Dust Dynamics: Distribution Patterns of Semi-Volatile Organic Chemicals across Particle Sizes in Varied Indoor Microenvironments

doi:10.21203/rs.3.rs-3748518/v1

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Dust Dynamics: Distribution Patterns of Semi-Volatile Organic Chemicals across Particle Sizes in Varied Indoor Microenvironments

https://doi.org/10.21203/rs.3.rs-3748518/v1

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published 10 May, 2024

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An extensive analysis of the distribution patterns of three distinct classes of semi-volatile organic chemicals (SVOCs)—phthalates (PAEs), organophosphate flame retardants (OPFRs), and polycyclic aromatic hydrocarbons (PAHs) — across four discrete size fractions of dust (25, 50, 100, and 200 μm) was conducted. The dust samples were sourced from AC filter, covered car parking lots, households, hotels, mosques, and car floors. To generate the four fractions, ten dust samples from each microenvironment were pooled and sieved utilizing sieving apparatus with the appropriate mesh size. Selected SVOCs were quantified utilizing gas chromatography-mass spectrometry in electron impact (EI) mode. Results revealed varying degree of contamination with dust fractions from car parking lots dust showed the lowest chemical contamination, while car floor dust was the most contaminated. SVOCs concentrations exhibited a tendency to increase as the size of dust particles decreased, particularly evident in car floor and car parking lot dust across the microenvironments analyzed. Among SVOCs, PAEs emerged as the predominant contributors with >90% followed by OPFRs and PAHs. Among analyzed microenvironments, car floor dust exhibited the highest levels of PAHs and OPFRs, this align logically with the fact that numerous interior components of cars are treated with OPFRs to adhere to fire safety regulations. Furthermore, petroleum products are a major source of PAHs in the environment and all the sampled cars in the study had combustion engines. Consequently, car dust is more likely to be polluted with PAHs stemming from petroleum combustion. Heavy metals and brominated flame retardants have increased with decreasing dust particles in a few investigations, however, this is the first study analyzing these SVOCs in different fractions of dust from various microenvironments. However, aside from two specific microenvironments, the observed pattern of escalating SVOC concentrations with smaller dust particle sizes was not corroborated among the examined microenvironments. This divergence in concentration trends suggests the potential involvement of supplementary variables in influencing SVOC distributions within dust particles.

SVOCs

Polycyclic aromatic hydrocarbons

Phthalates

Dust fractions

People spend the majority of their time indoors in modern times, whether at home, at work, participating in indoor leisure activities, or traveling [Kelly and Fussell, 2019]. While indoors, people get exposed to a number of chemicals that are used in consumer products, such as halogenated flame retardants, plasticizers such as phthalates, pesticides, etc. [Salthammer, 2020]. Other chemicals, like polycyclic aromatic hydrocarbons (PAHs), are made indoors by things like cigarette smoke, the smoke from stoves and fireplaces, and so on [Singh and Agarwal, 2022]. A number of studies have reported settled dust as a surrogate for indoor pollution [Ali, 2019; Bu et al., 2020]. Indoor dust is a complex mixture of several things, such as outdoor dust, wear and tear of textiles and furniture, dead skin, hair, dandruff, and emitted organic chemicals [Escher et al., 2020]. After emission from the source, semi volatile organic chemicals (SVOCs) adsorb to the surfaces of particulates, which indicates settled dust as a sink for these environmental chemicals [Escher et al., 2020]. Of the SVOCs, organophosphate flame retardants (OPFRs), and phthalates added to a number of household items and building materials, over time many of these chemicals are released from the treated products and partitioned among air, suspended particles, and settled dust according to chemical properties [Ali et al., 2023; Tang et al., 2020]. Several studies have reported the ubiquitous occurrence of PAHs, OPFRs, and phthalates in indoor settled dust [Ali et al., 2023; Tang et al., 2020; Wang et al., 2021]. Of particular concern is the escalating utilization of OPFRs, given their increasing production and usage in recent years. The annual production volume of OPFRs alone has surged, reaching an estimated 598,422 metric tons [Huang et al., 2022]. This rise in usage and production of OPFRs is primarily due to regulations that restrict the use of commercial formulations of polybrominated diphenyl ethers (PBDEs) [Huang et al., 2022]. Consequently, indoor monitoring of these chemicals becomes imperative as their exposure has been associated with various adverse health effects. Exposure to OPFRs and phthalates has been linked to detrimental impacts on the immune system, thyroid function, respiratory health, reproductive health, and neurodevelopment, including attention-deficit/hyperactivity disorder (ADHD) in children [Golestanzadeh et al., 2020; He et al., 2018; Kim et al., 2013; Kojima et al., 2013; Kumar et al. 2020; Zhao et al., 2022]. Similarly, exposure to PAHs is associated with carcinogenicity and deleterious effects on the respiratory, cardiovascular, reproductive, developmental, and immune systems [Kim et al., 2013; Wang et al., 2021]. Humans are exposed to these chemicals on a daily basis by involuntary ingestion, inhalation, and dermal contact of dust particles. This is especially concerning for the most vulnerable groups, such as infants, young children, and the elderly, who spend most of their time indoors with developing and compromised immune systems. Young children are especially vulnerable to a greater intake of chemicals due to their hand-to-mouth behavior and rapidly developing bodies; therefore, they are at a higher risk of developing side effects due to rapid cell proliferation [Het et al., 2018; Isa et al., 2020]. Therefore, it is important to understand the occurrence of various chemicals in our indoor settings so that we lower the exposure by taking various measures, such as legislative ones.

