 7.00 ±

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0.12)8,9, while the remainder may be exhaled in subsequent breaths3. Following the

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displacement of particulate matter from air into the aqueous phase, Pb solubilization in lung

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fluid may result in absorption into the blood via the air-blood barrier8. Solubilized transition

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metals may cause localised pulmonary toxicity10, translocate to extra-pulmonary organs (e.g.

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heart and liver)11, and exert systemic toxicity (e.g. DNA and hepatic damage by oxidative

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stress)12-14.

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Unlike the ingestion pathway, where Pb absorption may be influenced by nutritional status

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(i.e. fed vs unfed, cations and lipid composition of diet)15,16, a higher fraction of solubilized


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Pb may be absorbed in the lungs due to the high surface area and extensive vascularisation of

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the respiratory system, coupled with the short air-blood barrier separation17. Inhalation

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studies in humans using Pb particles ( 1 µm) depositing in the back of the nose may be swallowed, and those depositing

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in the tracheobronchial region may be cleared from the lungs via the muco-ciliary escalator,

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reaching the gastro-intestinal (GI) system24. Further solubilisation of Pb in the low pH

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environment of the stomach may contribute to the fraction available for absorption in the

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intestine depending on nutritional status25. Poorly soluble Pb particles that deposit in the

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alveolar region may not be cleared by muco-ciliary action and can remain in the lungs for

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months to years24.

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Although inhalation exposure to inorganic and tetra-ethyl/methyl Pb has been well-studied in

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human experiments as part of the toxicity assessment of Pb additives to gasolines3, Pb

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bioavailability (absorption into systemic circulation) from the inhalation of Pb impacted dust

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has received little attention. This is particularly important as dust Pb concentrations in

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mining/smelting regions or in houses containing Pb based paint are often elevated26,27.

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Moreover, the majority of in vitro bioaccessibility (metal dissolution in simulated lung fluid)

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studies have focused on Pb extraction in the lung phase only, with tremendous variation in

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extraction time-frame (from 1h – 360 days) to accommodate the long lung dust retention

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time28. Lead bioavailability and retention/clearance with mining/smelting impacted dust, and



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the link between Pb inhalation and ingestion scenarios are currently not well understood.

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Consequently, this study investigated the dynamics of Pb absorption from Pb contaminated

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house dust following instillation into C57BL/6 mouse lungs. Changes in blood Pb

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concentration were monitored over a 24 h exposure period. X-ray Fluorescence (XRF)

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microscopy of whole organs was then utilized to visualize particle distribution and clearance

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from the lungs into the GI tract. Changes in Pb speciation in vivo were also assessed using X-

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ray Absorption Near Edge Structure (XANES) spectroscopy.

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Materials and Methods

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PM10 collection and characterization

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Port Pirie, South Australia is home to one of the largest primary Pb smelters worldwide. As a

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consequence of fugitive dust emission, areas of Port Pirie have been impacted with Pb-dust

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resulting in elevated PbB in some residents. Household dust from Port Pirie, South Australia,

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was collected from an abandoned house in Port Pirie, South Australia by the South Australian

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Department of Health using a Nilfisk vacuum fitted with a canister. Following drying at

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40°C, household dust was initially hand sieved to < 53 µm (PM53, stock sample) to remove

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oversized material (hair, fluff etc.). The PM53 stock was further sieved to < 10 µm (PM10, PP-

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D) to recover the inhalable fraction using an Endecotts Octagon digital shaker. PM10 was

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homogenized by end-over-end rotation (45 rpm) for 24 h Further details of collection and

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processing can be found in the Supporting Information (SI). Particle size distribution was

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analysed in Accusizer (n = 3) after overnight dispersion of PP-D (5 mg) in 0.1 M NaCl

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solution (50 mL)., and the remainder of the particles were stored at 20 °C prior to use for

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physico-chemical characterization and in vivo assessment.

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Total metal(loid) content of PP-D was determined using inductively coupled plasma mass

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spectrometry (ICP-MS) (Agilent 8800) according to USEPA method 6020A29 following


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aqua-regia digestion in a MARS-6 microwave (CEM) (USEPA method 3051)30. Digests were

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syringe-filtered (0.45 µm) and stored at 4°C prior to analysis.

