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Samplings of indoor PM were conducted at three behavior spaces of a factory worker working in Mando Corp., i.e., the living room of his house (35.946N; 126.955E), the target room in factory (35.960N; 127.008E), and a restaurant (35.964N; 126.958E) using mainly on weekdays (see Fig. 1).

Fig. 1. 
Locations of the selected three behavior spaces of a factory worker in Iksan, Korea.

His all daily life was made within Iksan city. Iksan city is located in the southwestern part of South Korea, and it is a modern industrial city and headquarters to numerous factories producing textiles, jewelry, metal, electrical products, and machinery. Mando Corp. completed an auto parts (e.g., steering/suspension system) manufacturing plant in Iksan city in september of 1995. His living room with tobacco smoke-free, fireplace-free, and moderately maintained floor, wall/ceiling coverings, and furnitures was selected as the place of home measurement. The popular restaurant he was using mainly on weekdays was equipped with an wood interior, an open kitchen with wood cabinet and modern appliances, wood tables, plastic cover chairs, and tiled floor.

An intensive field survey of PM was conducted at each site described above. For the sampling of PM as a function of its size, an Andersen air sampler (Tokyo Dylec Co., AN-200) was operated at each site (see Fig. 1). PM was collected onto a 80 mm diameter Nuclepore ^® filter at the 3rd and the 5th stages. Sampling duration time was 24 h and the airflow was maintained at approximately 28.3 L min^-1. The 50% cut-off sizes of the 3rd and the 5th stages are 3.3 and 1.1 μm, respectively.

After sampling, the filters were placed in clean sterilized petridishes. And then each petridish was sealed with Teflon tape and wrapped with aluminum foil and was placed in a cold storage bag during air transportation. Blank filters were handled in the same manner as the samples.

Elemental specification (i.e., relative elemental content and distribution in and/or on individual particles was performed by the micro-PIXE system equipped at the facilities of the TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) in Japan atomic energy research institute.

Micro-PIXE relatively recently debuted by the combination of PIXE analysis and the micro-beam technique is one of the most powerful micro-analytical techniques having an excellent sensitivity. Minimum Detection Limits (MDL) (for a typical 3- to 4-minute analysis of an accumulated charge of 2.5 microcoulombs) are in the range of sub-ppm-10 ppm, with precisions of 1-10% for a surface layered sample. Quantity of an element in a sample can be measured by counting the number of characteristic X-rays. The X-ray yield for the characteristic X ray of element A (Y_A) can be calculated by following equation:

where N_A is the number of element A in sample, Q is the total number of incident particle, dΩ is a solid angle, E is detecting rate of detector, R_a is the absorption rate of characteristic X-ray, σ^x is a section area of characteristic X-ray generation, S is beam spot, respectively.

The overview of the analysis facility and analytical principle/conditions of TIARA’s micro-PIXE can be referred to other papers (Ma, 2010; Sakai et al., 2005).

By means of micro-PIXE analytical technique, a total of 350 particles randomly sectected from each coarse and fine PM collected at three different sampling sites were analyzed.

Since the particles often overlap and form clusters on the impaction media of Andersen sampler, the edge portion of a particle conglomeration was the target of micro-PIXE analysis.

An example of micro-PIXE X-ray spectrum (top) and elemental maps (bottom) for several individual particles in PM_2.1-1.1 is shown in Fig. 2. It was possible to draw obviously separated elemental spectra from the micro-beam scanning of a 10×10 μm² area. Corresponding to micro-PIXE imaging with scanning transmission ion microscopy, it also enables us to obtain the distributions of trace elements of individual particles. Ca, Cl, P, and Fe were principally detected in and/or on several individual particles and their localizations were varied in each particle. From the visualized elemental distribution for individual particles, it is possible to know the particle-to-particle variation in composition.

Fig. 2. 
A micro-PIXE X-ray spectrum (top) and elemental maps (bottom) drawn from several particles (PM_2.1-1.1) collected at a home.

The number of X-ray counts for each element of individual particles and filter background at the region of interest (ROI) (i.e., energy range of the target element) could be calculated by normalizing with the dose (nC) of 3.0 MeV H⁺ micro-beam. Fig. 3 shows the box plots (left) and scatter grams for several selected elements (right) of micro-PIXE elemental net-counts of individual particles in PM_4.7-3.3 collected at a home.

Fig. 3. 
Box plots (left) and scatter grams (right) of micro-PIXE elemental net-counts of individual particles (PM_4.7-3.3) collected at a home.

