Introduction

The risk of exposure to metals found in the environment is a health concern for the rural communities in the southwest/mountain west region of the United States (U.S.) due to abundant mineral deposits and a land-use ethic that intimately connects many communities to the environment [1]. The potential for daily activities, and traditional cultural practices, to result in community-level non-occupational exposure to metal mixtures is a new and recently identified public health concern in the region. A recent review of the epidemiological literature demonstrated consistent adverse health outcomes associated with arsenic (As) and cadmium (Cd) exposures among rural, minority populations living in this region [2]. This review also determined that the exposure range and assessment methods varied widely across the epidemiology studies. Additionally, exposure assessment studies have been conducted for other metals common to the region, but are not represented in the epidemiological literature alone. Thus, a review of these exposure-specific results was needed to support future epidemiologic health studies of metals and metal mixture exposure. Our objective is to conduct a systemized review of environmental metal exposure studies conducted in rural areas of the southwestern/mountain west region of the U.S. published between June 2013 and June 2018.

Methods

Database Search and Eligibility Criteria

The authors met to discuss inclusion/exclusion criteria and scope of the search prior to database searching. Searches were performed in PubMed, Web of Science, and Google Scholar in May 2018 using controlled and keyword terms for environmental exposure, rural, minority, and various monitoring terms such as blood, urine, water, soil, and biomonitoring. Searches in each database were limited to those studies published in the 5-year period between June 2013 and June 2018 in English. The full search strategy is available in supplemental information.

Studies were eligible for inclusion if they included an environmental exposure assessment; conducted in the Southwest and Mountain West geographical locations of the U.S., an area including the states of Arizona (AZ), Colorado (CO), Nevada (NV), New Mexico (NM), Texas (TX), and Utah (UT), and reported metal contaminant exposures. These states were selected because of the prevalence of mining-related exposure sources, substantial representation of isolated, rural, and minority populations with distinct exposure risks not represented in other geographic areas of the U.S., and environmental health research in this area is underrepresented in the published literature [2]. Studies that were conducted in urban locations were excluded. Three investigators independently screened titles and abstracts against the exclusion criteria. The same investigators then screened full-text articles against the inclusion criteria and met in person to discuss and resolve any discrepancies. The review process was managed with the systematic review application Rayyan [3].

Data Abstraction, Evaluation, and Synthesis

Data were abstracted from the records to capture the metal, study location, population impacted, type of exposure, exposure metric, exposure assessment methods, exposure estimates, and any human health outcomes that were examined. The main results during the article evaluation and selection phase were defined as the measures of exposure. Records were also classified by exposure type and the data were organized into tables.

Results

The systematized review of environmental exposure studies yielded information about (1) the study area and populations investigated; (2) the analytical methods used to quantify exposure; and (3) exposure estimates in biological and environmental media. The results included studies from eight states that most commonly measured arsenic (As), uranium (U), or cadmium (Cd) in environmental or biological media using spectroscopy methods.

Search Results

Our search identified 165 studies via database searching and hand-searching relevant publications, which included scanning reference lists; keyword searching in Google, Google Scholar, and PubMed; and citation searching in Web of Science and Google Scholar. After removing duplicates, 139 records remained to be screened by review of the title and abstract, of which, 81 were excluded. After screening the remaining studies by reading the full text (N = 58), we further excluded 31 that did not fully fit the above inclusion criteria, as they were not a rural population (N = 3), were an incorrect study design (N = 7), were outside of the geographical region of interest (N = 1), or did not evaluate exposure to a metal (N = 20). Twenty-seven studies were included in our final narrative synthesis. Figure 1 is a flowchart of the search, screening, and inclusion/exclusion process. Tables 1, 2 and 3 describes the data organized by exposure category: air, water/soil, and biomarker.

Fig. 1
figure 1

Flowchart of the search, screening, and inclusion/exclusion process. For more information, visit www.prisma-statement.org

Full size image
Table 1 Water/Soil/Sediment. Summary of environmental exposure studies among rural, minority populations in the southwest/mountain west region of the United States published between 2013 and 2018 by media used to assess exposure
Full size table
Table 2 Biological. Summary of environmental exposure studies among rural, minority populations in the southwest/mountain west region of the United States published between 2013 and 2018 by media used to assess exposure
Full size table
Table 3 Air. Summary of environmental exposure studies among rural, minority populations in the southwest/mountain west region of the United States published between 2013 and 2018 by media used to assess exposure
Full size table

Study Locations and Populations

The results included studies from eight states (AZ, CO, NM, TX, Oklahoma (OK), North Dakota (ND), South Dakota (SD), and UT). Five of these states were included among our original six target states and OK, ND, and SD were included as these populations were aggregated with a large rural Native American cohort from AZ; it was not possible to disaggregate the results by geographic location.

