Abimbola O.
Famuyiwa
*ab and
Jane A.
Entwistle
b
aDepartment of Science Laboratory Technology, Moshood Abiola Polytechnic, Abeokuta, Ogun State P.M.B 2210, Nigeria. E-mail: abimbola.famuyiwa@gmail.com
bDepartment of Geography and Environmental Sciences, Northumbria University, Newcastle Upon Tyne NE1 8ST, UK
First published on 22nd April 2021
Ambient and indoor air pollution results in an estimated 7 million premature deaths globally each year, representing a major contemporary public health challenge, but one poorly quantified from a toxicological and source perspective. Indoor exposure represents possibly the greatest potential overall exposure, yet our indoor environments are still poorly understood, modelled and characterized. In rapidly growing cities, such as Lagos, Nigeria, environmental monitoring can play an important role in establishing baseline data, monitoring urban pollution trends and in environmental education. Classroom dust samples were collected from 40 locations from across the twenty local government areas (LGAs) of Lagos, in June 2019. The aim of the study was to assess the potential hazard posed by PTE in indoor dusts and to develop a suitable risk communication strategy to inform and educate the public, promoting environmental health literacy. Concentrations of total PTE in indoor dusts were assessed using Energy Dispersive X-Ray Fluorescence (ED-XRF) spectrometry. Oral bioaccessibility determinations using the unified BARGE method, and analysis by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) were also performed on the dust samples to determine the fraction available for absorption in the gastrointestinal tract. Results showed that the indoor dust samples were largely uncontaminated, with only few exceptions (2 samples). Enrichment factor pollution trend for the total PTE concentrations was in the order of Pb > Zn > U > Cr > Cu > Ba > Mn > V > As > Cd > Ni > Al. Source apportionment studies using factor analysis suggests concentrations of Al, As, Fe, Mn, Ni, and U may be influenced largely by lithogenic factors, while Cd, Cu and Pb originated principally from anthropogenic sources. Chromium, V and Zn appear to originate from mixed sources of both lithogenic and anthropogenic origin. Our oral bioaccessibility determinations indicate that the assumption of 100% bioavailability based on pseudototal or total concentrations would overestimate the hazard potential of PTE in these indoor dusts. Zinc was the most bioaccessible PTE (mean of 88%), with Mn (57%), Pb (48%), Ba (48%), Al (41%), As (37%), Cu (36%), Ni (28%), Cr (10%) and Fe (7%) the least bioaccessible. Human health risk assessment, for both children and adults using the bioaccessible fraction, showed values to be within acceptable risk levels.
Environmental significanceExposure to particulate matter can initiate or enhance disease in humans, yet the nature of the hazard that indoor dust presents remains poorly characterized from a toxicological and a source perspective. Our research provides key information about the spatial distributions and concentrations of contaminants in indoor dusts from school environments in Lagos, Nigeria, providing geochemical baseline information and source characterization of 12 Potentially Toxic Elements (PTE). A range of contamination assessment criteria were employed, as well as a modified approach incorporating oral bioaccessibility measurements into human exposure risk models. The online ‘dust atlas’ and ancillary supporting information enables participants to interactively view the levels of PTE in indoor dusts across all participating regions and the range of practical actions to improve indoor environmental quality. |
Dusts contain harmful agents, including metals/metalloids, organic substances, microbes and allergens. Indoor dust is a crucial, often ignored, part of the daily exposure to PM given the amount of time people spend indoors (in homes, offices and schools); estimated to be about 88% of the day for adults and 71–79% of the day for children.9 Potential sources of dust in the indoor environment include ingress of ambient particles, cooking, smoking, cleaning, wear and tear, and soils transported on clothes and footwear. However, incidental or deliberate ingestion of indoor dust can be an important non-dietary pathway by which PTE get into human body, especially children. Involuntary inhalation of resuspended dust particles in the indoor environment coupled with dermal contact are other PTE exposure pathways to humans.10
It is increasingly recognized that the human health risk associated with PM is not solely related to the physical abundance of the PM, or total or pseudototal PTE concentration, but with the biologically available portion of the contaminant.11 Bioaccessibility is a surrogate in vitro estimation of bioavailability. Oral bioaccessibility of a contaminant is the fraction extractable in the human gastro-intestinal (GI) system.12 Indeed, a significant portion of the PM inhaled also reaches the GI system through particle clearance mechanisms in the human pulmonary system.13 In addition to lung clearance mechanisms, indoor dust has a direct oral ingestion pathway to the GI system through hand-to-mouth activity. When these fractions reach the GI tract the PM reacts with gastric juices and the PTE released represent the orally available (bioaccessible) fraction.14