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Lead speciation in PP-D was determined using extended x-ray absorption fine structure

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(EXAFS) at the Materials Research Collaborative Access Team (MRCAT) beamline 10-ID at

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the Advanced Photon Source (APS) of the Argonne National Laboratory (ANL), USA31,32.

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Details of the EXAFS methodology can be found in the Supporting Information.

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In vivo instillation exposure

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This study was conducted with approval from the Animal Ethics Committee of the Women’s

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and Children’s Health Network, Adelaide, South Australia (WCHN AEC project No.

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AE1044). Female C57Bl/6 mice (8-10 weeks old) were randomly assigned to treatment

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groups (n = 6 per group). Instillation was demonstrated to be an effective surrogate technique

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for studying exposure to ambient particulate matter of PM1011. Additionally, this exposure

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technique is capable of delivering precise doses, which was deemed appropriate for

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comparing PbB among different time points. Mice were anaesthetized with an intraperitoneal

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(i.p) injection of a 10 μL g/body weight mixture of medetomidine (0.1 mg/mL, Orion

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Corporation, Finland) and ketamine (7.6 mg/mL, Parnell Laboratories, Australia). Intubation

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was performed using an endotracheal tube (18G BD Insyte i.v. cannula) and fibre optic light

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(4Dx Pty Ltd, Australia) as described previously, with the endotracheal tube marked to

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ensure reliable placement depth33.

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Prior to instillation, PP-D (20 mg of dust/mL) were dispersed in 0.9% NaCl (w/v) and

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continuously stirred to ensure that PP-D remained in suspension according to Cmielewski et

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al.33. A 20 µL bolus dose was administered directly into the trachea via the endotracheal tube

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over ten seconds using a micropipette and a thin gel tip (Microloader, No: 5242 956.003,

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Eppendorf). Mice were constantly monitored and maintained anaesthetized on a 37°C heated



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warming blanket for up to 2 hours, or allowed to fully recover following reversal of

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anaesthesia with a 2 µL/g i.p. injection of Antisedan (atipamezole 0.5 mg/mL, Orion

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Corporation, Finland). Mice that were allowed to recover were monitored 1-2 times hourly

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for 8 h and then returned to the holding room overnight.

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Groups of mice were humanely killed by CO2 asphyxiation at 0.25, 0.5, 1, 2, 4, 8 or 24 h after

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Pb instillation. Blood was removed via cardiac puncture and placed into heparinized tubes

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and stored at 4°C until analysed. Lungs and the entire GI tract were removed and

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immediately frozen at -86°C (Shuttle ULT-25E, Ultracold Portable freezer, Stirling, Ohio,

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USA). Organs were stored at -80°C until the next stage of analysis.

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Determination of Pb concentration in organs and blood

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Four of the whole lung and GI tracts were utilized during the determination of Pb

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concentration in tissues, reserving 2 of each for XRF microscopy described below. GI tracts,

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samples were sectioned into stomach, small intestines and large intestines. GI components

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and lungs were freeze dried (Modulyod Freeze Dryer), then digested using concentrated nitric

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acid (70% HNO3) using a block digester ramped to a maximum temperature of 180°C (A.I.

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Scientific AIM500). Once dissolved and the volume reduced to 1-2 mL, samples were cooled

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to room temperature, diluted with Milli-Q water (10 mL), filtered through 0.45 µm cellulose

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acetate filters (Millipore Millex-HV) and stored at 4°C until analysed by ICP-MS.

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Immediately prior to ICP-MS analysis the blood was diluted 10-fold in diluent solution

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containing 1-butanol (2% w/v), EDTA (0.05% w/v), Triton X-100 (0.05% w/v), and

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ammonium hydroxide (1% w/v) in Milli-Q water34.