On the basis of box plots of elemental net-counts, every element was classified into three groups, i.e., a group of high net-counts (Na, Al, and Si), a group of intermediate net-counts (Mg, S, Cl, K, and Ca), and a group of minor trace elements (P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb). Among a group of high net-counts, Si shows a preponderantly high net-count compared with other two elements. In view of the particle size, it is suggested that the mineral elements such as Mg, Al, Si, K, and Ca were originated from soil.

Household PM may be accumulated from both internal and external sources over long periods of time. Soil particles can be easily drawn into the indoor from outside by one’s shoes, especially in the case of no partition between the front door and the living room. Na and Cl showing high interrelationship with 0.85 R² level were considered to be of marine origin because the target home is geographically close to the sea, A good correlation (R²=0.68) shown between Co and Fe was attributable to the fact that cobalt and iron were chemically alike in many respects and occur together in many geologic materials (Kubota, 1958).

One of high-hazard characteristic sources in home is a coat of paint on the walls. If people live in an older home, many of paint chips get pulverized to microscopic particles that become part of the interior dust people breathe (Salo et al., 2008). In the present study, to classify the paint origin into non-paint sources for the elements in individual coarse particles collected at a home, the enrichment factors (EF) was applied. As a simple means to assess the sources of elements in PM, the EF has long been used (Reimann and Caritat, 2000).

Although the non-lead paint is widely used recently, Ti and Ca are known as its major components. Fe and S are also the trace element found in lead-free paint (Aichi electrical engineer, 2008). EFs of several elements including Ti, Ca, Fe, and S were calculated by the following equation.

where, n-C_x,p and n-C_ref,p are the net-counts of x element and the referantial element in indoor particles, respectively. C_x,paint and C_ref,paint are the concentrations of x element and the referantial element in paint, respectively.

As the suitable referential marker for paint source, Ti was selected in this study. The relative concentrations of elements in lead free paint were based on the data reported by Aichi electrical engineer (2008). Fig. 4 shows the EFs of individual coarse particles collected at a home. The highest EF was shown at Si and relatively high EFs were found at P, S, and Ca, while Fe and Zn showed very low EFs. The fact that Fe is one of trace elements in lead-free paint is probably why its EF was very low. As mentioned above, although Ca and S are also detected lead-free paint, the results of EF indicate that they were not originated lead-free paint.

Fig. 4. 
Enrichment factors of individual coarse particles (PM_4.7-3.3) collected at a home.

Rasmussen et al. (2001) investigated the element profiles of indoor dust versus exterior soils from 50 residences located in Ottawa, Canada and reported that the level of heavy-metal contamination in household dust was higher than that in garden soil. The results of EF therefore suggest that several heavy metals like Fe and Zn can be generated from the sources within the house itself including paint.

In general, it is expected that larger particles are more easily generated than small ones in industrial work places like auto parts manufacturing plant and electronic waste recycling facility. Kim et al. (2015) reported that the mass concentration of coarse particle (PM_10-2.5) was 3-4 times more abundant than those of fine/ultrafine PM (<2.5 μm) in electronic waste recycling facility.

Fig. 5 shows the box plots and scatter grams of micro-PIXE elemental net-counts of individual coarse particles (PM_4.7-3.3) collected at the indoor of Mando Corp.

Fig. 5. 
Box plots (left) and scatter grams (right) of micro-PIXE elemental net-counts of individual particles (PM_4.7-3.3) collected at the indoor of factory.

Compared to the same size range PM collected at home, one of unusual points is that the net-counts of mineral components were none too high. The strict measure not carrying exterior soil dust into indoor workplace might have led to this result. The correlation plot between Na and Cl and those net-counts indicate that sea-salts, on the other hand, might not be completely blocked. High elemental net-counts of Na and Cl also suggest that their other sources might exist in indoor workplace. Another exceptional feature shown in the individual PM in workplace is that Cr, Mn, and Co clearly formed their ne-counts. Among them, a significant correlation was found between Cr and Mn (R²=0.99). Co net-counts were found to correlate more highly (R²=0.97) with the Fe in PM of workplace than that in PM of home (R²=0.68). Ibanez et al. (2010) suggested that although, the bioaccessibility of heavy metals depends on chemical speciation and particle size, those of Cr, Mn, and Co are 10-50%, 40-60%, and 15-50%, respectively. The health effects resulting from breathing air containing chromium (chromium(III) and chromium(VI)) particles are well known. Longterm deposition of high levels (100 to 1,000 times higher than those found in the natural environment) in lung has been associated with lung cancer in steel workers (Toxicological profile for chromium, 1992).