The 2019 United Nations world population statistics revealed that more than half of the world population growth will occur in Africa between now and the year 2050.15 Environmental degradation, with accompanying health impacts, may be one of the resultant effects of the projected population growth, and associated urbanization and industrialization. Several studies which include oral bioaccessibility and risk assessment of PTE in indoor dusts have been reported from a range of countries globally,16–21 however previous studies on PM in Nigeria have largely focused on the outdoor environment, with soil and street dusts the prime research focus.22,23 Recent relevant PM concerns for the Nigerian community include release and resuspension of mining and smelting sources, power generation emissions, traffic, and major fire incidents.24 Lagos is Nigeria's most populous city and seventh fastest growing city in the world.25 Lagos, like many other urban centres across Nigeria, has experienced an unprecedented increase in urban migration, but one amplified by the fact that it serves as the commercial nerve centre of Nigeria.10 Lagos State Government estimated the population of Lagos as 17.5 million during a parallel census conducted in 2006 with over 12 million people living in the urban areas. A more recent account reported its population as 21 million, making it the largest city in Africa.26 In comparison to most developed countries, developing countries typically lack suitable public and environmental health standards and thus residential, community and educational indoor environments can be a source of unacceptable exposure to PM and to PTE in these indoor dusts. This problem can be exacerbated due to the lack of, or poor implementation of, environmental sanitation policies. The aim of this current study is to (i) assess the potential hazard posed by PTE in indoor dusts from schools across Lagos and, (ii) to develop and implement a risk communication strategy to help inform and educate the public around the potential hazards of PM, promoting environmental health literary to reduce exposure. Specific objectives include: (1) to assess the level of PTE in indoor dusts collected from nursery and primary schools in all 20 local government areas of Lagos state, (2) application of an oral bioaccessibility procedure to indoor dusts (250 μm fraction) to determine the fraction of PTE available for absorption, (3) assessment of the human health risk of PTE in classroom dust to children and adults in Lagos environs, and (4) dissemination of the research findings to the local communities in an accessible format. To widen the reach of our environmental health literacy message the focus of our indoor dust collection was on schools.
Equipping the next generation, and more broadly the wider public, with better information about dust-related hazards in their local environmental has the potential to improve the health and wellbeing of communities by empowering citizens to take ownership of response strategies.
The in vitro oral bioaccessibility extraction protocol employed in this work was based on the unified BARGE method (UBM).27 This in vitro extraction method has been widely employed and validated for oral bioaccessibility measurements in soil and dusts.28–30 The simulated digestive fluids were prepared and stored in the refrigerator 1 day before extraction commences.31 Approximately 0.6 g (weighed to 4 decimal places) of dust sample was weighed into individual polycarbonate extraction tubes and mixed with 9.0 ml of saliva. After a quick manual shaking (10 seconds), 13.5 ml of gastric solution was pipetted and added. The pH of the solution was adjusted to 1.20 ± 0.05 with HCl (37%). The tubes were shaken using an end-over-end rotator inserted into a thermostatic water bath (@ 37 °C) for 1 hour and then centrifuged at 4500 g for 15 minutes. The supernatant was thereafter carefully pipetted and was acidified using HNO3 and stored in a refrigerator prior to inductively coupled plasma optical emission spectrometry (ICP-OES) using a PerkinElmer Optima 8000 (configured with software Winlab 32 for ICP, version 5.5), Waltham Massachusetts USA.
Determination of the pseudo-total metal concentration was undertaken using a microwave digestion technique. Approximately 0.50 g (weighed to 4 decimal places) of dust was weighed into Teflon digestion vessels and 12.0 ml of aqua-regia mixture was added. The sample was heated in a temperature-controlled microwave digestion system (CEM MARS 6™, Buckingham UK). After digestion, any violent chemical reactions were allowed to subside as the vessels cooled to near room temperature. Digests were filtered and diluted using deionized water and stored in a refrigerator prior to ICP-OES analysis.