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Scanning X-ray fluorescence (XRF) microscopy


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In preparation for XRF microscopy, GI tracts (n = 2) were thawed, unravelled and

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sandwiched between Kapton tape and re-frozen. Each frozen lung (n = 2) was thawed,

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dissected and the left lobe was suspended on a Lecter coat hanger over 50 mL Falcon tubes in

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order to retain morphology, and then re-frozen. GI tracts and lungs were freeze dried

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(Modulyod Freeze Dryer) to minimise self-absorption issues particularly when operating in

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tomographic mode. Prior to XRF microscopy, GI tracts and lungs were sandwiched between

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ultralene or individual lung samples were mounted onto Kapton tubing. XRF microscopy of

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the lung and GI tract were performed at an incident energy of 18.5 keV (velocity of 2 mm/sec

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and pixel sizes of 5 µm for lungs and 20 µm for GI tracts) at the x-ray fluorescence

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microscopy (XFM) beamline at the Australian Synchrotron.

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Additionally, tomography of the left lobe of the lung exposed to PP-D for 0.25 h was

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undertaken to visualise the spatial distribution of Pb particles in the deeper regions of the

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lung (9-18 slices, 500 rotations at a velocity of 0.5-1 mm / sec and a pixel size of 2 µm).

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Tomographic images were reconstructed as a multi-step process, using the mango software

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package35: (i) any misalignments of the imaging system were detected and corrected using

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the “autofocus” approach of Kingston et al.

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system and sample was detected and corrected using the re-projection approach of Latham et

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al.37; (iii) the 3D volume was estimated using filtered back-projection38; and (iv) the

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reconstructed 3D volume was refined using Simultaneous Iterative Reconstruction

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Technique38. The final images were generated by overlaying images of Compton and three

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metals (Pb, Zn and Fe) using Image J39. Regions of interest (0.2 x 0.2 mm) of the lungs (0.25

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h, 2 h and 8 h) and small intestines (8 h) were also selected for XANES analysis (90 energy

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steps, mapped at a velocity of 0.5 mm/sec and a pixel size of 2 µm)40.

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XRF spectra were analysed using GeoPIXE according to Ryan et al.41,42. XANES spectra

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were normalised using Athena (Demeter 0.9.25 using IFEFFIT 1.2.12)43. LCF was conducted

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; (ii) intra-experiment motion of the imaging



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in a relative energy range of 13015 to 13090 eV using the smallest R-factor values to assess

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the best fit using 25 standards used during the aforementioned EXAFS analysis. More

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information about the XRF and XANES methodology is given in the SI.

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Quality Assurance and Quality Control

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The accuracy of the PP-D and organ digestion process was confirmed using Pb recovery from

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National Institute of Standards and Technology standard reference material (NIST SRM)

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2710a and NIST SRM 2976 respectively (n = 3). The quantitative average Pb recovery (mean

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± SEM) from SRM 2710a (5520 ± 30 mg/kg) was 5489 ± 55, while that from SRM 2976

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(1190 ± 180 mg/kg) was 1210 ± 200 mg/kg (101.4%). During the determination of Pb

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concentration using ICP-MS, duplicate analysis, check values and spiked samples were

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included. The average deviations in Pb concentration from duplicates were 2.4% (n = 17) and

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check values were 4.7% (n = 58). The average recovery of Pb spiked samples was within the

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limits specified in USEPA Method 6020A29.

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Results and discussion

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Characterization of Port-Pirie dust (PP-D)

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Household dust, collected from an unoccupied house in Port Pirie, South Australia, was

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utilized as the test material for the instillation experiments. Following screening to a particle

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size of < 10 µm, analysis was undertaken to determine the concentration of key elements of

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concern. The primary target for instillation experiments was Pb; Port Pirie is home to one of

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the world’s largest Pb smelters, which has been in continuous operation since 1889. As a

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consequence of smelter emissions, Pb bearing dust may be transported to houses directly via

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atmospheric deposition or indirectly through windblown transport of deposited Pb via fine


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soil particulates. Table 1 details elemental concentrations, particle sizes and Pb speciation in

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PP-D. The concentration of Pb in PP-D was elevated (32,045 ± 392 mg/kg) with Pb

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predominantly bound to organic matter (44% weighted contribution) as determined by

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EXAFS. The remainder of the Pb species were comprised of

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[Pb3(CO3)2(OH)2,

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Pb5(PO4)3Cl, 10%] and mineral sorbed Pb (Pb oxides and adsorbed onto clays, 13%). Lead