Dust is made up of different-sized particles, and a few recent studies have shown that the amount of chemicals in dust may change depending on the size of the particles [Cao et al., 2019; Zhou et al., 2019]. The particle size of most of the settled dust may range from > 2 mm to 25 µm, with approximately one third of the dust consisting of dust particles smaller than 500 µm [Mercier et al., 2011]. However, this may vary for different indoor microenvironments, e.g., households, offices, public places, etc. The finer fractions of the settled dust are of great interest due to their smaller particle size, which is available for uptake via inhalation, ingestion, or dermal contact [Mercier et al, 2011]. The composition of settled dust is a mirror of the nearby surroundings; therefore, it is important to study the chemical profiles of different indoor microenvironments to understand a thorough risk assessment for exposure to indoor pollutants [Mercier et al., 2011; Zhou et al., 2019]. However, one important factor to understand is the presence of organic pollutants in various sizes of dust particles. The size dependence of organic pollutants is not well studied but is very important because the specific area of particles is indirectly proportional to the particle size. The mass transfer of chemicals is reported to be higher with finer particles, which make hazardous chemicals attached to them readily available for uptake [Mercier et al., 2011; Zhou et al., 2019]. Therefore, the main aims of this study was to conduct a comprehensive comparison of different SVOCs among different size fractions (< 25 (F1) µm), 50 − 25 (F2), 100 − 50 (F3), and 200 − 100 (F4)) of settled dust and study the factors for these differences. At the same time, morphological difference, and possible impact of different environmental factors on the levels of SVOCs in different fractions size was studied. To our knowledge, the novelty of present study emerges from the explicit focus on SVOCs that, to date, have not been investigated with respect to dust particle size in various indoor microenvironments. These dust samples were collected from different indoor microenvironments (household, mosque, hotel, AC filter, car, and car parking) in Saudi Arabia. This unique approach serves to expand our understanding of the intricate relationships between SVOCs and particle dimensions, filling a significant gap in the existing body of research.

2.1. Chemicals and solvents

Tris (1-chloro-2-propyl) phosphate (TCPP), Tris (2-chloroethyl) phosphate (TCEP), Tris (1, 3-dichloro-2-propyl) phosphate (TDCPP), and Triphenyl phosphate (TPhP) were bought from AccuStandards as analytical standards for OPFRs. Analytical standards for PAHs and phthalates were purchased from Sigma Aldrich in a mixture named EPA-525 semi volatiles and quantified using internal standards (i) acenaphthylene (Acy-d10), phenanthrene (Phe-d12), and chrysene (Cry-d12). PAHs namely acenaphthylene (Acy), anthracene (Ant), benz (a) anthracene (BaA), benzo (a) pyrene (BaP), benzo (b) fluoranthene (BbF), benzo (g,h,i) perylene (BghiP), benzo (k) fluoranthene (BkF), chrysene (Chr), dibenz (a, h) anthracene (DahA), fluoranthene (Flu), indeno (1,2,3-cd) pyrene (IcdP), phenanthrene (Phe), and pyrene (Pyr) were analyzed along with benzyl butyl phthalate (BzBP), bis (2-ethylhexyl) adipate (DEHA), bis (2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), diethyl phthalate (DEP), and dimethyl phthalate (DMP). Acetone, dichloromethane (DCM), n-hexane (n-Hex), and iso-octane were of analytical grade and obtained from Sigma Aldrich. All the glass used was baked at 400°C overnight and kept at 100°C until use.

2.2. Sampling

Households (n = 10), AC filters (n = 10), cars (n = 10), mosques (n = 10), hotels (n = 10), and covered car parking lots (n = 5) in Jeddah, Saudi Arabia, were used to collect dust samples for this study. Ali et al. [2016; 2018] have presented a comprehensive methodology for sampling indoor environments, encompassing mosques, homes, AC filters, cars, and hotels, to analyze a range of SVOCs. For collecting dust samples from individual households and mosques, freshly accumulated dust from the top of the vacuum cleaner bag was taken. The collected dust was carefully placed on aluminum foil, securely wrapped, and preserved in sealed zip bags. In the case of hotel floor dust, a thorough cleaning of the vacuum cleaner was performed prior to use. The entire room floor was then vacuumed to have enough samples size. When dealing with AC filters, a different technique was employed. Dust samples were collected by gently brushing the filters onto aluminum foil. To prevent any cross-contamination, brushes exclusively designated for each respective location were utilized after thorough pre-cleaning.

To collect dust samples from car, the interior of the vehicle, including the dashboard, seats, and trunk, was subjected to vacuuming. Importantly, the car floor was excluded from this process because of tracked in dust with shoes. Prior to each sample collection, the vacuum cleaner underwent meticulous cleaning to eliminate any potential sources of cross-contamination. For car parking, a vacuum cleaner was used to collect dust for 5 minutes, and before each sample, the vacuum cleaner was well cleaned to prevent cross contamination. We carefully selected covered car parking facilities from various malls in Jeddah to collect dust samples. All the collected samples were immediately transported to the lab and stored at -20°C. For this study, dust samples from each microenvironment were pooled to get enough sample size to have four fractions. Different sieving apparatuses were used to fractionate the dust according to particle size. The small fraction < 25 microns (µm) was the most difficult to obtain because the fractions of dust with smaller dust particle sizes were less dense than the other three fractions. The sieved dust sample was kept at -20°C until analysis.