For improved short-term precision (repeatability), each pellet is rotated during acquisition of the spectra in the Xepos, and as part of our quality control procedures we additionally ran selected sample pellets three times to monitor the % relative standard deviation (% RSD). Where sample concentrations were above 1.0 mg kg−1, % RSDs ranged from <1% to 10% for all of the elements of interest. Instrument limits of detection (LOD, determined for a pure quartz sample) are: Al 27 mg kg−1; As 0.1 mg kg−1; Ba 2.0 mg kg−1; Cd 0.2 mg kg−1; Cr 0.2 mg kg−1; Cu 0.5 mg kg−1; Fe 1.2 mg kg−1; Mn 0.2 mg kg−1; Ni 1.0 mg kg−1; Pb 0.2 mg kg−1; V 0.3 mg kg−1; Zn 0.2 mg kg−1, and U 0.2 mg kg−1, respectively, with all determinations made using the instruments TurboQuant II software (Spectro Analytical Instruments, XRF-113-AB).32
All sample extractions were run in triplicate with procedural blanks (i.e. reagents only, used for blank subtraction and to monitor for any contamination introduced during the laboratory processing) included in each batch. For ICP-OES analysis of the extracts, calibration curves were prepared for the PTE under investigation, using aqueous standards across the calibration range from 0 to 100 mg L−1. The ICP-OES six-point calibration curves produced linear graphs over the determined ranges, with regression coefficient (R2) data of at least 0.998 for all PTE analysed. An internal standard solution (indium) was used to monitor and correct for instrument drift and 2% nitric acid was used to achieve appropriate dilutions. Instrument drift was additionally monitored by analysing calibration standards after every tenth sample measurement. BGS 102 guidance material was used to determine the accuracy of the determinations in the UBM procedure33 (Table 1), and BCR 143R for the aqua-regia digests (Table 2).
Al | As | Ba | Cd | Cr | Cu | Fe | Mn | Ni | Pb | V | Zn | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
n = 5 | Determined mean | 2500 ± 198 | 5.5 ± 0.51 | 118 ± 6.73 | 3.54 ± 10.6 | 36.7 ± 2.58 | 5.82 ± 1.41 | 545 ± 299 | 3470 ± 268 | 13.1 ± 1.34 | 17.1 ± 1.53 | 1.0 ± 1.24 | 46.2 ± 3.49 |
n = 50 | Reported data33 | 2568 ± 245 | 3.9 ± 0.36 | 72.2 ± 6.02 | 0.24 ± 0.03 | 36.7 ± 2.48 | 8.5 ± 1.03 | 854 ± 237 | 2979 ± 294 | 13.0 ± 1.27 | 15.2 ± 2.97 | 6.1 ± 0.91 | 41.5 ± 4.45 |
% recovery | 97 | 140 | 163 | NIL | 100 | 68 | 64 | 117 | 101 | 112 | 16 | 111 |
Cd | Cr | Mn | Ni | Pb | Zn | |
---|---|---|---|---|---|---|
Determined mean (n = 6) | 65.1 ± 4 | 458 ± 69 | 783 ± 120 | 272 ± 13 | 160 ± 7 | 1060 ± 120 |
BCR 143R certificate values | 72.0 ± 1.8 | 426 ± 12 | 858 ± 11 | 296 ± 4 | 174 | 1063 ± 16 |
% Recovery | 90 | 109 | 91 | 92 | 92 | 100 |
Enrichment factor (EnF) of PTE in the indoor dust samples were calculated using eqn (1):
(1) |
Zhang and Liu41 noted that EnF values between 0.5 and 1.5 indicate PTE is entirely from natural origins whereas EnF values greater than 1.5 suggests source/s are more likely to be anthropogenic. Five contamination categories have been proposed to indicate the degree of pollution: no or minimal enrichment (EnF < 2); moderate enrichment (EnF = 2–5); significant enrichment (EnF = 5–20); very high enrichment (EnF = 20–40); extremely high enrichment (EnF > 40).
Whilst dust metal loadings are important for assessing potential exposure to indoor dust, the resultant concentration data provides us with a useful indicator of the presence of different sources of metals across the Lagos schools investigated. This concentration data has also been shown to be of use for comparing indoor dust with outdoor sources such as street dust and soil.43,44 Where only metal concentration data exist (without loadings data), the approach commonly adopted by other studies is based on the health risk assessment method developed by United States Environmental Protection Agency.45–49
Human exposure to PTE can occur via three main routes; (a) inhalation of particulates through the nose and mouth (b) dermal absorption adhered to exposed skin (c) through ingestion of particles. For this study, we computed the carcinogenic and non-carcinogenic risks of PTE exposure to two age groups; children and adults.