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concentration in PP-D was approximately 30-fold higher compared to values determined for

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outdoor soil of the house (data not shown) and 4.5 fold higher compared to < 10 µm outdoor

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dust collected from another Port-Pirie location close to the smelter as described in Kastury et

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al.26. The elevated Pb concentration in PP-D suggests that Pb sources other than smelter

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emissions may have contributed to the household dust Pb burden. Screening of indoor

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surfaces (using portable XRF) identified that Pb-based paint was present in the house while

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principal component analysis of stable Pb isotopes (208/206Pb versus 207/206Pb) from household

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dust, paint, soil and ore determined that approximately 65% of the household dust Pb burden

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could be apportioned to Pb-paint contributions (personal communication, data not shown).

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The Pb concentration observed in PP-D resembled the median Pb concentration (35,000

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mg/kg) in a household dust study from old houses impacted by Pb based paint in New

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Orleans, USA27. Among other elevated co-contaminants, Arsenic (As), Cadmium (Cd) and

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Zn were present at 586 ± 16,231 ± 4.9 and 32,045 ± 630 mg/kg respectively. Similar to Pb, the

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concentration of these three metal(loid)s were between 3.1 - 6.1 fold higher than Port-Pirie

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outdoor dust reported previously26.

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Pb bioavailability following PP-D instillation

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PP-D was administered into mouse lungs via intra-tracheal instillation in order to assess Pb

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bioavailability over a 24 h exposure period using PbB as an endpoint. Although deposition of

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particles via fluid instillation is thought to differ considerably from dry particle inhalation44,

21%],

galena

(PbS,

11%),

Pb-phosphates


hydrocerussite

[chloropyromorphite,


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the choice of instillation was deemed appropriate in this study as it allowed for a more precise

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dose delivery, and enabled effective comparison of the Pb absorption at different time points.

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Figures 1A and 1B show PbB (µg/L) and tissue Pb concentrations (lungs, stomach, small

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intestines; µg/organ) over a 24 h exposure period following the instillation of PP-D (lung Pb

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concentration: PbL, stomach Pb concentration: PbS, and small intestine Pb concentration:

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PbSI). A rapid increase in PbB was observed during the first 0.25 h of exposure (from 0.29

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µg/L at time = 0 to 16.2 ± 1.6 µg/L at time = 0.25 h), indicating fast initial Pb solubilization

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and absorption in the lung. The high surface area due to the small particle size of PP-D (Table

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1) may have facilitated Pb dissolution and absorption in the first fifteen minutes of the study.

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Figure 1A shows that between 0.5 to 2 h, PbB rapidly increased (~ two-fold in 1.5 h: from

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32.7 ± 4.4 to 63.3 ± 3 µg/L) while PbL fluctuated between 5.9 to 6.9 µg/organ (Figure 1B).

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Although mucociliary clearance of PP-D from the tracheobronchial region most likely

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commenced soon after instillation, Pb was not detected in the stomach in the first 2 h (0.05

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and 0.06 µg/organ respectively at 0.5 and 2 h) (Figure 1B).

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Between 2 - 8 h, PbB increased at a slower rate compared to the first 2 h (~ 1.5 fold in 6 h:

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from 63.2 ± 2.9 at 2 h to 96.6 ± 10 µg/L at 8 h), with a corresponding reduction in Pb L (from

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6.2 ± 0.6 at 2h to 2.7 ± 0.6 µg/organ at 8 h). Whole organ digest data (Figure 2B) also

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showed an increase in PbS from 0.06 ± 0.02 to 1.5 ± 0.3 µg/organ and in PbSI from 0.06 ±

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0.01 to 0.14 ± 0.01 µg/organ. At 24 h PbB declined to 31.8 ± 3.3 µg/L (Figure 1A), however,

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it did not return to the baseline level observed at time = 0. High lung Pb retention at 24 h post

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instillation with particulate matter has been reported in Kastury et al.45 and Wallenborn et

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al.11. Similar to this study, Kastury et al.45 instilled mining/smelting impacted PM10 from

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Broken Hill, Australia, into mice and reported a 71.5% retention of Pb in the lung at the end

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of 24 h (1.13 µg of Pb at 0.25 h to 0.81 µg of Pb at 24 h). Wallenborn et al.11 instilled

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particulate matter collected from residual oil from Boston power plant into Wistar Kyoto rats


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and reported that ~ 50% of the Pb observed in the lung at 4 h was retained at the end of 24 h.

