2. Materials and Methods
2.1. Study Design
This was an epidemiological and cross-sectional study for quantifying blood Ba levels in populations exposed to industrial pollutants in nine localities in Barcarena, namely, Acuí (ACUI), Bairro Industrial (BIN), Vila de Itupanema (VIT), Comunidade Laranjal (CLA), Bairro Canaã (BCA), Vila do Conde (VCO), Dom Manoel (DMN), Curuperê (CUR), Ilha São João (ISJ), and one from Abaetetuba known as Vila de Beja (VBJ) (
Figure 1).
The collection of blood samples and epidemiological surveys were conducted through home visits between February 2012 and February 2013. The city of Barcarena has a territory of approximately 1310 km
2 and is located in the Amazon River Delta region in northern Brazil [
14]. In the early 1980s, Barcarena had approximately 20,000 inhabitants (22.37/km
2), and currently, the population is over 120,000 inhabitants (93.32/km
2) [
15]. Due to the local industrial activity, it is among the cities with the highest economic development indicators, with a per capita income of BRL 39,732.60.
Abaetetuba is a municipality located next to Barcarena, with an estimated area of 1610 km², a population of 141,000 inhabitants (87.61/km²), and a per capita income of BRL 8718.18. The selection of these study areas was motivated by geospatial factors and previous reports of risks to human health from the consumption of water contaminated by toxic metals and inhalation of particulate matter (soot) emitted by industries, as shown in
Table 1 [
16,
17,
18,
19,
20,
21,
22].
The districts of ACUI (N = 88), BIN (N = 188), VIT (N = 209), DMN (N = 43), CUR (N = 22), ISJ (N = 31), VCO (N = 230), and BCA (N = 75) were located up to 1 km from the site where industrial kaolin processing occurs, where there are reports indicating that there is atmospheric emission of particles from boiler chimneys and possible contamination of groundwater used for human consumption from settling basins of liquid effluents from kaolin processing in these districts.
The districts of CLA (N = 296) and VBJ (N = 295) were selected because these districts are relatively distant (3 to 7 km) from where these anthropogenic activities occur and would be parameters for comparing concentration levels by proximity. During the sampling, a portion of the total population of Barcarena, approximately 20,000 inhabitants, was used to calculate the sample size (Sample N = 1477) sufficient for statistical inference, 95% confidence interval for significance.
2.2. Epidemiology
The study was approved by the Ethics Committee of the Instituto Evandro Chagas (IEC) (0010/2009). Participants included in this study were individuals of both sexes who resided in the 10 districts studied. This study involved the application of an epidemiological survey and sampling of biological material (blood). Participation was voluntary, and after all individuals signed the Free and Informed Consent Form (TCLE), an epidemiological form was applied with sociodemographic data, habits, and lifestyle variables (schooling, gender, consumption of alcoholic beverages, and the use of cigarettes, or the habit of dyeing hair). In addition, dietary variables (daily consumption of fruits, vegetables, red meat, fish, and seafood or canned food) and variables related to the morbidities reported by the interviewees were included, type of housing, forms of drinking water treatment (the use of mineral water, boiling, coagulation, use of hypochlorite, filtration or the non-treatment of consumed water) and the origin of the water consumed by the interviewees were also included.
2.3. Blood Sampling
Blood samples were collected through venipuncture on the forearm (5 mL) using needles and vacuum tubes containing high-purity K2EDTA anticoagulant, indicated for metal analysis. After collection, the samples were immediately transported in thermal boxes. Following this, after thawing and homogenizing the samples, 200 μL of the whole blood was transferred to 15 mL polytetrafluorethylene (PFA) tubes, and 200 µL of Triton X-100 solution (0.1%) and 1% HNO
3 were added. Subsequently, the sample solutions were stirred (VORTEX, QUIMIS, Diadema, Brazil) and left to stand for 5 min, following which they were adjusted to 4 mL with 1% HNO3 solution. The solutions were again homogenized and centrifuged at 6000 rpm for 10 min (Z206A, HERMLE, Gosheim, Germany). Finally, the supernatant was transferred to 15-mL PFA tubes [
28]. The metals were quantified using an inductively coupled plasma mass spectrometer (ICP-MS, Bruker, Billerica, MA, USA, model 820-MS).