The average daily dose values for the three main exposure routes were calculated as follows:
(2) |
(3) |
(4) |
C (exposure point concentration, mg kg−1) in eqn (2)–(4) is an estimate of reasonable maximum exposure, but in this current study is replaced with bioaccessible concentration values (mg kg−1) which represents the fraction that is available for absorption in the gastro intestinal tract of humans. For non-carcinogenic risk estimation each PTE average daily dose was divided with corresponding reference dose (RfD) to yield a hazard quotient (HQ) as shown in eqn (5). The hazard index (HI) was estimated to be equal to the sum of the HQs calculated. The HI index evaluated the overall potential for non-carcinogenic risks to man from the three exposure routes considered (eqn (6)). If the HI value is greater than 1, there is a probability of non-carcinogenic risks occurrence in a lifetime. Carcinogenic risk is the probability of an individual developing any type of cancer during a lifetime exposure. Risk was evaluated by multiplying the average daily dose values against oral slope factor (SF) (eqn (7)).
(5) |
HI = ∑HQi | (6) |
CR = D × SF | (7) |
Only As, Cr, Ni and Pb were considered since oral slope factors are only available for these elements. Calculated cancer risks above 1 × 10−4 were considered unacceptable and risks below 1 × 10−6 were considered not to have the capability of triggering any health effect, while values between 10−6 to 10−4 were considered to be within acceptable limits.23
Al | As | Ba | Cd | Cr | Cu | Fe | Mn | Ni | Pb | U | V | Zn | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a https://www.esdat.net/environmentalstandards/canada/soil/rev_soil_summary_tbl_7.0_e.pdf. b https://www.miljodirektoratet.no/globalassets/publikasjoner/klif2/publikasjoner/andre/1691/ta1691.pdf. c Contaminated Land: Applications in Real Environments (CLAIRE) consortium, SP1010 Final Project Report (Revision 2). Development of Category 4 Screening Levels for Assessment of Land Affected by Contamination. DEFRA R&P Project Report, 2014a. d V. Alekseenko, A. Alekseenko, The abundances of chemical elements in urban soils, Journal of Geochemical Exploration, 2014, volume 147, part B, pages 245–249. e https://pubs.geoscienceworld.org/gsa/gsabulletin/article/72/2/175/5262/Distribution-of-the-Elements-in-Some-Major-Units. | |||||||||||||
Minimum | 13200 | 1.4 | 97.4 | <LOD | 39.8 | 9.6 | 10000 | 106 | 10.5 | 14.9 | 2.2 | 15.9 | 66.9 |
Mean | 32000 | 3.3 | 254 | 0.5 | 130 | 28.1 | 24500 | 368 | 20.9 | 47.4 | 6.1 | 52.4 | 208 |
Median | 30800 | 3.2 | 212 | 0.4 | 76.9 | 23.0 | 21100 | 339 | 20.1 | 43.7 | 5.5 | 49.4 | 163 |
Max | 60700 | 4.8 | 1560 | 1.3 | 606 | 110 | 42400 | 764 | 40.3 | 114 | 15.0 | 103 | 622 |
SD | 10300 | 0.96 | 233 | 0.29 | 114 | 17.6 | 8530 | 141 | 6.34 | 20.1 | 2.76 | 22.4 | 129 |
Canada SGVa | 12 | 10 | 0.4 | 63 | 50 | 140 | 200 | ||||||
Norway SGVb | 12 | 12.2 | 75 | 150 | 90 | 90 | 300 | ||||||
CLEA, UKc | 40 | 150 | 310 | ||||||||||
Earth's crustd | 80500 | 1.7 | 650 | 0.13 | 83 | 47 | 46500 | 1000 | 58 | 16 | 90 | 83 | |
Continental Shalee | 80000 | 13 | 580 | 0.3 | 45 | 45 | 47200 | 850 | 68 | 20 | 3.7 | 130 | 95 |
Aluminium, Ba and V concentrations were compared against the typical background soil concentration. None of these three PTE (Al, Ba and V) were elevated when compared to these typical soil concentrations/crustal abundances. This suggests, levels of Al, Ba and V may largely be from natural sources, as anthropogenic contributions are not pronounced considering comparisons with respective background values.
Cadmium measurements ranged from below the LOD (0.1 mg kg−1) to 1.3 mg kg−1, with a mean concentration of 0.5 mg kg−1. None of the Cd measurements exceeded the Norwegian and Canadian soil guideline values (SGVs) but nearly all of the sample measurements were higher than the typical soil background value. The highest Cd measurement in a previous study of residential soils from Lagos and Ibadan (Nigeria) was 2.8 mg kg−1, with an average concentration of 0.94 mg kg−1,58 and indicated a similar range of Cd measurements to this current study.