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Additionally, previous inhalation experiments in humans using Pb(OH)2 and PbCl2 have

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reported evidence of high PbB for up to 4 days post exposure7. Investigations into

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mucociliary clearance patterns in human lungs using ‘symmertric and stochastically

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generated asymmetric models of the conducting tree’ by Asgharian et al.46 similarly predicted

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significant lung burdens beyond 24 h, however, this simulation was based on particles alone

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and not specific to metals associated with PM10. Therefore, it is likely that a fraction of the

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PM10 may reside in the lungs for longer than 24 h, contributing to Pb absorption beyond the

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24 h exposure period; however, future studies should be designed with longer exposure

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period (e.g. 7-10 days) to investigate Pb bioavailability from mining/smelting/paint impacted

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dust to confirm this theory.

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XRF microscopy of Pb dynamics in the lung and GI tract

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In order to visualize the clearance of dust from the lungs and its passage into the GI tract,

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XRF microscopy was undertaken using freeze dried left lung lobes (at 0.25, 0.5, 1, 2, 4, 8 and

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24 h) and freeze dried GI tracts (including stomach, small and large intestines at 2, 4 and 8 h).

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The left lung lobe was chosen for analysis because of its uniformity in shape, which aided

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comparison between lungs from multiple time points. Despite fluorescent imaging techniques

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having been utilized to visualize the deposition of airborne particulate matter in the lungs

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before47, to the best of our knowledge, this is the first study to report the clearance patterns of

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metals in dust particles utilizing XRF microscopy at a synchrotron-based X-ray source.

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During the analysis of XRF microscopy results, the spatial distributions and clearance

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patterns of Pb, Zn and iron (Fe) were investigated. Although Pb was the primary focus of this

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study, dynamics of Zn in the lungs was of particular interest because Zn is also smelted at

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Port Pirie and is present in PP-D at elevated levels (Table 1). Because of the high potential

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for lung inflammation and fibrosis when exposed to Zn48, exposure from PP-D may be a



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significant health concern. The dynamics of Fe absorption was investigated because of its

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propensity to co-precipitate with Pb at neutral pH49, which is relevant to this investigation

280

because both lung epithelial fluid and intestinal solutions are pH neutral9,50.

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Four horizontal slices of the 0.25 h lung (left lobe) were examined using XRF microscopy

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based tomography to establish the suitability of XRF imaging to visualize lung structure and

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elemental distribution within it. Figure 2A illustrates the image of the lung, marking the

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positions of the four slices corresponding to Figures 2B (i- iv). The elements of interest are

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indicated by tri-colour maps (Pb = red, Zn = yellow, Fe = blue), while the position of Pb in

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Figures 2B (i-iv) is indicated by orange arrows. Figure 2A and 2B (i) suggested that 15

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minutes post instillation, the majority of the Pb particles were located close to the trachea.

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However, Figures 2B (ii – iv) suggest that lung structure was significantly affected during

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sample preparation. Post collection and storage at -20°C, lungs were thawed to dissect the left

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lobe prior to re-freezing and freeze drying for XRF microscopy preparation. This process

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may have resulted in the formation of crystals in the lungs during the re-freezing step which

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may have ruptured some of the delicate alveoli structures, giving the lung an emphysematous

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appearance. Nonetheless, elemental distributions in these images were clearly visible,

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particularly in the deep alveolar regions of the lung in Figure 2B (iv). These four tomographic

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images of the lung indicate that while Pb distribution was largely restricted close to the site of

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instillation at 0.25 h, a fraction of Pb particles had been distributed to the distal parts of the

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lungs within this time-frame. Therefore, although the structure of the alveolar region of the

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lung may have been altered during the freezing and freeze drying process, XRF microscopy

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was effective at illustrating the spatial distribution of elements of interest (e.g. Pb, Zn and

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Fe).