The operating conditions of the ICP-MS were 1400 W radio frequency power, 15 L min−1 argon flow, Meinhard nebulizer with 0.98 L min−1 nebulization flow, and a cyclonic chamber and Scan mode: peak hopping. All blood samples were analyzed in duplicate, with five readings per sample. The Ba isotope measured was Ba-137, and the concentration in the calibration curve varied between 0.600 and 9.600 µg·L−1. The quantification limit for Ba was 0.598 µg·L−1, and the detection limit was 0.179 µg·L−1. The mean recovery percentage of the reference materials used in the quantification of blood Ba was 80.10% for Whole Blood Lyophilized L-1 and 75.6% for Whole Blood Lyophilized L-2.
The samples were prepared, and readings were performed in a clean room environment (class 1000) to minimize the risk of contamination. All blood samples were analyzed in duplicates, with five readings per sample. As an internal standard, multi-element solutions of yttrium, gallium, lanthanum, rhenium, and rhodium were used at a concentration of 5 µg·L−1. The equipment ICP-MS was calibrated using the standard addition method with the multi-element solutions,
2.4. Statistical Analysis
The results obtained were organized and stored in Microsoft Excel 2013 spreadsheets. Statistical analysis was performed using the Minitab 17.0 and BioStat 5.2 software. Values below the LOQ were considered to be ½ LOQ, as used in other studies [
29,
30,
31]. The Grubbs test was applied to verify the presence of outliers, and these were excluded. Ba concentrations were described using the arithmetic mean (AM), geometric mean (GM), and median (MED). The distribution of normality of variables by groups was determined using the Kolmogorov–Smirnov test (K–S).
The variables studied were sex, age group (>10, 10 to 19, 20 to 59, and ≥60 years), duration of residency in the study area (2 to 5, 6 to 9, and ≥10 years), drinking water source (well or general network and river), alcohol consumption (consumes or does not consume alcohol), smoking (smoker or nonsmoker), and eating habits related to food items (consumption of meat, chicken, fish, seafood, Brazil nuts, vegetables, and fruits), as they could be associated with blood Ba levels.
To compare blood Ba levels among two or more groups, an analysis of variance using the non-parametric Mann–Whitney and Kruskal–Wallis tests was performed. Dunn’s test was also used as a post-hoc test to identify the pairs different from samples with more than three groups.
To verify possible associations between Ba contents and predictive variables, Spearman’s Correlation Test was used. Variables that showed a significant correlation with Ba levels in the sequence were evaluated by simple regression analysis. The concentration of Ba was defined as a dependent variable, with the independent variables: proximity to impacted rivers (next door, near, and far), proximity to the kaolin industry (next door, near, and far), water consumption (deep wells, wells shallow, and river water consumption), and frequency of weekly fish consumption (<1 time, 2 to 4 times, and <4 times).
All tests were performed using a 95% confidence interval and a p-value of 0.05 for significance.
3. Results
The study involved 1477 individuals.
Table 2 shows the general characteristics of the population assessed from each locality. Most individuals were women (54.91%), adults aged 20–59 years (47.33%), and had lived in the area for >5 years (81.04%). The location with the highest number of individuals evaluated was CMA (N = 296), and the one with the lowest participation was CUR (N = 22).
The K–S normality test for blood Ba levels of all evaluated individuals showed a skewed distribution (KS = 0.290,
p < 0.010).
Table 3 shows the AM, MED, GM, standard deviation, and percentage distributions of blood Ba levels for all localities. The results showed differences in the median concentration ranges among the localities, based on which they were divided into two distinct groups: Group 1, localities wherein the median blood Ba levels ranged from 0.299 to 1.330 μg·L
−1 (ACUI, BIN, VIT, CLA, and VCO) and Group 2, localities wherein the median blood Ba levels ranged from 9.570 to 37.300 μg·L
−1 (BCA, VBJ, DMN, CUR, and ISJ).