In order to classify individual particles collected at the indoor of factory, the micro-PIXE elemental net-counts were subjected to cluster analysis. The agglomeration scheduled hierarchical cluster analysis can identify relatively homogeneous groups of variables based on selected characteristics. In this study, the individual particles that all components (from Na to Pb) were detected were the subject of the cluster analysis. Table 1 summarizes the result of cluster analysis for the individual coarse particles (PM_4.7-3.3) at the indoor of factory. It shows that individual particles were categorized into three clusters. Cluster 1 with a large portion of total particle population (73.5%) was found to be the particles with sea-salt/mineral>>sulfur>>> trace (Co, Cr, and Mn). Cluster 2 with a small account rate of total particle population (18.4%) was showing sea-salt>>mineral/sulfur>trace (Co, Cr, and Mn). Cluster 3 with a poor particle number (8.1%) had high mass for sea-salt and mineral with coexisting of trace elements (Co, Cr, and Mn). Clustering of individual particles suggests that the individual coarse particles (PM_4.7-3.3) collected at the indoor of factory were chemically heterogeneous and most particles could be clustered with highly dissimilar dendrogram which expresses the modification of individual particles with sea-salt, mineral, and artificially derived elements.

Compared to labor and sleep, even though mealtime is short, it is meaningful to investigate PM properties collected at restaurant used by a factory worker in a regular every day. Since the popular restaurant was equipped with an open kitchen with the most thorough ventilation, it can be thought that PM were mainly the externally inflowed coarse PM due to customer access.

Fig. 6 shows box plots and scatter grams of micro-PIXE elemental net-counts of individual coarse particles (PM_4.7-3.3) collected at the indoor of restaurant. A notable feature is the enrichment of mineral components like Si, Al, and Ca, Na and Cl, as the next major components, were presumably derived from sea-salt. Their interrelationship was very high with 0.91 R² level. The scattering plot between Co and Fe shows a significant positive correlation (R²=0.78). As a result, the principal components in individual PM collected at restaurant were sea-salt and mineral without mixing with harmful trace elements like Cr and Mn.

Fig. 6. 
Box plots (left) and scatter grams (right) of micro-PIXE elemental net-counts of individual particles (PM_4.7-3.3) collected at the indoor of restaurant.

Due to the limitations of current micro-PIXE technologies for elemental quantification, the sufficient assessment of the absolute elemental amount (e.g., μg m^-3 or μg m^-2) according to the PM sampling place could not be performed in the present study. However, the valued elemental net-count of individual PM at high sensitivity will be very useful indicator for the evaluation of the PM characteristics of each indoor place.

Fig. 7 shows the comparison of micro-PIXE elemental net-counts of individual fine particles (PM_2.1-1.1) collected at three indoor locations. The elements related to marine and soil still tend to have a overwhelmingly higher net count than others at all three sites. Among them, the net-counts of Cl and Na appeared unusually high in the PM collected at the indoor of factory. Owing to geographical features of sampling locations, it is easily taken into account that sea-slats will have a strong impact on the elemental composition in the indoor PM. Add to this, sodium carbonate used as machine dishwashing products and surface cleaners might have contributed to the unusually high net-counts of Cl and Na because its impurities may include NaCl, Na₂SO₄, CaCO₃, NaHCO₃, and Fe. Among various heavy metals, the net-count of Cr in the PM collected at the indoor of factory was noticeably higher than those of two other indoor sites. Cr (ZnCr₂O₇ and Cr₅O₁₂) is used as a corrosion-resistant coatings on numerous vehicle components. A thorough measure is needed in terms of workers’ health hazards because the elemental Cr and Cr(VI) can have adverse human health effects (Jacobs and Testa, 2004). In addition, several trace elements, including Pb and Co, also showed high net-counts in the PM collected at factory. Pb as an alloy in Al widely used in wheel rims, engine parts, and window levers. Co is a metal used in corrosion- and wear-resistant alloys and it is also used to make airbags in automobiles (Cobalt statistics and information, 2017).

Fig. 7. 
Comparison of micro-PIXE elemental net-counts of individual fine particles (PM_2.1-1.1) collected at three indoor locations.

Comparing restaurant and home indicates that relatively light elements (from Na to Ca) were more enriched in the PM of home. Meanwhile, in the case of some toxic heavy metals (e.g., Co, Ni, and Cu), the elemental net-counts were much higher in the PM of restaurant than in those of home. If a factory worker breathe the same amount of particles from home and factory in his daily life, the PM of factory containing much more harmful heavy metals have far greater influence on his health.