2.3. Sample preparation and quantitative analysis

The samples extraction and purification are described elsewhere [Ali et al., 2016; 2018]. Briefly, an accurately weighed aliquot of dust samples (typically between 50 and 75 mg) were spiked with internal standards and 3 mL of hexane: acetone 4:1, v/v added. Samples were covered with aluminum foil and left overnight at room temperature to equilibrate. Samples were extracted three times by ultra-sonication for 20 min each time, with 3 mL of hexane/acetone (4/1, v/v), followed by centrifugation at 3500 rpm for 10 min. The pooled extracts were evaporated to incipient dryness under a gentle stream of nitrogen and re-solubilized in 0.5 mL of hexane and 50 L of isooctane. The extracts were purified for instrumental analysis using pre-cleaned silica cartridges. Hexane/Dichloromethane (DCM) (4/1, v/v) 8 mL followed by 6 mL Ethyl acetate was used for elution; all elutes were evaporated under nitrogen to incipient dryness and were re-solubilized in 100µL of isooctane prior to gas chromatography–mass spectrometry (GC–MS) analysis. Na₂SO₄ (100 mg), as replica dust, was spiked with a known concentration of selected SVOCs [Ali et al., 2016; 2018]. Spiked Na₂SO₄ samples and procedural blanks for each batch of 5 samples were extracted the same way as other dust samples. The final levels were blank corrected and the recovery of spiked levels were within acceptable range of 77–110% for analyzed SVOCs. To minimize photo-degradation of chemicals all procedures were performed in fume hood without light.

For quantitative analysis, GCMS- TSQ™ 8000 Evo triple quadrupole GC-MS/MS (Thermo Fisher Scientific, Waltham, MA, USA) was employed in selective ion monitoring (SIM) mode. For chemical separation, a fused silica capillary column (Rtx-MS 30 M x 0.25 mm x 0.25 m) was used. For phthalates and PAHs, the injector and ion source were maintained at temperatures of 230°C and 280°C, respectively. Helium served as the carrier gas at a flow rate of 1.5 mL/min. The oven temperature was programmed to start at 100°C and held for 4 minutes, then ramped up to 310°C at a rate of 8°C/min, and held for 10 minutes. PAHs were quantified through monitoring ions with m/z values of 152, 162, 178, 188, 202, 228, 240, 252, 276, and 278. For phthalates, ions m/z 163, 279, and 149 were monitored, except for DZBP where ion m/z 206 was used. For OPFRs, the injector temperature was held constant at 230°C, while the ion source temperature was set at 280°C. Helium was employed as the carrier gas, flowing at a rate of 1.5 mL/min. The oven temperature was programmed to ramp up from 90 to 300°C at a rate of 15°C/min, with a subsequent one-minute hold period at the maximum temperature. Ions with m/z 247, 249 (TCEP), 379, 381 (TDCPP), 277, 279 (TCPP), 325, 326 (TPhP), and 261, 263 (TECP-d12) were monitored for OPFRs quantification.

3.1. Rationale behind selecting indoor microenvironments

The collection of dust samples from varied indoor environments serves a fundamental purpose: the elucidation of the presence and composition of SVOCs, which operate as pivotal indicators of indoor environmental quality. Given the substantial duration that individuals spend indoors in contemporary times, comprehending pollutant levels within these spaces assumes paramount importance.

The selection of dust sampling locations is guided by a rationale that encapsulates the multifaceted nature of indoor environments. The analysis of dust extracted from household floors offers a window into pollutants that originate from routine activities encompassing cooking, cleaning, the interactions of building materials and household articles, and overall occupancy [Ali et al., 2023; Bu et al., 2020; Mercier et al., 2011; Tang et al., 2020; Zhou et al., 2019]. In a parallel vein, the assessment of dust captured by AC filters facilitates the evaluation of filtration system efficiency and the potential presence of suspended contaminants in the indoor air circulation steered by air conditioning systems [Ali et al., 2016]. The inclusion of dust collected from mosques' floors provides a vantage point for understanding the influence of communal spaces on indoor air quality, entailing the intricate interplay of variables like foot traffic and congregational activities [Ali et al., 2012; 2018]. Likewise, the investigation of hotel floor dust patterns contributes to the unraveling of indoor pollutant dynamics, influenced by the transient nature of guests and the varying intensity of activities that ensue [Ali et al., 2018].

Car interiors are confined spaces where individuals spend considerable time during commutes. Dust accumulation on car floors, from shoes, clothing, and external sources (exhaust and road dust), makes it a pertinent medium to study pollutants impacting occupants' health. Along with this car interiors harbor pollutants from diverse sources, including vehicle materials like fabrics and plastics that release compounds based on temperature and humidity [Ali et al., 2016; Yoshida et al., 2006]. The rationale underpinning the selection of covered car parking lots is twofold. Primarily, the substantial influx of shoppers results in numerous individuals utilizing these areas while engaging in shopping activities, potentially contributing to dust accumulation. Furthermore, the presence of drivers awaiting family members introduces an additional exposure pathway, wherein they might come into contact with the chemicals present in the dust particles.