Iron and Mn are major components of soil minerals and have been noted to be abundant elements in the earth crust.59 Our Fe and Mn concentrations were within typical soil concentration ranges, with all measurements for both of these PTE falling below the earth's crust and continental shale values respectively. Our dust samples were similarly not contaminated with Cu and Ni, with concentrations below typical soil concentrations and the reported SGVs (Table 3).
Zinc, Pb and Cr displayed a rather different behaviour among the PTE. Nearly all of the Zn concentration measurements in the dust samples exceeded the background values (99%) and about 33% were higher in value than Canadian SGV; some multiple times higher. Several samples did exhibit slightly elevated Cr and Pb concentrations when compared to the typical background concentrations (earth crust and continental shale values), but only Cr had dust samples which exceeded the reported SGVs.
To provide further context for the data, PTE in indoor house dusts from selected studies are reported (Table 4). Direct comparisons however are again somewhat limited as these studies focus on house dust, and differences in sample preparation and analysis also exist. However, our Lagos indoor school dust samples typically had PTE concentrations that fell within the mean/geomean data reported in these other studies (Table 4). Our observed As concentrations ranged between 1.4–4.8 mg kg−1; comparable to concentrations measured in classrooms and offices from Ogun state, Nigeria by Olujimi et al.10 and below representative countries soil guideline values (Table 3). Our Cr concentrations were also largely comparable with those data reported in Table 4. An average concentration of Cr in cement typically ranges from 179–257 mg kg−1.60 The aging concrete (cement) floors within the classrooms may have contributed to Cr concentrations measured in the samples. Whilst the majority of PTE we investigated in this study did not indicate elevated concentrations when compared to our selected SGVs (with the exception of Zn and Cr), many were higher than those measured in other Nigerian cities. For instance, Cd, Cu, Fe, Mn and Pb measurements in Ilorin city,61 Makurdi,62 and Kano city63 (other cities in Nigeria) were all lower than concentrations measured in this current study. Lagos is the commercial nerve centre of the country and over sixty percent of all industrial activities in Nigeria are domiciled in Lagos. The huge industrialization and urbanization in Lagos may have contributed to the higher PTE load in our indoor dusts in comparison to these other Nigerian cities. Indeed, in a recent study in the Niger delta area of Nigeria,64 classroom dusts collected from schools along major roads with high traffic density showed elevated PTE concentrations. In keeping with this study, several of our sampled schools were located in close proximity to busy markets (e.g. Ojuwoye and Balogun markets in Mushin, and Lagos Island LGA), and so elevated PTE might be expected in our indoor dust samples attributed to traffic sources. The high influx of vehicles, emissions from small and medium scale enterprises, alternative power supply (use of gasoline or diesel electricity generators) and artisan workshops around the vicinities of our sampled schools may all have contributed to the PTE load in the dust samples. Activities in the local vicinity will influence the PTE load in indoor dusts. Potentially toxic elements find their way into buildings through a range of mechanisms, such as backtracking of particles on people and animals, as airborne dust particles (such as through open windows), as well as from numerous indoor sources, such as building materials, furnishings and consumer products. Darus et al.71 noted that indoor dust or particle load can be associated with street dust or soil and are transported via wind blow through building openings and by the movement of occupants in and out of the building. On average, the As, Cd, Cu, Ni, Pb and Zn contents obtained in this study on Lagos were lower, and in some instances multiple times lower, than ones reported for Xian, China;1 Guangzhou, China;67 Istanbul Turkey;68 Japan,69 Stratoni Greece;70 (see Table 4). The higher PTE levels in indoor dust from these cities, compared to that of this study (Lagos), are likely connected to the long histories of industrialization and urbanization in these countries. In contrast, the industrialization of Lagos only started in the post-independence era (1960 to 70 s).72 Of particular concern is the apparent build-up of PTE in the Lagos environment over a relatively short period of time, as observed when we compare our data (on samples from 2019) with those of a previous study across Lagos24 from 2012 (although we acknowledge that these were on a different set of 20 public primary schools). Making such data publicly available can raise awareness of environmental issues and drive appropriate mitigation strategies.