The Kruskal–Wallis test demonstrated that there were significant differences (H = 567.29,
p = 0.000) in the median blood Ba levels among the 10 localities evaluated. In addition, Dunn’s test was used to determine which pairs of localities had similar median values (
Table 4). These results showed that there were no significant differences in the median blood Ba levels between BIN and ACUI (
p = 0.549) or between CLA and VCO (
p = 0.918), with both pairs belonging to Group 1.
There were also no significant differences between the medians of BCA and DMN (p = 0.129), VBJ and DMN (p = 0.894), VBJ and CUR (p = 0.338), DMN and CUR (p = 0.469), and CUR and ISJ (p = 0.241). All localities in the second group had higher blood Ba levels. All individuals with the lowest median blood Ba concentrations resided in locations that were different from the locations of individuals with the highest concentrations, confirming that the categorization of groups was appropriate.
The Grubbs outlier test was performed to determine outliers for each location. The blood Ba level of each outlier, results of the Grubbs test, and the level of significance were as follows: ACUI (4.867 µg·L−1; G = 4.61, p > 0.001), BIN (50.451 µg·L−1, G = 13.07, p > 0.001), VIT (46.606 µg·L−1; G = 6.04; p > 0.001), CLA (113.698 µg·L−1; G = 5.44, p > 0.001), VCO (27.033 µg·L−1; G = 5.67; p > 0.001), BCA (84.266 µg·L−1; G = 5.32, p > 0.001), VBJ (55.653 µg·L−1; G = 4.71, p > 0.001), DMN (21.131 µg·L−1; G = 4.47; p > 0.001), CUR (62.269 µg·L−1; G = 3.26; p > 0.001), and ISJ (135.850 µg·L−1; G = 3.33; p > 0.001). After determining the outliers, these were removed to perform the statistical analysis for each locality.
Figure 2 shows the frequency of distribution of blood Ba levels over four concentration ranges: ≤1 µg·L
−1, >1 to ≤10 µg·L
−1, >10 to ≥50 µg·L
−1, and >50 µg·L
−1. The distribution frequencies of the blood Ba levels were different among the ranges mentioned above, based on which the 10 localities could be divided into two different groups: Group 1 showed a frequency of Ba blood levels with the lowest concentrations < 1 µg·L
−1, mainly in ACUI and BIN, with concentrations ranging from >1 to ≤10 µg·L
−1 in VIT, CLA, and VCO. In Group 2 (BCA, VBJ, DMN, CUR, and ISJ), the frequency with the highest concentrations for blood Ba levels ranged from >10 to ≥50 µg·L
−1, with concentrations reaching > 50 µg·L
−1.
Table 5 and
Table 6 show data for all districts with Ba concentrations stratified per the co-variables studied (sex, age, duration of residency in the study area, alcohol consumption, smoking status, and source of drinking water).
The results showed significant differences among different sources of drinking water in BIN (
p = 0.002), between male and female residents in CUR (
p = 0.009), and among age groups in DMN (
p = 0.041).
Figure 3 depicts the trend in water consumption.
Table 7 and
Table 8 show the data for all locations with Ba concentrations stratified per eating habits. Significant differences were found in individuals from VIT who consumed Brazil nuts (
p = 0.035); individuals in CLA who consumed fish (
p = 0.034); individuals from VCO who consumed meat (
p = 0.050), fish (
p = 0.020), seafood (
p = 0.010), vegetables (
p = 0.031), and fruits (
p = 0.020); and individuals from VBJ who consumed Amazon nuts (
p = 0.052) and fruits (
p = 0.018), compared with the individuals who did not consume these food items.
Significant differences were found in blood Ba levels between individuals from CLA and VCO who consumed fish more than four times a week and individuals who did not.
Figure 4 shows the associations between blood Ba levels and fish consumption for all locations.