The evaluation of SVOCs across diverse indoor settings yields critical insights with profound implications for the assessment of potential health risks and the formulation of strategies aimed at augmenting indoor air quality and public health standards. Armed with a comprehensive comprehension of indoor environments and the intricacies of SVOC presence, health professionals and policymakers are empowered to make well-informed decisions, thereby upholding the well-being of occupants and fostering the creation of healthier living environments.

With an understanding that fine particles can have profound health implications upon ingestion, a fractionation approach was employed to separate the dust samples into four distinct particle size categories: F1 (< 25 µm), F2 (25–50 µm), F3 (50–100 µm), and F4 (100–200 µm). This fractionation strategy allowed us to study how the distribution of these compounds varies with respect to different particle sizes in selected indoor microenvironments. Remarkably, analysis revealed the presence of selected SVOCs across all analyzed dust fractions from the chosen microenvironments.

3.2. Concentrations and profile of SVOCs in different fractions of dust from different microenvironment

3.2.1. Phthalates

The levels of phthalates in all fractions and microenvironments studied ranged from 19 to 985 ppm (Table 1). All individual phthalates were found in all dust samples with varied concentrations; this indicates wide application of phthalates, which has resulted in their ubiquitous occurrence across indoor environments. Car parking dust exhibited a relatively low concentration of phthalates, ranging between 19 and 65.6 ppm, which is understandable as car parking places are semi covered areas, with limited emission sources of phthalates, and the temperature is also relatively high compared to other indoor microenvironments. Covered car parking areas typically have fewer indoor sources of phthalates compared to other indoor environments because of limited emission sources of phthalates, which are commonly used in consumer products like plastics, vinyl flooring, furniture, household products and personal care items [Erythropel et al., 2014]. Covered car parking areas are often unoccupied or have limited human activity compared to other indoor spaces. Phthalates may be introduced into indoor dust through the shedding of particles from treated products, the reduced human presence and possible emission sources in covered car parking lots may lead to fewer contributions of phthalates to the dust.

Of all the individual phthalates, DEHP emerged as the predominant compound, showcasing its overwhelming presence in all the analyzed dust samples and fractions (Fig. 1; Figure S1-6). This preeminence is attributed to DEHP's extensive use in various polyvinyl chloride (PVC) products, where it is often incorporated without being chemically bound to the materials [Erythropel et al., 2014]. Moreover, DEHP is considered a heavy molecule with a high molecular weight and low vapor pressure [Schripp et al., 2010; Weschler and Nazaroff, 2010]. This might provide possible justification for why it is usually found as the most abundant phthalate in several studies, as these physical parameters help in easy adsorption onto dust and, as a result, deposition on floors, walls, and other surfaces [Shinohara et al., 2019]. High levels of DEHP in studied microenvironments are understandable, since in the present study, dust samples were collected from carpeted hotels, mosques, and semi carpeted houses. Studies have reported a significant association between high concentrations of DEHP in dust and the presence of carpets in homes [Bi et al., 2018]. DEHP was relatively lower in hotel dust than house and mosque dust, even though it is an indoor environment that has similarities with households and mosques indoor settings. This might be explained by the fact that hotels are usually kept clean in order to gain customer loyalty, with fewer plastic products and regular vacuuming. According to Bi et al. [2018], their results showed decreased phthalates and OPFRs in settled dust as a result of frequent vacuuming of carpets. Phthalates with a low boiling point and high vapor pressure such as DEP and DBP were expected to have greater concentrations in finer particles as suggested by some studies; however, it was not very obvious among studied microenvironments, which indicates other unknown factors for their deposition on the dust particles [Białecka-Florjańczyk and Florjańczyk, 2007]. DEHA and DBP were the other prominent phthalates in all studied microenvironments and different dust fractions (Fig. 1; Figure S1-6). This can be primarily explained by the fact that these are the most common plasticizers and are predominantly used in the manufacturing of PVC products (flooring, wallpaper, shower curtains, upholstery, cosmetics, and food containers) as additives in order to provide flexibility [Kawakami et al., 2011].