Twelve of the 27 reviewed articles investigated metal exposure and health outcome using an epidemiological study design. Of these, ten studies were from the Strong Heart Study cohort of Native Americans living in non-urban locations included in the geographic area of this review [4,5,6,7,8,9,10,11, 12•, 13]. Additionally, two studies were included from the Facing Rural Obstacles Now Through Intervention, Education, and Research (FRONTIER) study, based in rural western TX and focused on Hispanics [14, 15]. Fourteen of the reviewed studies quantified metal exposure based on concentrations in environmental media (water, air, soil), vegetation, or livestock meat and organs that may be consumed by local communities, but did not associate exposure levels with health outcomes. These studies were included in our review because they assessed environmental metal exposure potential for a rural population in the study region. Two of these studies reported associations between environmental metals in water and markers of potential health effect [16, 17••], and two other studies reported associations between As in water with levels in blood or nail clippings [18, 19••].

Analytical Methods Used to Quantify Exposure

Results indicate that inductively coupled plasma (ICP)-optical emissions spectroscopy and ICP-mass spectroscopy were most commonly employed to measure metal concentrations in environmental and biological media. Use of more specialized analytical methods, such as X-ray fluorescence, X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy were used in environmental studies.

Methods Used to Measure Metals in Environmental Media

Concentrations of As, U, Cd, and other metals were reported for public water sources and unregulated water sources (e.g., private groundwater wells). Sampling methods were conducted in a prospective fashion including one-time grab samples and repeat sample collection to observe seasonal variability. Chemical concentrations were measured using inductively coupled plasma (ICP)-optical emissions spectroscopy [16, 17••, 20, 21••, 22], ICP-atomic emissions spectroscopy [16, 17••, 20, 21••], ICP-mass spectroscopy [14, 16, 17••, 18, 20, 21••, 22,23,24,25], or graphite furnace atomic absorption spectroscopy [19••]. Additionally, concentrations of As, U, chromium (Cr), lead (Pb), iron (Fe), and vanadium (V) were measured in mine waste, soil, sediment, and other solid material using X-ray fluorescence [22, 23], X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy [22]. ICP-MS was also used to measure As, U, Cd, selenium (Se), Pb, and molybdenum (Mo) in soil and in grasses consumed by sheep [26•].

Methods Used to Measure Metals in Particulate Matter

Metal concentrations in particulate matter (PM) were reported for two studies. Gonzales-Maddux et al. (2014) used a filter-based chemical speciation monitor to collect PM2.5 samples [27]. Subsequently, concentrations of 64 trace elements were measured using a double-focusing magnetic sector ICP-MS. Beamer et al. (2014) used ICP-MS to quantify As, Cd, aluminum (Al), beryllium (Be), Cr, Pb, manganese (Mn), and nickel (Ni) in PM collected using dust fall passive samplers and active air samplers [28••].

Methods Used to Measure Metals in Biological Media

Spot urine samples were collected and analyzed for total As, U, Cd, and other metals using ICP-mass spectroscopy (ICP-MS). Additionally, As species concentrations were measured with high-performance liquid chromatography coupled with ICP-MS [4, 5, 9, 13] or ion-pair chromatographic separation with hydride generation-atomic fluorescence detection [19••]. Blood As, Cd, and Pb were also measured using ICP-MS [18] and toenail total As was measured by instrumental neutron activation analysis [19••]. Samuel-Nakamura et al. (2017) quantified accumulated As, U, Cd, Se, Pb, and Mo in sheep muscle, bone, intestine, lung, liver, kidney, and wool using ICP-MS [26•].

Exposure Estimates for Metals and Metal Mixtures

Results indicated that As, U, and Cd were the most commonly quantified metal exposures in the geographic study area. One or more of these metals were analyzed in each study reviewed. Reviewed articles also reported metal concentrations of Al, Be, Cr, copper (Cu), Cr, Fe, Pb, Mn, Mo, Ni, Se, and V.

Public Water Supply

Arsenic was the only metal evaluated in regulated, public water sources (Table 4). The median As concentrations reported from two studies of public water supplies in west Texas ranged from 3.8–7.8 μg/L [18, 25] (Table 5). Calderon et al. (2014) also measured As concentrations in public water supply sources in Nevada but did not report ambient concentrations [19••].