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In addition to the tomography performed with the 0.25 h lung, 2D scanning XRF images

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were also obtained to visualize the dynamics of residual metals of the left lung lobes at 0.5, 1,


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2, 4, 8 and 24 h and GI tracts (stomach, intestines, cecum and colon) at 2, 4, 8 and 24 h,

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(Figure 3A and 3B), with elements of interest depicted by tri-colour maps (Pb = red, Zn =

305

yellow and Fe = blue). At 0.25 h, Pb was more prominent in the upper part of the lobe, which

306

was previously observed in Figure 2A and 2B (i). Although the tri-colour maps should not be

307

used to assess co-localization of Pb, Zn and Fe due to the different effect that self-absorption

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has on the fluorescence of these 3 elements, Figure 3A indicates that a fraction of Pb was

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sorbed onto Zn (indicated by the yellow) closer to the trachea. Because of the high Zn

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concentration in PP-D, it is conceivable that Pb was sorbed onto Zn oxides.

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Images of the 0.5, 1 and 2 h lungs demonstrates that Pb became widely dispersed in the lungs,

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presumably due to the penetration of dust particles deeper into the distal parts of the lungs as

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a result of breathing and gravity51 and the concurrent displacement of Pb from PP-D into lung

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surfactants and the underlying epithelial fluid. During this time period, Pb was not detected in

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the XRF images in the GI tracts (Figure 3B), indicating that the mucociliary escalator had not

316

yet resulted in clearance of particles from the lung, which supports the aforementioned results

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of no increase in PbS in the first 2 h in Figure 1B. As a result, the increase in PbB during the

318

initial two hours of the assay may be attributed to Pb absorption from the lungs alone.

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During 4 and 8 h, XRF images showed a marked reduction in fluorescence intensity in the

320

lungs (Figure 4A), indicating that particles were being cleared from the lung during this time,

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which is also supported by the reduction in PbL in Figure 1B during this time-frame. Figure

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3B demonstrates that Pb particles appeared in the stomach at 4 h and in both the stomach and

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small intestines at 8 h, suggesting that PP-D had reached the pharynx via the mucociliary

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escalator within this period and was swallowed. Because of the time taken to detect Pb in the

325

stomach (~ 4 h), it was deemed that regurgitation/swallowing during the initial delivery of

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PP-D or the withdrawal of the extra-thoracic tube was not the cause of Pb to reach the

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stomach and that mucociliary clearance was the main executor of this result. Similar to the



328

lung epithelium, Divalent Metal Transporter 1 is known to mediate Pb absorption in the small

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intestines52. Therefore, the presence of Pb in the small intestine suggests that Pb absorption

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during 4-24 hours post instillation may be attributed to the combined absorption from the

331

lungs and small intestines. Although metal absorption from the small intestine may be

332

considered more efficient because it follows solubilization in the acidic stomach

333

environment, the presence of small quantities of residual Pb in the small intestines was most

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likely the result of re-precipitation during the change in pH from 1.5-2.5 to 750.

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Despite the XRF image of the 24 h lung indicating that the majority of particles were cleared

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from the lungs (Figure 3A), PbL at 24 h and 8 h were comparable (2.8 ± 0.4 and 2.6 ± 0.6

337

µg/organ respectively) (Figure 2B). Based on PbL at the start (T = 0.25 h) and the end of the

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assay (T = 24 h), 40.4% of Pb was retained in the lung. High lung Pb retention was also

339

reported by Wallenborn et al.11 after the instillation of oil combustion particulate matter in the

340

lungs of WKY rats. Similarly, high retention of inhaled Pb was reported in human studies of

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Pb industry workers exposed to Pb fumes (e.g. 39-47% in Mehani et al.53 and 30-50% in

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Kehoe et al.54). The extended Pb lung retention suggests that in addition to causing systemic

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toxicity as a result of Pb absorption and translocation to extra-pulmonary organs (e.g. heart,

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liver, kidney)11, continued exposure to Pb and Zn contaminated dust in situ after deposition in

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the lungs may also pose chronic toxicity effects, such as lung fibrosis and inflammation.48