It was noted that individuals in 8 of the 10 localities had average blood Ba concentrations above the normal reference values reported in the literature (
Figure 5).
Statistical analysis in
Table 9 demonstrates the Spearman correlation coefficients and linear regression to identify the influence between Ba and the independent variables, also considering statistical significance (
p = 0.05). The correlation analysis showed that Ba concentrations in the blood were strongly affected in the vicinity of the kaolin factory by water consumption and the frequency of weekly fish consumption.
4. Discussion
This is the first study regarding environmental exposure to Ba in the Amazon in almost 50 years since the beginning of mineral mining and industrial development in the region. Blood is the most frequently used biological sample to measure exposure levels of metals in an organism. However, metal exposure can also be evaluated using other samples such as hair, bones, urine, and feces [
9]. Previous studies have shown that the blood levels of metals tend to be higher in populations living close to industrial areas [
12,
17,
32].
When comparing the results of the mean Ba levels in the blood of the study population with the results of studies of unexposed populations, we can observe that 8 of the 10 districts studied (VIT, CLA, VCO, VBJ, BCA, DMN, CUR and ISJ) exhibit higher concentrations than studies of unexposed populations such as Italy (AM = 1.2 µg·L
−1; N = 590), Argentina (AM = 0.48 µg·L
−1; N = 31) and Guinea in Africa (AM = 0.43 µg·L
−1; N = 70) [
22,
33,
34].
However, these levels were lower than those found in a population of mining workers in Russia (AM = 84.5 µg·L
−1; N = 50) and occupationally exposed individuals [
35]. These data indicate that most locations had greater environmental exposure to Ba than that reported in other countries and that living in industrial areas is a factor that contributes to increased blood Ba levels (
Figure 5).
Among the localities in Group 1, most individuals from ACUI (91.95%), BIN (88.30%), VIT (71.29%), CLA (57.09%), and VCO (48.70%) had blood Ba levels in the range of normal reference values for environmentally unexposed populations. However, notably, in VIT, CLA, and VCO, a high percentage (28.71–51.30%) of individuals presented with higher concentration ranges, indicating that blood Ba levels of individuals in these three localities may be gradually increasing over time, and if exposure to the source continues, individuals with normal Ba levels may eventually present with the profile of environmentally exposed populations.
These observations correspond with the records of significant increases in industrial production in Barcarena in the last decades [
26,
36]. The increase in production generates a greater amount of waste, leading to a greater production of effluents and expanding the need for larger areas for waste deposition [
3]. The data show that the areas marked for waste deposition have expanded in recent years. Additionally, the distance of these areas from residential localities in the region has gradually decreased [
27,
36].
In Group 2, most individuals in BCA (79.73%), VBJ (87.46%), DMN (100%), CUR (100%), and ISJ (96.77%) had blood Ba levels higher than the reference range for environmentally unexposed populations. The comparison tests between groups (
Table 5 and
Table 6) were indicative of a possible relationship between higher blood Ba levels and drinking water sources. Water sources are among the main sources of exposure to metallic elements [
32,
37].
BIN and VBJ showed blood cadmium (Cd) levels that were 1.5–3 times higher in individuals who consumed well water than in those who consumed general network water and also demonstrated higher blood lead (Pb) concentrations in consumers of well water than in those consuming network water in the DMN locality [
17,
38].
Ba is known to become naturally bioavailable in the environment from weathering processes, which are normally more intense in tropical regions with high temperatures and humidity [
2,
8,
32,
39]. However, Ba can also be introduced into the environment through effluent discharge and waste generated from industrial processes [
9,
22,
40].
Although only the data for BIN indicated a significant association (
p = 0.002) with drinking water sources,
Figure 3 indicates a trend of increases in the median blood Ba levels that were associated with the main sources of drinking water in these localities. Notably, the levels were higher in localities where the main sources of drinking water were wells or rivers. Importantly, ISJ, CUR, and BCA, all in Group 2, are located on the banks of the Curuperê and Dendê rivers, and individuals here presented with higher than normal blood Ba levels.