As | Cd | Cr | Cu | Mn | Ni | Pb | Zn | |
---|---|---|---|---|---|---|---|---|
Ogun state, Nigeria10 | 0.96–5.33 | 82.9–8595 | 17.7–105 | 2.37–250 | 81.7–600 | 2.37–58.4 | 3.80–127 | 15.2–480 |
Niger-delta Nigeria45 | 0.00–22.8 | 0.00–1.12 | 1.33–153 | 0.31–2.60 | 0.00–34.6 | 0.06–1.63 | 5.00–149 | |
Lagos, Nigeria24 | 0.01–8.28 | 3.45–17.4 | 0.06–82.5 | |||||
Xian, China1 | 6.0–38.3 | 88.9–751 | 33.2–164 | 398–824 | 20.5–104 | 87.5–593 | 184–1838 | |
Sri Serdang, Malaysia49 | 0.4–2.65 | 6.44–86.5 | 0.17–68.3 | |||||
Canada20 | 0.1–153 | 0.5–223 | 0.5–2930 | 24–4880 | 17.3–2300 | 14.2–7800 | 144–6630 | |
Port Harcourt, Nigeria65 | 0.07–1.53 | 0.53–3.52 | 19–45.2 | 10.3–25.4 | ||||
Kumasi, Ghana66 | BDL | 0.00–33.2 | 0.00–41.9 | |||||
Guangzhou, China67 | 3.30–155 | 355–2745 | 17.7–281 | 178–3394 | 331–3928 | |||
Istanbul, Turkey68 | 0.4–20 | 2.8–460 | 62–1800 | 8.0–1300 | 120–2600 | 3.0–300 | 210–2800 | |
Japan69 | 0.18–5.62 | 14.8–285 | 66.8–7720 | 38.6–679 | 11.9–3730 | 188–5920 | ||
Stratoni, Greece70 | 3–28 | 71–8390 | 390–6920 | |||||
This study | 1.4–4.8 | 0–1.3 | 39.8–606 | 9.6–110 | 106–764 | 10.5–40.3 | 14.9–114 | 66.9–622 |
Moderate enrichment of Cu was found in samples A2 (3.31), A19 (1.51) and A39 (2.02). Anthropogenic Cu sources have been linked to car components, tyre abrasion, corrosion of galvanized metal parts in cars, lubricants, engine wear, brake dust and thrust bearings.74 So, it is not surprising to see enrichment of Cu in a number of samples taken from our urban schools, it is perhaps more surprising that we do not see this moderate enrichment more widespread across the sample set.
The Pb, U and Zn EnF values all indicated moderate to significant enrichment, with the majority of the Cr samples (63%) also indicating either moderate or significant enrichment. Lead and Zn enrichment in indoor dusts have been linked to biomass burning, vehicular traffic and industrial activities18 whilst releases from mill tailings, combustion of coal and other fuels, mining of phosphate ores, use of phosphate fertilizers and emissions from nuclear plants have variously been blamed for the anthropogenic loading of U in soils.75 Although we can rule out a nuclear source, no nuclear power plants exist in Lagos, the combustion of coal and other fuels are likely a key contributor to the widespread enrichment of Pb, Zn and U in our indoor school dusts.
Variable | Factor 1 | Factor 2 | Factor 3 | Communality |
---|---|---|---|---|
As | 0.748 | −0.283 | −0.151 | 0.662 |
Al | 0.832 | 0.068 | 0.161 | 0.722 |
Ba | 0.080 | −0.261 | −0.544 | 0.370 |
Cd | −0.095 | −0.872 | −0.104 | 0.780 |
Cr | 0.122 | −0.271 | 0.858 | 0.825 |
Cu | 0.252 | −0.832 | −0.009 | 0.756 |
Fe | 0.786 | −0.481 | 0.154 | 0.873 |
Mn | 0.737 | −0.012 | −0.073 | 0.548 |
Ni | 0.855 | −0.327 | 0.199 | 0.877 |
Pb | 0.468 | −0.793 | 0.155 | 0.871 |
V | 0.739 | −0.093 | 0.630 | 0.952 |
U | 0.780 | −0.252 | 0.020 | 0.672 |
Zn | 0.661 | −0.657 | 0.097 | 0.877 |
Variance | 5.0467 | 3.1490 | 1.5910 | 9.7868 |
% Var. | 38.8 | 24.2 | 12.2 | 75.3 |
Factor 1 was dominated by Al, As, Fe, Mn, Ni, V and U accounting for 39% of the total variance. This suggests concentrations of these elements may be influenced by a common factor or source, which given the crustal abundance of many of these elements is likely to be the local lithology of the study area. Indeed, previous studies have observed this PTE grouping and their association with natural sources.18
The second component explains 24% of the total variance and was negatively loaded on Cd, Cu and Pb. The close association between these elements in urban soils and dusts have been observed and identified as ‘typical urban metals’.76 Indeed, these PTE are typically associated with releases from leaded gasoline, braking, engine wear and other traffic-related activities,68,69 and have been reported to have their sources from pesticides, coal and biomass burning, sewage sludge, and industrial emissions.18 Factor 2 thus suggests PTE originating predominantly from anthropogenic sources.