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Change in Pb speciation in the lungs and small intestinal tract

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Recent research in metal exposure assessment has highlighted the significant role of Pb

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speciation on its bioavailability49,50,55,56. To elucidate changes in Pb speciation in vivo

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following the instillation of PP-D, XANES analysis was performed in selected regions (0.2 x

350

0.2 mm) of the 0.25, 2 and 8 h lungs and 8 h small intestines. These regions of interest were

351

selected based on the prevalence of Pb, Zn and Fe particles. As spectra corresponding to the


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Page 17 of 32


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entire region selected for XANES analysis were unusable for LCF fitting (due to low overall

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Pb concentrations), spectra from five Pb particles from each region were used. Because of the

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small area selected for XANES analysis, results may not reflect localized Pb species

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heterogeneity (e.g. Pb speciation in the residual particles inside the acidic lysosome of

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alveolar macrophage may be different from that found deposited on the neutral lung lining

357

fluid). However, undertaking XANES analyses of larger areas or the whole tissue was

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impractical due to the time required for such analyses. Nevertheless, the results obtained in

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this study revealed vital information regarding the change in Pb speciation in the lung and

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following its solubilization and re-precipitation in the stomach and small intestine

361

respectively.

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Figure 4 details 2D XRF images of particles selected for XANES analysis (left, Pb = red, Zn

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= green, Fe = blue), the LCF fits (middle) and Pb speciation (right, weighted %) in the lung

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(A = 0.25 h, B = 2 h, C = 8h) and small intestinal tract (D = 8 h). Figure 4A shows that at

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0.25 h, Pb speciation in the lung was predominantly galena (21-80.1%) and mineral sorbed

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species (20.3-77.1%), while anglesite was present in one particle. Pb speciation in the 0.25 h

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lung differed from that observed in PP-D prior to instillation, in particular, the absence of

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organically bound Pb, Pb-phosphates [pyromorphites + Pb3(PO4)2] and hydrocerussite (Table

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1). Higher weighted % of galena and mineral sorbed species in lung residuals were expected,

370

as these Pb species exhibit low solubility57. However, because the 2D XRF image of the 0.25

371

h lung (Figure 2A) showed that Pb was not evenly distributed, the speciation results at this

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initial time point may not be representative of the whole lung. Although no major change in

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Pb speciation was observed in the 2 h lung (anglesite = 74.3%, mineral sorbed Pb = 18.4 -

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53.5% and galena = 10.5 - 79.8%), organically bound Pb (17.2%) and Pb-phosphates (6.3 –

375

35.5%) were present (Figure 4B). In contrast, XANES analysis of the 8 h lung revealed that

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Pb speciation was predominated by mineral sorbed Pb (27.4 - 60.4%) and Pb phosphates (18



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– 32%), with low levels of galena (6.3 and 48%), organically bound Pb (16.7 and 37.9%) and

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Pb(OH)2 (39%) (Figure 4C). Lung fluid is rich in organic molecules (e.g. high molecular

379

weight glycoconjugates, albumin, mucins and enzymes) and phosphorus as glucose-6-

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phosphates58,59. An increase in the weighted % of organically bound Pb and Pb-phosphate

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species at 8 h suggests that a fraction of Pb ions that were not absorbed during this time may

382

have reacted with phosphates and organic ligands in lung fluid (Figure 4C). The increase in

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mineral sorbed Pb in lung residual particles may be a result of sorption of Pb onto Zn or Fe

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oxides, which has been reported to occur at neutral pH49.

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In addition to lungs, XANES analysis of particles in the small intestine at 8 h was also

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undertaken to identify Pb species following lung clearance and ingestion of PP-D post

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instillation. Figure 4D indicates that low amounts of mineral sorbed Pb and Pb-phosphates

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were present in the small intestine, presumably due to the dissolution in the acidic conditions

389

of the stomach and re-precipitation in the small intestine. Although Pb-phosphates are

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considered to exhibit low solubility in stomach acid57, dissolution of hydroxypyromorphite in

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vivo (Balb/C mice) has been previously reported50. In Juhasz et al., Pb contaminated soil (

Dynamics of lead bioavailability and speciation in ... - ACS Publications

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