Table 1

Levels (ppm) of PAHs, OPFRs, and phthalates in dust fractions of various microenvironments of Saudi Arabia.
Chemicals	LOQ	Car floor dust				Car parking dust				AC filter dust				House floor dust				Mosque floor dust				Hotel floor dust
Chemicals	LOQ	25	50	100	200	25	50	100	200	25	50	100	200	25	50	100	200	25	50	100	200	25	50	100	200
Acy	0.002	0.05	0.04	0.03	0.02	0.04	0.03	0.01	0.01	0.01	0.01	0.01	0.04	0.03	0.03	0.03	0.04	0.03	0.04	0.17	0.26	0.02	0.03	0.03	0.03
Flu	0.002	1.00	0.63	0.49	0.41	0.12	0.36	0.24	0.10	0.16	0.17	0.15	0.11	0.38	0.22	0.22	0.26	0.29	0.29	0.24	0.33	0.24	0.32	0.38	0.44
Phe	0.005	7.54	5.29	3.03	2.22	0.66	0.36	0.31	0.33	0.74	1.94	0.63	0.47	1.26	0.80	1.07	1.36	1.99	2.43	2.06	3.01	1.72	1.25	1.37	1.61
Ant	0.005	1.60	0.76	0.39	0.30	0.03	0.02	0.01	0.01	0.91	2.22	0.04	0.54	0.08	0.04	0.07	0.06	0.07	0.09	0.08	0.16	2.74	2.31	1.05	1.77
Pyr	0.002	5.94	3.92	2.28	1.51	1.27	0.65	0.36	0.40	0.44	0.49	0.31	0.17	0.60	0.39	0.39	0.62	0.77	1.01	1.18	1.56	1.24	0.74	0.73	0.87
BaA	0.005	1.32	0.87	0.54	0.35	0.26	0.11	0.09	0.11	0.54	0.31	0.40	0.22	0.27	0.20	0.22	0.23	0.32	0.35	0.34	0.42	0.27	0.27	0.27	0.32
Chy	0.002	2.50	1.63	1.13	0.76	1.00	0.40	0.25	0.33	1.79	0.91	1.22	0.56	0.62	0.45	0.45	0.46	1.17	1.30	1.21	1.53	0.82	0.76	0.83	0.87
BbF	0.005	0.73	0.32	0.22	0.14	0.54	0.20	0.12	0.18	0.20	0.10	0.15	0.62	0.87	0.64	0.53	0.60	0.64	0.61	0.30	0.31	0.28	0.25	0.12	0.43
BkF	0.005	2.46	1.62	0.99	0.38	0.15	0.12	0.19	0.07	1.89	0.94	1.26	0.31	0.26	0.21	0.27	0.21	1.35	1.47	1.44	1.51	0.52	0.44	0.32	0.55
BaP	0.005	2.56	1.52	0.75	0.16	0.20	0.08	0.13	0.07	0.08	0.41	0.47	0.23	0.51	0.36	0.11	0.11	0.67	0.60	0.59	0.38	0.30	0.29	0.27	0.07
IcdP	0.01	0.30	0.95	0.44	0.22	0.25	0.11	0.06	0.10	1.04	0.52	0.75	0.32	0.45	0.29	0.29	0.25	0.33	0.33	0.31	0.32	0.18	0.20	0.17	0.18
DahA	0.01	0.21	0.33	0.13	0.08	0.09	0.05	0.04	0.06	0.18	0.12	0.17	0.07	0.13	0.11	0.13	0.15	0.12	0.14	0.14	0.18	0.09	0.17	0.08	0.09
BghiP	0.01	2.08	1.61	0.72	0.35	0.85	0.36	0.21	0.35	0.57	0.31	0.43	0.19	0.84	0.57	0.53	0.60	0.62	0.64	0.62	0.59	0.27	0.33	0.31	0.31
∑PAHs		28.3	19.5	11.2	6. 9	5. 5	2.8	2.1	2.1	8.5	8.4	6.1	3.9	6.3	4.3	4.3	4.9	8.4	9.3	8.7	10.6	8.7	7.4	5.9	7.5
TCEP	0.015	30.6	17.6	12.9	12.9	0.03	0.00	0.00	0.00	7.73	6.39	7.76	7.68	3.90	3.64	3.39	2.16	1.67	1.61	1.25	0.98	0.71	0.88	0.95	0.63
TCPP	0.015	3.85	3.69	1.84	1.36	0.76	0.27	0.13	0.00	1.81	1.69	2.40	2.08	3.27	3.33	5.14	3.53	1.99	2.26	1.96	3.49	1.57	2.09	1.99	3.30
TDCPP	0.015	8.52	8.19	4.43	2.88	0.05	0.05	0.26	0.05	1.39	1.13	1.98	1.53	1.38	1.17	2.16	1.93	0.89	1.17	0.55	0.99	1.48	1.60	1.76	1.70
TPhP	0.045	0.20	0.20	0.13	0.12	0.00	0.09	0.83	0.20	0.18	0.05	0.18	0.09	0.75	0.85	0.85	0.21	1.78	2.53	0.36	2.63	3.08	3.16	3.11	3.01
∑OPFRs		43.2	29.7	19.3	17.2	0.84	0.41	1.22	0.25	11.1	9.27	12.3	11.4	9.31	8.98	11.5	7.83	6.32	7.57	4.12	8.09	6.85	7.73	7.81	8.64
DMP	0.01	0.41	0.36	0.29	0.54	0.04	0.05	0.10	0.07	0.35	0.34	0.31	0.32	0.72	0.51	0.42	0.32	0.44	0.42	0.53	0.32	0.15	0.15	0.21	0.10
DEP	0.015	2.13	1.42	1.55	1.32	0.35	0.22	0.26	0.14	1.23	1.08	1.40	1.45	2.75	2.17	2.23	2.29	9.57	9.98	11.7	13.6	3.46	3.05	3.51	4.46
DBP	0.4	27.1	21.0	28.9	21.2	7.96	2.49	3.50	1.65	3.40	2.50	3.26	4.87	13.0	10.1	11.1	22.3	25.5	26.6	27.5	39.6	25.7	23.3	30.0	32.0
DEHA	0.015	52.1	35.0	23.5	15.9	7.41	5.98	4.57	2.09	19.8	19.5	23.7	23.6	8.73	6.26	6.87	13.0	62.0	71.8	76.4	103	48.3	46.0	50.0	57.4
DZBP	0.01	1.04	0.77	0.94	0.56	0.79	0.51	0.43	0.33	2.53	2.22	2.51	2.05	0.58	0.42	0.15	0.71	0.69	0.68	0.83	0.93	0.78	0.69	0.83	0.83
DEHP	1.8	693	482	386	273	49.1	36.0	28.1	14.8	520	469	558	554	821	691	653	694	550	607	583	825	368	350	365	461
∑PAE		776	541	441	312	65.6	45.3	36.9	19.0	548	495	589	585	846	710	673	732	648	716	700	985	446	423	450	555
∑Chemicals		845	591	472	337	72.0	48.5	40.2	21.4	568	512	607	601	861	725	690	745	663	733	712	999	462	438	465	572
∑PAHs (%)		3.34	3.30	2.36	2.05	7.58	5.84	5.03	9.93	1.50	1.65	0.99	0.64	0.73	0.59	0.62	0.66	1.26	1.27	1.22	1.05	1.89	1.68	1.28	1.32
∑OPFRs (%)		5.10	5.02	4.09	5.12	1.17	0.84	3.03	1.18	1.96	1.81	2.03	1.89	1.08	1.24	1.67	1.05	0.95	1.03	0.58	0.81	1.48	1.77	1.68	1.51
∑PAE (%)		91.6	91.7	93.5	92.8	91.3	93.3	91.9	88.9	96.5	96.5	97.0	97.4	98.2	98.2	97.7	98.3	97.8	97.7	98.2	98.1	96.3	96.6	97.0	97.2