Table 4 Metals in environmental and biological media
Full size table
Table 5 Median concentrations of directly measured arsenic, uranium, and cadmium in water, soil/sediment, urine, or particulate matter in exposure studies conducted in the rural, southwestern/mountain west region of the U.S.
Full size table

Unregulated Water Sources

In unregulated water sources (UWSs), primarily groundwater wells in AZ and NM, median total As and U concentrations ranged from 1.95–6.4 and 2.3–3.8 μg/L respectively. Minimal seasonal/temporal variability was reported for As and U measured in UWSs in the Arizona portion of Navajo Nation [24] and UWSs across the Navajo Nation [20]. Blake et al. (2017) observed As concentrations less than 10 μg/L in replicate surface water sources located downstream of a U mine in New Mexico [23]. In the same samples, however, U concentrations ranged between < LOD and > 700 μg/L and exhibited a strong season effect. Two additional studies measured As and U concentrations in both public supply and UWSs in the Navajo Nation [16, 17••]. These measurements were used to estimate annual oral intake of As and U for modeling and were not reported as ambient measurements.

The reviewed studies indicated that Cd water concentrations were generally low in the study area. Hoover et al. (2018) reported Cd concentrations were less than the limit of detection (1 μg/L) for more than 70% of unregulated water sources on the Navajo Nation tested between 1998 and 2010 [21••]. Samuel-Nakamura et al. (2017) reported Cd concentrations ranging from 0.03 to 0.65 μg/L for eight water sources used by sheep in a Navajo community in New Mexico [26•]. Four studies reported concentrations of other metals in UWSs (Table 4) including iodine in groundwater in west TX [15, 29]; V [22], Al, Fe, Mn, Pb, Se, Mo, and other metals [21••, 22, 23, 26•].

Soil, Sediment, and Mine Waste

Three studies were conducted in or near abandoned U mining sites. Blake et al. (2017) reported 9300 mg/kg of U in unremediated mine waste materials mixed with soil and 320–1400 mg/kg in sediment samples. Concentrations of other metals were at or below the limit of detection in soil and sediment samples. At an abandoned uranium mine site in AZ, Blake et al. (2015) reported U, V, and As concentrations of 6614, 15,814, and 40 mg/kg respectively. Samuel-Nakamura et al. (2017) also reported soil concentrations of U, As, Cd, and Pb to range from 0.36–1.15, 1.20–4.53, 0.05–0.17, and 3.91–9.07 mg/kg respectively.

Ambient and Indoor Particulate Matter

Metals concentrations in particulate matter (PM) were quantified in two studies. Beamer et al. (2014) reported concentrations of eight metals in particulate matter samples collected by active and dust fall samplers in a mining community located in southern AZ. Reported mean concentrations were 972 μg/m3 for Al, 1.39 μg/m3 for As, 0.385 μg/m3 for Cd, and 0.028 μg/m3 for Be. Gonzales-Maddux et al. (2014) used active sampling methods to collect PM2.5 inside of homes in the Navajo Nation in northern NM and reported analytical results for 64 elements. The average PM2.5 concentrations across all indoor samples were 7.0 μg/m3; the geometric means of As and U were 0.18 and 0.01 μg/m3 respectively. Principle components analysis indicated that indoor PM likely originated from local soil, coal combustion, industrial activity, and sea salt.

Urine

Median total As concentrations (creatinine adjusted results) ranged from 9.7 μg/g in Strong Heart Study participants (AZ, OK, ND, and SD) to 14 μg/g among Hispanics non-White men and women living in southern NM [5, 10, 13, 30], and 4.5 μg/L on the Navajo Nation and 39.0 μg/L in Nevada among non-adjusted results [16, 19••]. Urine As results were also reported as the sum of inorganic and methylated species [9] and as the percentages of inorganic, methylarsonate, and dimethylarsinate to their sum [4], among Native American participants of the Strong Heart Study.

Adams et al. (2015) reported a median creatinine-corrected U concentration of 0.013 μg/g for Hispanic non-White men and women in southern NM. For Navajo Nation residents living in New Mexico, Harmon et al. (2018) reported that 14.6% of study participants had urine U concentrations exceeding the NHANES 95th percentile (0.031 μg/L) for the 2003–2004 cycle.

