Impact of humidity on speciation and bioaccessibility of Pb, Zn, Co and Se in house dust

Abstract

Understanding bioaccessibility, defined as the fraction of an element that is soluble under gastrointestinal conditions, has become an important component of human health risk assessments. This study investigated the impact of humidity on the bioaccessibility of lead (Pb), zinc (Zn), cobalt (Co) and selenium (Se) in house dust. Three dust samples (two vacuum samples and NIST 2584 Indoor Dust) were weathered in an oxygenated humidity chamber for four months. Bioaccessibility was determined in original and weathered samples using a simulated gastric extraction followed by Inductively-Coupled Plasma Optical Emission Spectrometry (ICP-OES) and/or Mass Spectrometry (ICP-MS) as required. Exposure to humid conditions significantly increased bioaccessibility in all samples by 45–93% for Pb and by 21–65% for Zn. In contrast, bioaccessibility of Se was significantly decreased in all three weathered samples (by 75–93%), whereas bioaccessibility of Co displayed a variable response (16% increase; no significant change; 35% decrease respectively). Synchrotron XAS analysis of Pb speciation in the original and weathered dust samples determined that Pb transformed from less bioaccessible inorganic species towards more bioaccessible organic species under humid conditions. These results indicated that chemical transformations in damp microenvironments where house dust accumulates, such as window troughs or the base of carpets, may increase the bioaccessibility of particle-bound elements. Changes in bioaccessibility caused by weathering were shown to be significant in the context of the nationally representative Canadian House Dust Study (n = 1025). The national datasets indicated that bioaccessibility is element-dependent (median 59% for Pb; 67% for Zn; 33% for Co; and 13% for Se). Variations in the frequency distributions were attributed to site-specific differences in speciation and the varied bioaccessibility of individual species. The importance of speciation in determining overall bioaccessibility was further demonstrated by measurements of dozens of Pb, Zn and Co compounds that could feasibly occur in house dust. It was concluded that interactions between these elements and the organic matter content of house dust are key to understanding element bioaccessibility and accumulation behaviour in the indoor environment.