3.2.2. Organophosphate flame retardants

OPFRs were detected at multifold lower levels than phthalates (Table 1). The results here also show no inverse relation between dust sizes and OPFRs concentrations in all microenvironments except for car dust samples, where OPFRs concentration decreases with increasing particle size (Table 1). Some studies have reported an inconsistency in the correlation between OPFRs concentrations and particle size in indoor environments and household dusts, whereas other studies showed an obvious inverse relation between concentrations and particle size [Li et al., 2019]. Similar to phthalates, levels of OPFRs were highest in car floor dust and lowest in car parking dust (Table 1). In car dust fraction, OPFRs contributed between 4–5% of the total chemical load which was highest contribution of OPFRs among the studied indoor microenvironments (Table 1). This is logical since interior of car is a compact environment and car manufacturers often incorporate flame retardant materials in the interior components of vehicles to comply with safety regulations and reduce the risk of fire hazards. These materials, such as seat cushions, carpets, dashboard components, and other interior surfaces, may contain OPFRs as flame retardants [Ali et al., 2021; Lexen et al., 2021; Zulauf et al., 2019]. Over time, these chemicals may migrate from treated materials to nearby surfaces like car floor dust through wear and tear or the release of dust particles. Compared to other indoor environments, vehicles have relatively limited ventilation and air exchange [Ali et al., 2021; Lexen et al., 2021; Zulauf et al., 2019]. As a result, OPFRs that are released into the cabin air, whether from car interior materials or occupant activities, can accumulate in dust rather than being quickly removed from the environment Lexen et al., 2021].

In the car floor and AC filter dust samples of this study, TCEP showed the highest concentrations among all OPFRs, while TCPP and TDCPP were the other two important OPFRs (Table 1; Fig. 2). The levels of OPFRs also showed an inverse relation between particle sizes for car floor dust. While for other studied microenvironments, no such trend was observed. At the same time, the profiles of the studied OPFRs were also different in other microenvironments, e.g., TPhP was the most important OPFRs in hotel floor dust, while TCPP was the major OPFRs in house and mosque floor dust [Figure 2; Figure S1-6]. This variation in the profile indicates the preference for the use of certain OPFRs in different indoor microenvironments. In car parking dust, OPFRs were detected at the lowest concentration, which is understandable since not many emission sources are used in car parking areas, and plausibly contaminated dust from the interior of cars with shoes tracked in is the major source of these chemicals in car parking dust. The high levels of OPFRs in car floor dust are understandable since a number of electronics parts and other car interiors, such as seating, are treated with these chemicals to comply with fire safety regulations [Ali et al., 2021; Tokumura et al., 2017; Zulauf et al., 2019]. Recent studies have also suggested the presence of higher levels of OPFRs than brominated flame retardants in indoor dust from different countries [Ali et al., 2012; Dirtu et al., 2012; Mizouchi et al., 2015; Tokumura et al., 2017].

3.2.3. Polycyclic aromatic hydrocarbons

Like OPFRs, PAHs were found at multifold lower levels than phthalates. Likewise, for other SVOCs, there was no clear inverse relation between dust sizes and PAH concentrations found for dust fractions from house, mosque, and hotel floor dust. However, PAHs were present at high levels in finer dust fractions collected from AC filters, cars, and car parking areas (Table 1). The compositional pattern of individual PAHs in dust fractions among studied microenvironments was different (Table 1 and Fig. 3). The major contribution to the compositional pattern of PAHs came from 3–4-ring PAHs, with Phe, Pyr, and Chy contributing the major share. However, 5–6 ring PAHs also significantly contributed, especially car and car parking floor dust. Overall, PAHs were present at higher levels in the dust from car floors, which is quite understandable as one of the primary sources of PAHs in car floor dust is automotive exhaust emissions [Dat et al., 2017]. PAHs are formed during the incomplete combustion of fuel in internal combustion engines, particularly in vehicles powered by gasoline or diesel [Dat et al., 2017]. These exhaust emissions contain a wide range of PAHs, which can settle onto the car floor and become part of the accumulated dust. PAHs in car floor dust can undergo re-emission into the vehicle's cabin air under certain conditions, especially in warm weather [Oliveira et al., 2019]. This re-emission process can further increase PAH levels inside the vehicle. Other sources such as human activities inside vehicles, such as smoking, vaping, and using air fresheners or other chemical products, can contribute to PAHs in car floor dust [Kumar et al., 2023; Oliveira et al., 2019]. Smoking, in particular, is a significant source of PAHs, and the resulting smoke particles can be deposited on interior surfaces, including the car floor. Concomitantly, vehicles have limited ventilation compared to other indoor spaces, which can lead to the higher accumulation of airborne PAHs and particulate matter inside the cabin. Therefore, the reduced air exchange in vehicles can contribute to higher PAH level in car floor dust [Kumar et al., 2023; Matt et al., 2008].