Median-adjusted urinary Cd concentrations ranged from 0.30–0.94 μg/g creatinine. Four studies measured urinary Cd in samples from 2864 to 3792 Native American adult participants of the Strong Heart Study, and all reported a median adjusted Cd concentration of 0.94 μg/g in their epidemiological analyses [6,7,8, 11]. An additional study examining 1725 Strong Heart Study participants reported a lower median creatinine-corrected urinary concentration of 0.44 μg/g among the subset of participants in their analyses [12•]. Adams et al. (2015) reported a median adjusted urine Cd concentration of 0.30 μg/g for Hispanic residents of southern NM. The same study also reported adjusted urinary concentrations of lead. Harmon et al. (2018) reported urinary Cu, Ni, and V results for participants in a Navajo cohort.

Vegetation, Livestock Tissue, and Organs

Samuel-Nakamura et al. (2017) reported that the kidney cortex had greater uptake of U, Se, Mo, and As compared to the kidney medulla. The liver uptake of Se and Mo was observed, as well as Pb accumulation, in wool. The authors noted that the mean concentrations of these metals did not exceed National Research Council maximum tolerable concentrations in the shoots or roots of the collected plants [31]. It was also observed that roots generally had higher metal concentrations of U and As than the above-ground shoots. Cd, Se, and Mo were both observed to accumulate at higher concentrations in shoots compared to roots.

Conclusions

Results indicated that exposure to As, U, Cd, either alone or in combination, were quantified in each study reviewed. Exposure was also assessed for other metals including Al, Be, Cu, Fe, Pb, Mo, Ni, Se, and V. Although small, the current body of literature suggests that rural populations in the southwest, mountain west, and adjacent regions of the U.S. experience exposures to mixtures of environmental metals. Rural populations in this region experience exposure to metals through unique pathways and sources, which differ from those in urban areas. Sources include active and abandoned mining and smelter operations, locally grown foodstuffs (plants and free-range livestock), and contaminated natural materials, such as the wool from locally raised sheep for weaving.

Most of the reviewed studies applied direct measurements to assess metal exposures. Cumulative, body burden of exposure was assessed through measurement of metals accumulated in biological samples (e.g., urine, blood, and nail clippings from humans) and in livestock tissues and organs used for food. Exposure to metals was also directly measured in environmental media such as public water supplies, unregulated water sources (e.g., private wells), soil, indoor and outdoor particulate matter, local vegetation used for food, and in wool collected and used by local weavers. Indirect exposure assessment methods incorporated directly measured concentrations of metals to model representative exposures based on geographic proximity to sources or to more precisely estimate individual-level exposures by applying survey-reported intake of the potentially contaminated media.

Six studies illustrated how individual survey results and spatial analysis methods can model exposure estimates using direct measurements. In the FRONTIER study in west TX, direct measurements were used in geospatial models to estimate groundwater concentrations at unsampled locations [14, 15]. Hoover et al. (2017) used geospatial spatial analysis methods to assess the influence of distance from an abandoned U mining (AUM) site on levels of As and U measured in UWSs on the Navajo Nation [20]. Additionally, Hoover et al. (2018) identified metal mixtures found in UWSs on the Navajo Nation and evaluated the geographic distribution of these metal mixture clusters using spatial analysis methods [21••]. Also on the Navajo Nation, Harmon et al. (2017) used residential proximity to AUMs sites, weighted by surface area of each AUM, to assess exposure in a cohort study [17••]. Harmon et al. (2017, 2018) also estimated annual As and U intake modeled from self-reported volume of water consumed and the measured metal concentrations for each water source used [16].

A previous review of the epidemiological literature in this study area reported consistent adverse health outcomes associated with environmental exposure to particulate matter, As and Cd, for rural, minority populations [2]. The health effects associated with metal exposure in this region are comparable in magnitude to those reported in urban settings, often at lower exposure levels. This observation highlights a gap in the current understanding of the role of exposure duration in rural communities given their prolonged, close contact with the natural environment. Our current review emphasizes the unique exposure pathways and the significant risk of exposure to environmental metals, especially U, Cd, and As, in this region. The studies reviewed attempted to identify potential sources of metal exposure and provide information about the levels of exposures experienced by these populations through a variety of methods including spatial analytical methodologies.

It is important to emphasize that gaps remain in our understanding of the associations between environmental metal exposures and health effects, especially effects from long-term exposures, as the current body of environmental epidemiologic studies in this region is sparse. There remains an opportunity to expand the use of existing exposure assessment methods into population studies in the region. In addition, population-representative exposure assessments may require novel refinements to modeling methods used in more urban and densely populated settings to account for the different resource- and land-use patterns among rural minority populations in the southwestern/western region of the U.S.