The third component explains 12% of the total variance and is primarily loaded on Cr, with a slightly lower loading on V (<0.700 but >0.600). As factor 1 has been associated with natural origins for the different PTE, and factor 2 as predominantly anthropogenic sources, factor 3 may suggest a more mixed source. Here, we should also note that Zn occurs, at moderate loadings on both factors 1 and 2 (0.661 on factor 1 and −0.657 on factor 2; Table 5). It is likely that Cr, V and Zn are indicating a mixed origin, rather than dominated by either natural or anthropogenic sources. Geochemical association of Cr with Fe and Mn hydroxides and silicate minerals may suggest a natural source, while urban sources of Cr include cigarette smoke and ash, abrasion of stainless steel, cooking pans, yellow/red/green chromium-bearing paints as well as chrome plated fixtures and fittings.77 Urban anthropogenic releases of V are typically small in comparison to lithogenic releases, with oxides and hydroxides of Fe identified as a common primary source of V, but the increased presence/use of V has been reported within urban areas.78 Zinc occurs naturally in the earth's crust with trace levels found in water, air, soil and food, but contamination often occurs in relation to industrial processes, and industrial wastes. Compounds containing Zn are commonly found in association with anthropogenic activity, as Zn is used in wood preservatives, fabric dyes, in paints, and is typically associated with the construction and the automobile industry.18
Based on the findings of our multivariate analyses we allocated each PTE to one of three groups, identifying the dominant source for that PTE in indoor school dusts across Lagos:
A predominantly lithogenically derived group: Al, As, Fe, Mn, Ni, U.
A predominantly anthropogenically derived group: Cd, Cu and Pb.
A highly mixed origin group: Cr, V and Zn.
Bioaccessibility in this study is expressed as metal solubilized in the UBM stomach phase relative to the pseudototal concentration and expressed on percentage basis (see eqn (8)):
(8) |
Vanadium, Cd were below detection in the UBM extract and are not considered further. Of the remaining PTE investigated, Fe, Cr and Ni were the least bioaccessible PTE, with mean% BAF of 7%, 10% and 28% respectively. This indicated only a small fraction of these PTE would be available in the case of accidental ingestion.
Copper (36%), As (37%) and Al (41%) indicated higher mean bioaccessible concentrations in the indoor dusts, while Pb (48%), Ba (48%) and Mn (57%) were higher again, with mean bioaccessible concentrations of close to 50% or above. Zinc was the most bioaccessible PTE (mean of 88%), with bioaccessibility of around 100% in half of the indoor dust samples studied, suggesting that the simulated UBM gastric solution was as efficient in solubilizing Zn as the aqua-regia. Assuming 100% bioavailability based on pseudototal or total concentrations would overestimate the risk of potential of these PTE.
Our observed trend in indoor dust bioaccessibility was thus, in ascending order, Fe < Cr < Ni < Cu < As < Al < Ba < Pb < Mn < Zn. Such a trend is largely comparable to the order for mean values of bioaccessibility of the elements observed in urban street dusts from a number of studies (e.g. street dust from Nanjing, China: Cd > Zn > Pb ≈ Mn > As > Cu > Ni > V > Cr ≈ Fe80). Indeed, mean Pb bioaccessibility is commonly reported to be around 50% across urban soils and street dusts (e.g. 47% bioaccessibility in urban street dusts Nanjing,79 China, 53% in urban street dusts of Newcastle upon Tyne, UK80) with typically low Cr bioaccessibility reported (e.g. 4% in some urban soils81). Studies on indoor dusts have similarly reported high bioaccessibility of Zn, Mn and Al.19 About 80 to 90% of Zn content were bioaccessible in household dusts of Estarreja, Portugal.18
Whilst we acknowledge the challenges in comparing bioaccessibility extraction test results due to the array of extraction protocols employed, differences in sample composition, environmental media (rural, urban, street dust, indoor dust, sediments etc.), geological profile of study areas etc. there do appear to be common trends in PTE bioaccessibility reported in the literature that are also evidenced in our school dusts.