The Curuperê and Dendê rivers were, for almost a decade, the final destination of untreated effluents and debris from countless dam ruptures in the basins where kaolin processing plants had been constructed close to the spring sources of these water bodies since the 1980s. Data from 2003 and 2007 showed that there was a continuous discharge of untreated effluent on the margins of these water bodies, and the concentrations of Ba in surface waters reached 744 µg·L
−1 [
3,
23].
These levels were higher than those found in other rivers in the Amazon, for which surface water levels ranged from 3.1 to 56.1 µg·L
−1 [
41]. They were also lower than the results for drinking water in Santana in Amapá State (56–179 µg·L
−1), where the drinking water sources were underground wells or rivers (Amazon River) [
42]. Studies on the Murucupi River, also located in Barcarena, showed that the Ba levels varied from 14.6 to 76.7 µg·L
−1, indicating that the levels in the Curuperê and Dendê rivers were higher than the natural levels in the region [
18].
Only a study of the population of Kandal, a locality in the Mekong River basin in Cambodia with 1028 µg·L
−1 of Ba in the drinking water, reported values higher than those in our findings [
43]. Therefore, this information indicates that the higher blood Ba concentrations in the ISJ, CUR, and BCA localities may potentially be associated with the use of water from the Curuperê and Dendê rivers, which are water bodies that have received large amounts of effluent discharge and waste for years, without implementation of remediation measures [
3].
ISJ is a riverside locality on the banks of the Dendê River, which the population uses for leisure, fishing, and transportation; CUR is located adjacent to the kaolin processing industry and uses the Curuperê River for fishing. The wells in this region are considered to be shallow [
23,
44]. In cases of man-made disasters such as effluent spills, there are reports of changes in the color and odor of the rivers in these locations, as well as increased fish mortality rates [
3,
23,
25,
42].
Owing to the deposition of contaminants in these rivers over the years, the highest levels of Ba in these localities may be associated with particular sources of drinking water and fish consumption. The correlation and regression tests reinforce this assertion: whoever lives next to or close to the kaolin industry and consumes water from shallow wells or rivers is directly linked to having higher concentrations of Ba in the blood. The higher frequency of weekly fish consumption is also directly associated with higher concentrations of Ba in the blood of the population.
Geological studies of the kaolin extracted in the Capim region in the Amazon showed Ba levels between 15.8 and 39.8 mg kg
−1 in the ores [
8,
45]. In Botswana, up to 5806 mg kg
−1 of Ba was found in samples of kaolin deposits [
46], and in western Cameroon, there are reports of concentrations up to 699 mg kg
−1 [
47]. Similarly, the chemical profile of trace elements in the bauxite ore mining in Paragominas City, Pará State, showed BA levels in the range of 17 to 70 mg kg
−1 [
41]. Therefore, the presence of high Ba concentrations in the residues and effluents from kaolin and bauxite processing is expected.
The occurrence of the greater availability of Ba in drinking water has been shown in similar situations in Texas, Georgia, New Mexico, Kentucky, and Pennsylvania in the USA [
48]. Ingestion of Ba, even for relatively short periods, can cause gastrointestinal disorders, nausea, vomiting, colic, and muscle weakness. Further, ingestion of high levels for a long time can result in cardiovascular, kidney, metabolic, neurological, and mental disorders [
11,
49,
50].
Kaolin is also present in the geological layers of the subsoil in Barcarena and Abaetetuba. Georgia has the second-largest kaolin deposit worldwide [
3,
45]. However, this study showed that greater proximity to industrial areas and commercial ports contributes to high barium exposure, owing to the impact of large industries on the aquifers and nearby rivers. Among these industrial enterprises, kaolin processing and aluminum production have the greatest environmental impact.