A recent study on indoor dust from automobile workshops reported high levels of PAHs that were much higher than those reported from households in the same city [Ali et al., 2017; Escher et al., 2020]. This indicates vehicles are a major source of PAHs in the surrounding environment; consequently, higher levels in car parking dust are not surprising which were multifold higher than OPFRs found in dust fractions from car parking. The difference in the levels and profiles of studied PAHs among microenvironments indicates different emission sources for these chemicals (Fig. 3). Although there is not enough data on PAHs among different dust fractions in the literature, it has been generally suggested in a few studies that PAHs decrease with the increase of dust particle size [Siciliano et al., 2010; Wang et al., 2013].

3.3. Factors affecting SVOCs deposition on dust fractions

When cars are used, especially when they are parked in the sun, the temperature inside the car goes up. This helps these chemicals get out of the treated products and leads to a higher number of SVOCs. At the same time, tracked-in dust and soil from the car parking areas results in cross contamination of the car floor dust with car parking dust, which might dilute the concentrations of chemicals in coarse dust particles. This could plausibly be one of the reasons for the high levels of studied SVOCs in fine dust particles from the car floor. While in other studied indoor environments, less coarse dust particles were tracked in from outside since most of the sampled indoors were carpeted and inhabitants did not wear outdoor shoes inside. Moreover, samples from AC filters, household, mosque, and hotel floor dust showed phthalates, OPFRs, and PAHs with almost similar concentrations among the 4 dust fractions, suggesting that different dust particle sizes have no effect on filtered samples and the origin of dust particles inside these microenvironments has similar sources. It was suggested that factors such as surface area could have an effect on chemicals adhered to different dust particles. It has been reported in several studies that contaminants accumulate on larger surfaces within fine particles [Cao et al., 2013; Weiss et al., 2018].

Changes in temperature also have a significant influence in SVOCs deposition rates from sources to various media, including dust. When temperatures decline, the concentration of vapors in the atmosphere decreases, leading them to condense into aerosols. These aerosols subsequently settle under the force of gravity, depositing the chemicals on the surface/dust when they reach the ground [Bi et al., 2020; Weschler and Nazaroff, 2010]. The low concentrations of SVOCs in parking lot dust and comparable levels in hotel, mosque, and residential dust may suggest that temperature plays a significant role in their deposition of these chemicals, given that the temperature in parking lots is higher than in other indoor microenvironments [Bi et al., 2015]. For vehicles, high temperatures during parking may aid in the release of SVOCs from the interior materials, and when the air conditioner is put on, the colder air helps the chemicals settle on the dust [Ali et al., 2021; Lexén et al,. 2021; Tokumura et al., 2017; Zulauf et al., 2019]. This could be one of the reasons for higher levels of chemicals in finer dust fractions from cars, as finer dust is primarily airborne during air conditioning.

Considering that SVOCs which are additive chemicals such as OPFRs and phthalates can potentially be emitted from products into the surrounding air through volatilization, therefor it is essential to account for their vapor pressure (Vp) [Bi et al., 2020; Weschler and Nazaroff, 2010]. The relative partitioning between suspended and settled indoor dust can be influenced by factors like the octanol-air partition coefficient (KOA), dust-air equilibrium partition coefficient (KDA), dust diffusion coefficient (Dd), and dust-source material partition coefficient (KDS) [Bi et al., 2020; Weschler and Nazaroff, 2010]. While Vp and KOA pertain to the physicochemical attributes of SVOCs, Kda, Dd, and Kds are also contingent upon material and dust properties [Liu, 2022]. Once these chemicals undergo volatilization, their deposition onto both suspended and settled indoor dust is depend on the properties of the dust particles themselves [Liu et al., 2022; Weschler and Nazaroff, 2010]. Therefore, this interplay of volatilization and deposition becomes more pronounced for SVOCs with elevated vapor pressures and lower KOA values. In contrast, the presence of chemicals that are less volatile may be determined more by other processes, such as direct contact between the chemical source and dust or abrasion of the components that comprise the chemical source [Rauert and Harrad, 2015; Webster et al., 2009]. These factors, in conjunction with the fact that atmospheric deposition of SVOCs to dust particles will be greater for finer particle sizes due to their greater surface area to volume ratio, mean that significantly higher concentrations on finer dust particles would be anticipated for more volatile SVOCs, whereas this is likely to have less of an effect on their less volatile counterparts [Ali et al., 2018; Chen and Hu, 2014]. However, this may be contributed to the overall procedure by some flaws or inaccuracies or sample collection methods, as some studies have proposed that dusts collected with wipes are often double the dusts collected using vacuums and that vacuum cleaners can miss both fine and large particles through passing the cellulose thimble or by eliminating the sieving process [Raffy et al., 2017].