HQing | HQinh | HQder | ∑HI children | HQing | HQinh | HQder | ∑HI adult | CR children | CR adult | |
---|---|---|---|---|---|---|---|---|---|---|
Al | 1.70 × 10−2 | 7.24 × 10−6 | 5.76 × 10−2 | 7.50 × 10−2 | 2.20 × 10−3 | 2.17 × 10−6 | 3.64 × 10−3 | 5.85 × 10−3 | ||
As | 3.97 × 10−2 | 5.21 × 10−10 | 3.96 × 10−3 | 4.37 × 10−2 | 5.05 × 10−3 | 1.56 × 10−10 | 2.50 × 10−4 | 5.31 × 10−3 | 1.68 × 10−6 | 8.19 × 10−7 |
Cd | 6.92 × 10−2 | 4.53 × 10−9 | 1.08 × 10−1 | 1.77 × 10−1 | 8.79 × 10−3 | 1.36 × 10−9 | 5.81 × 10−4 | 9.37 × 10−3 | ||
Cr | 8.22 × 10−3 | 1.61 × 10−10 | 1.09 × 105 | 1.10 × 103 | 1.04 × 10−3 | 4.84 × 10−11 | 6.90 × 10−2 | 7.00 × 10−2 | 4.57 × 10−6 | 1.42 × 10−6 |
Cu | 2.36 × 10−3 | 7.86 × 10−3 | 1.02 × 10−2 | 3.01 × 10−4 | 4.97 × 10−4 | 7.97 × 10−4 | ||||
Mn | 7.72 × 10−3 | 1.42 × 10−8 | 2.56 × 10−2 | 3.33 × 10−2 | 9.82 × 10−4 | 4.25 × 10−9 | 1.62 × 10−3 | 2.60 × 10−3 | ||
Ni | 1.71 × 10−3 | 2.09 × 10−7 | 1.42 × 10−1 | 1.43 × 10−1 | 2.18 × 10−4 | 6.26 × 10−8 | 8.98 × 10−3 | 9.19 × 10−3 | 5.86 × 10−6 | 1.82 × 10−6 |
Pb | 4.68 × 10−4 | 1.33 × 10−1 | 1.33 × 10−1 | 5.95 × 10−5 | 8.42 × 10−3 | 8.48 × 10−3 | 4.42 × 10−7 | 1.37 × 10−7 | ||
Zn | 5.65 × 10−3 | 1.87 × 10−2 | 2.44 × 10−2 | 7.19 × 10−4 | 1.18 × 10−3 | 1.90 × 10−3 |
Since not all PTE are a cancer risk, only As, Cr, Ni and Pb were additionally considered for carcinogenic risks assessment, using the slope factors for these PTE available in literature. Contributions of each PTE to possible carcinogenic and non-carcinogenic risks are presented in Fig. 2 and 3. Cr and Ni were the prominent among the PTE considered. Chromium was the largest contributor to non-cancer risks (70% in children and 67% in adult; Fig. 2), whilst Ni was the largest contributor to the total cancer risk (with 47% in children and 43% in adults; Fig. 3). The mean cancer risks were within acceptable limit of no probable carcinogenic effects (10−4 to 10−6).
Fig. 2 Pie chart showing contributions of PTE to noncarcinogenic risks to children (a) and adult (b). |
Human health risk assessment, for both children and adults using the bioaccessible fraction, showed values to be within acceptable risk levels, however children appeared to be more susceptible to non-carcinogenic and carcinogenic risks than adults since all hazard quotient values, HI and carcinogenic risks evaluation in children exceeded that of adults for all PTE.
To maintain the PTE content in Lagos indoor dusts as low as practicable it is important that environmental laws are strictly adhered to and enforced, in tandem with ongoing improvements in the regulatory environment. Increasing levels of environmental awareness potentially empowers citizens to take actions to reduce exposure to indoor contaminants. An integral part of this project was public dissemination of our research findings to enhance the environmental health literacy of the staff and pupils within our participant schools.
As far as possible, ‘plain english’ was used to feedback results to the schools, with data made available on an interactive site at https://mapmyenvironment.com. Embedding citizen engagement is now acknowledged and accepted as an integral part of the research process as the potential exists for wider environmental benefits which result from a more environmentally aware society.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0em00445f |
This journal is © The Royal Society of Chemistry 2021 |