However, BIN, which is also located adjacent to an industrial kaolin processing area, showed the lowest median blood Ba levels (median 0.299 µg·L
−1 N = 120 for wells; median 0.299 µg·L
−1 N = 67 for general network). In BIN after 2007, when the kaolin tailing basin ruptured, resulting in effluent contamination of the drinking wells, a deep well was built for water collection and subsequent distribution through the general network. At the time of this study, most residents no longer used shallow wells and did not normally have direct contact with the river [
17,
51]. Therefore, despite the proximity to the kaolin processing industries, the locality did not have high exposure levels. This finding indicates that the use of deep wells may reduce Ba exposure in the region.
ACUI (MED = 0.299 µg·L−1; N = 87) is a locality with an indigenous population that practices rural habits of the local cultivation of fruits and vegetables and animal husbandry in their backyards. However, this locality is situated at some distance from the industrial areas. On the other hand, VIT (MED = 0.299 µg·L−1 N = 114 for wells; MED = 0.299 µg·L−1 N = 92 for the general network) and CLA (MED = 0.320 µg·L−1 N = 215 for wells; MED = 0.300 µg·L−1 N = 80 for the general network) are more urbanized localities and less dependent on rivers in the region.
VCO (MED = 1.432 µg·L
−1 N = 100 for wells; MED = 1.346 µg·L
−1 N = 125 for the general network) is one of the oldest localities in the region. It is situated on the banks of the Pará River and next to large ports that are used daily for the transport of raw materials and manufactured products [
52].
BCA (MED = 9.21 µg·L
−1 N = 74 for wells) and DMN (MED = 8672 µg·L
−1 N = 42 for wells) are located close to the kaolin processing industries and record high levels of atmospheric pollution and deposition of untreated effluents or industrial waste. However, most residents of these localities use water from shallow wells [
53].
In VBJ (Median 13,381 µg·L
−1 N = 156 for wells; 13,354 µg·L
−1 N = 138 for the general network), there were no significant differences in the median Ba levels between individuals who consumed drinking water from wells and those who consumed water from the general network. The local sanitation plan of the locality comprises a small water collection and distribution system. A semi-artesian well supplies water to only 26.66% of the population, whereas the rest use other wells or rivers [
54].
Although only data for CLA (
p = 0.034) and VCO (
p = 0.031) showed that there was a significant association of Ba levels with the weekly frequency of fish consumption (more than four times a week) (
Figure 4), there were significant increments in the blood Ba level in individuals from other localities (
Figure 2). Ba is known to accumulate in fish and other aquatic organisms [
11,
49]. Therefore, in communities with a high consumption of fish, this can be one of the main sources of exposure to toxic metals [
13,
55]. The sustained higher-than-normal Ba levels in surface waters may be indicative of the accumulation of this metal in the fish population, which can ultimately lead to high Ba levels in the local population.
Data for VIT and VBJ showed significant associations between Ba levels and consumption of Brazil nuts two to four times a week. This finding corroborates reports that show that Ba accumulates in Brazil nuts, fruits, and vegetables [
11,
13,
56,
57].
Although most participants were women, no significant associations were observed between blood Ba levels and sex, confirming that blood Ba levels depend mainly on possible sources of exposure relevant to the locality. In Cambodia, there were no significant associations of Ba concentrations in hair with age or sex, corroborating the assumption that these variables do not influence Ba exposure, and the main factor is the proximity of exposure sources [
43].
Alcohol consumption and cigarette smoking were not associated with increased blood Ba levels, demonstrating that these habits are not exposure sources. Studies show that blood Ba levels are primarily associated with the proximity of exposure sources. Although Ba is known for its acute toxicity, epidemiological data for chronic exposure are still scarce. There are also important gaps in the current literature with respect to the identification of vulnerable populations and the factors that make some individuals susceptible to Ba toxicity [
11,
32,
40].
We emphasize that the main effect of Environmental Exposure to Barium is associated with changes in the sodium–potassium pump that regulates the circulatory system and may be associated with possible changes in the blood pressure of exposed individuals. This information on blood pressure was not collected during this research, but it alerts health agencies to the need for better monitoring of cardiovascular diseases through a future biomonitoring program. Cardiovascular diseases are the main health indicators associated with exposure to barium.