Methodological factors influencing inhalation bioaccessibility of metal(loid)s in PM_2.5 using simulated lung fluid

Highlights

• Fine air particulate matter (PM_2.5) may act as a sink for toxic metal(loid)s.

• Solid to liquid ratio may affect bioaccessibility of metal(loid)s using ALF.

• ALF results in significantly higher metal(loid) bioaccessibility than PSF (p < 0.001).

• PM_2.5 assessment using ALF and 1:5000 for 24 h is a conservative bioaccessibility approach.

Abstract

In this study, methodological factors influencing the dissolution of metal(loid)s in simulated lung fluid (SLF) was assessed in order to develop a standardised method for the assessment of inhalation bioaccessibility in PM_2.5. To achieve this aim, the effects of solid to liquid (S/L) ratio (1:100 to 1:5000), agitation (magnetic agitation, occasional shaking, orbital and end-over-end rotation), composition of SLF (artificial lysosomal fluid: ALF; phagolysosomal simulant fluid: PSF) and extraction time (1–120 h) on metal(loid) bioaccessibility were investigated using PM_2.5 from three Australian mining/smelting impacted soils and a certified reference material. The results highlighted that SLF composition significantly (p < 0.001) influenced metal(loid) bioaccessibility and that when a S/L ratio of 1:5000 and end-over-end rotation was used, metal(loid) solubility plateaued after approximately 24 h. Additionally, in order to assess the exposure of metal(loid)s via incidental ingestion of surface dust, PM_2.5 was subjected to simulated gastro-intestinal tract (GIT) solutions and the results were compared to extraction using SLF. Although As bioaccessibility in SLF (24 h) was significantly lower than in simulated GIT solutions (p < 0.05), Pb bioaccessibility was equal to or significantly higher than that extracted using simulated GIT solutions (p < 0.05).

Introduction

Epidemiological studies have consistently linked chronic and short term exposure to fine particulate matter with an aerodynamic diameter of <2.5 μm (PM_2.5) to adverse health outcomes, e.g. cardiovascular and pulmonary morbidity, mortality, increased risk of chronic obstructive pulmonary disease (COPD), pneumonia and lung cancer mortality (Fajersztajn et al., 2017; Pinault et al., 2017; Pope and Dockery, 2006; Pun et al., 2017). The main constituents of airborne PM_2.5 include sulphate (20%), crustal materials (soil, sand, road and desert dust; 13.4%), equivalent black carbon (11.9%), NH₄NO₃ (4.7%), sea salt (2.3%), trace element oxides (1%), water (7.2%) and residual matter (40%) (Snider et al., 2016). A systemic review and meta-analysis of mortality and hospital admissions associated with daily PM_2.5 in 110 peer reviewed studies (until May 2011) found that an increase of 10 μg m³ PM_2.5 was linked to an increased risk of hospital admissions and 1.04% rise in mortality (Atkinson et al., 2014). Furthermore, 1.51% of the rise in mortality risk was associated with respiratory illnesses (e.g. COPD, asthma, lower respiratory infections) and 0.84% rise in mortality risk associated with cardiovascular causes (e.g. heart failure, stroke, ischemic heart disease and dysrhythmia) (Atkinson et al., 2014). Recent research highlighted that metals in PM_2.5 (e.g. Cu, Fe, Mn and Ni) collected at traffic intersections may have considerable reactive oxygen species generation capabilities (Fujitani et al., 2017). Additionally, metals in aqueous extracts of PM_2.5 were demonstrated to cause lung inflammation and injury, oxidative stress, lipid and protein damage and cardiovascular injury in mouse and rat models (Gavett et al., 2003; Pardo et al., 2016; Shuster-Meiseles et al., 2016). Similarly, Fe, Cu, Ni, Co and Cr in PM_2.5 were also associated with inflammatory responses in mouse type II alveolar cells by increasing the expression of pro-inflammatory genes and proteins (He et al., 2017). Therefore, although metal oxides may comprise a small proportion of PM_2.5, it may represent substantial potential to cause human health injuries. Furthermore, because of its small mass, PM_2.5 may stay airborne for extended periods and travel long distances, impacting communities far from point sources.

Instead of using total meta(loid) concentration for human exposure assessment, it is more relevant to use the concentration of metal(loid)s in PM_2.5 that may potentially dissolve in lung fluid and be absorbed into the blood (Leclercq et al., 2017; Li et al., 2016; Pelfrêne and Douay, 2017). Although using metal(loid) bioavailability (i.e. metal(loid)s absorbed into the systemic circulation in-vivo) remains the most appropriate for exposure assessment, metal(loid) bioaccessibility (i.e. metal(loid)s extracted using simulated lung fluid (SLF) in-vitro) is often more desirable as a rapid and cost effective approach (Mukhtar and Limbeck, 2013a; Stopford et al., 2003). PM_2.5 may stimulate engulfment by lung macrophages in the respiratory system (d'Angelo et al., 2014) and metal(loid) dissolution may take place within the acidic environment of phagolysosomes (Kanapilly, 1977). Several SLFs have been developed to simulate phagolysosomal fluid, e.g. simulated intracellular fluid (Thelohan and De Meringo, 1994), artificial lysosomal fluid (ALF) (Midander et al., 2007; Stopford et al., 2003) and phagolysosomal simulant fluid (PSF) (Stefaniak et al., 2005), the latter two being more popular in bioaccessibility assays (Kastury et al., 2017). However, the extraction efficiencies of these SLFs with an acidic pH of 4.5 have not been compared. Additionally, significant knowledge gaps exist in methods currently used to determine metal(loid) bioaccessibility in PM_2.5 (Kastury et al., 2017; Wiseman, 2015). For example, the solid to liquid (S/L) ratio ranged from 1:100–1:1163 (Hamad et al., 2014; Potgieter-Vermaak et al., 2012; Wiseman and Zereini, 2014) or not reported because a part of the filter paper with which the particles were collected was used directly in assays (Mukhtar and Limbeck, 2013a; Schaider et al., 2007). Varying agitation frequencies and types have been used in the literature, for example, occasional (Wiseman and Zereini, 2014), continuous (Hamad et al., 2014; Potgieter-Vermaak et al., 2012) or ultrasonic (Mukhtar and Limbeck, 2013a). Large variability in extraction time is also observed in the literature, such as, 1 h in Mukhtar and Limbeck (2013a), 120 h in Schaider et al. (2007) or 30 days in Zereini et al. (2012). Furthermore, incidental ingestion of surface dust may be considered an important pathway for Pb exposure in children due to frequent hand to mouth activity (Scheckel et al., 2013). However, limited research has been conducted on the oral bioaccessibility of PM_2.5 from Pb mining/smelting impacted region.

This study aimed to investigate how assay parameters affect metal(loid) bioaccessibility outcomes using PM_2.5 in order to recommend a standardised method. In addition, oral bioaccessibility of PM_2.5 metal(loid)s was also investigated to compare inhalation exposure to incidental ingestion of surface dust. To achieve this aim, the effect of solid to liquid (S/L) ratio, agitation, SLF composition and extraction time on metal(loid) bioaccessibility in PM_2.5 was assessed and compared to bioaccessibility results using simulated GIT solutions.