Scanning electron microscopy (SEM) analysis is a powerful technique employed to study the composition and morphology of particles in dust fractions. It enables researchers to observe detailed information about particle shapes and structures at extremely high magnifications. Figure 4 presents microscopic images of the finest size fractions obtained from various studied microenvironments, highlighting distinct differences between them. The particles in the finest size fractions from household floor and AC filter dust appear predominantly edgy and fragmented, while those from cars or parking lots exhibit mostly flat surfaces with some degree of roundness. The SEM analysis further revealed that dust particles found in cars' dust showed rounded edges similar to particles from car parking lots, suggesting that they likely originated from outdoor soil tracked indoors through cross ventilation. Interestingly, despite the similarities in dust particles between cars and car parking lots, the chemical profiles were markedly different. This difference can be attributed to the closed and compact nature of indoor car environments, leading to unique indoor particle compositions compared to other fine particulates found indoors, where airflows are generally less due to insulation against external weather conditions. These findings imply that indoor-outdoor particulate matter sources may differ even when originating from same urban areas. This variability could potentially explain the variation in the presence of SVOCs in different studies. The SEM analysis provided valuable insights into the morphological characteristics of dust particles from different microenvironments, shedding light on their likely sources and transport mechanisms. Further investigations and analyses in this domain may yield a deeper understanding of the intricate dynamics of SVOCs in indoor settings.

The total carbon content of dust is a factor that can influence the levels of SVOCs found in it [Chen and Hu, 2014; Weschler and Nazaroff, 2010]. In AC filter, household, hotel, and mosque floor dust, the total carbon content was in similar ranges that varied between 25 and 35%; however, it was 10–15% less in the coarse fractions (F3 and F4) of car and car parking area dust. The dust from car parking lots had less carbon than dust from other microenvironments, potentially explaining why there were less SVOCs in these microenvironments. It seems plausible that different sizes of fractionated particles might have an impact on SVOCs concentrations due to their varying carbon contents; however further research is needed before any conclusions can be drawn about this relationship with certainty or generality across contexts, since this finding is based on only a few dust samples. At this stage, the impact of factors like black carbon or total organic carbons towards influencing the occurrence of SVOCs within indoor environments remains unclear. Additional investigations into this matter are essential to comprehensively grasp the dynamics of their interaction with one another over time. In conclusion, while current evidence suggests that variations in particle size fractions could influence presence/absence patterns when looking at SVOCs within indoor. However, there is a significant need for deeper exploration to elucidate the roles of elements like black and total organic carbons in this context

Human exposure to chemicals via contaminated dust is unquestionable. A huge number of scientific studies from different parts of the world have reported the occurrence of SVOCs in indoor dust and calculated the health risk. Dust ingestion, inhalation, and dermal contact are considered the major sources of exposure routes for a number of SVOCs. Extensive research has been focused on investigating pollutant concentrations in dust, which is a mixture of fine and coarse particles. However, few studies have focused on understanding the occurrence of SVOCs in fine particles. It is very important to understand the fate, transport, and deposition of SVOCs in different fractions of dust to make accurate estimates of human exposure from dust. More studies are needed to generate sufficient information and data regarding the occurrence of various SVOCs in different fractions of dust from different indoor microenvironments. This is very important since the composition of dust and the transport and deposition of SVOCs in different microenvironments vary, as found in the current study. Understanding SVOCs in finer dust particles is of the utmost importance since these particles can easily enter the body through inhalation and ingestion. Therefore, this research will support scientific and public health institutions in understanding the occurrence of selected SVOCs in different fractions of dust in various indoor microenvironments and informing public health protection measures. However, further studies are needed that should include more classes of organic and inorganic pollutants in different indoor microenvironments, which are an important part of daily life.

Acknowledgments: This research work was funded by Institutional Fund Projects under grant no. (IFPIP: 2003-140-1443). Therefore, authors gratefully acknowledge technical and financial support from the Ministry of Education and King Abdulaziz University (KAU), DSR, Jeddah, Saudi Arabia.

Ethical Approval: Ethical approval was not required for this study.

Consent to Participate: Since no human participation was involved therefore no consent to participation was required. Detailed questionnaire from the sampled households, mosques, hotels, and cars were filled by the participants which fulfill the requirement for their consent to participate.

Consent to Publish: I give my consent to the publisher to publish this study.

Authors Contributions: Author has completed all aspects of this study.

Funding: “This research work was funded by Institutional Fund Projects under grant no. (IFPIP-2003-140-1443). Therefore, authors gratefully acknowledge technical and financial support from the Ministry of Education and King Abdulaziz University (KAU), DSR, Jeddah, Saudi Arabia”.

Conflicts of Interest:

“The authors declare no conflict of interest.”

“The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results”.

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Dust Dynamics: Distribution Patterns of Semi-Volatile Organic Chemicals across Particle Sizes in Varied Indoor Microenvironments

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Abstract

Figures

1. Introduction

2. Materials and Methods

2.1. Chemicals and solvents

2.2. Sampling

2.3. Sample preparation and quantitative analysis

3. Results and Discussion

3.1. Rationale behind selecting indoor microenvironments

3.2. Concentrations and profile of SVOCs in different fractions of dust from different microenvironment

3.2.1. Phthalates

3.2.2. Organophosphate flame retardants

3.2.3. Polycyclic aromatic hydrocarbons

3.3. Factors affecting SVOCs deposition on dust fractions

4. Conclusion

Declarations

References

Supplementary Files

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