**2.1 Measurement of trace element levels in soils**

The soil background concentrations/levels will depend on the mineralogical composition of the parent rock material and on the weathering processes that have led to its formation, the granulometry fractions, and the organic matter content [17–19]. These background measurements, which represent natural concentrations in unpolluted pristine soils, are very difficult to assess because they require a soil free of contamination. Given this difficulty, the measurement usually applied is the geochemical baseline concentration that represents an expected range of element concentrations around medium normal sample mean [20]. Although the TE baseline concentration levels in the soil may differ between countries and/or geographical regions, their assessment has been recognized as the only means to establish reliable worldwide elemental concentrations in natural materials [21, 22].

The measurement of TE in soils requires well-planned sampling strategies to achieve accurate data. There are several defined protocols for soil sampling and many digestion techniques to optimize the TE quantification [23]. The conventional methods are based on a regular soil sampling design, with soil sample collection at a depth of 0–20 cm and subsequent chemical analysis of the sampled soils in the laboratory,

followed by geostatistical interpolation of the data to obtain the spatial distributions of soil heavy metal content. For the assessment of TE in agricultural areas, the protocol of geochemical mapping of agricultural soils and grazing land of Europe (GEMAS) is the most used as the aim of the project is to provide harmonized geochemical data of arable land and of land under permanent grass cover at the continental European scale. The application of this protocol for meadows requires that all samples will be taken as rather large (2–2.5 kg) composite samples from one extensive field; the minimum size of field should be about 25 × 50 m [24]. The sampling stage is critical and it must take into account what we want to measure and the geological attributes of the site. Also, in order to avoid cross-contamination in the sampling of TEs, metal tools should not be used in the field or in the lab. The sample preparation and storage in the lab often require that the soil samples are air-dried and sieved to less than 2 mm [25]. Afterward, the total TE contents or the extractable fraction can be determined.

Given that the application of these methods has some disadvantages, since they are time-consuming and costly and cannot provide accurate estimates of soil heavy metal content over large areas, new approaches such as remote sensing are starting to be widely used as they can rapidly lead to spatially explicit estimates of soil heavy metal content and monitor their dynamics at a regional scale with low cost [26]. By capturing electromagnetic radiation reflected from the target, remote sensing can be used in the detection of heavy metals in soil and vegetation [27]. However, soil's properties cannot be easily assessed using hyperspectral sensing so the monitoring heavy metal contamination in soils has not been assessed comprehensively and it needs further studies [28].

The assessment of the total concentration of trace elements is required to: (i) determine the background (natural) TE levels in the soil; (ii) assess the total metal content; and (iii) evaluate if there has been TE accumulation over time [29, 30]. To assess the total concentration of TE, soils need to be digested to break down the primary silicate structures of the more resistant quartz and feldspar soil minerals and release the TE into solution. The most common types of digestion are carried with concentrated nitric acid and hydrogen peroxide or with a mixture of *aqua regia* concentrated nitric and hydrochloric acids.

Finally, the elemental concentrations of the digest solutions can be determined by spectroscopic methods, such as atomic absorption spectroscopy (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), or by plasma mass spectrometry (ICP-MS) [31]. ICP-OES and ICP-MS have more advantages when compared with the AAS, as they allow one to obtain numerous data from running the sample just once and have very low detection limits [32]. While ICP-OES is based on the measurement of excited atoms and ions at the wavelength characteristic for the specific element being measured, ICP-MS measures an atom's mass by mass spectrometry (MS). These distinct approaches result in different lower detection limits; the lower detection limit in ICP-OES is in parts per billion (ppb) while in ICP-MS can be extended to parts per trillion (ppt) [33]. On the study of trace elements in environmental samples, ICP-OES is more commonly used since it may be applied for samples with high total dissolved solids or suspended solids and is, therefore, more robust for analyzing groundwater, wastewater, soil, and solid waste. It is therefore, usually used to measure contaminants for environmental safety assessment and elements with a higher regulatory limit [31]; if the trace elements in study have very low regulatory limits, ICP-MS is adequate for quantification.

#### **2.2 Sources of trace elements**

Trace elements can enter the soil by natural or anthropogenic sources [34], and their behavior and fate in soils differ according to their source and species.

**105**

*Trace Elements in Volcanic Environments and Human Health Effects*

The most important anthropogenic sources of trace elements for soils include

There are also several natural sources (and processes) that contribute to TEs' deposition in soils. The main natural source is the soil parent material as the weathering of rocks and mineral deposits produces metals in dust, sediments, groundwaters, and surface waters [17]. Forest fires with the release of metal and mineral particulate matter in ash, gas, and aerosols or even sea spray with metals in the water droplets can also contribute to the deposition of TEs in soils, not only in the vicinity of their sources but also in areas far away. Another natural factor of major importance is geological activities; they are rather volcanic activities, earthquakes, landslides, debris flows, etc. all of which introduce major and trace elements into the environment [41]. The concentration of metallic TEs is the most affected by volcanic activity [42], resulting in high baseline concentrations of some metals in

The excess of metals in soils may affect the surrounding ecology and human health as these elements are non-biodegradable and therefore environmentally persistent [44]. While some of these trace elements are essential for plant growth and development, the monitoring of TE baseline values in soils is fundamental since at elevated levels they can become toxic. The toxicity of TEs will depend on the dose, exposure pathway, and duration of exposure [45, 46]. Regarding the bioavailability of TE in soils, the context is much wider as it includes chemical availability to a variety of biota [47]. Even with no universal definition, it is assumed that for the study of the environmental risk assessment of TEs in soils, the bioavailability assessment should include their soluble and solid-phase-associated labile fractions. Although the dissolution-desorption, transport, and uptake are very complex processes, the Committee on Bioavailability of Contaminants in Soils and Sediments [48] considered that bioavailability is the fraction of the total concentration of contaminants present in soil (solid and solution phases) which is potentially available for plant

Considering that the excess of TEs in soil due to volcanic activity cannot be controlled, and that TE ingestion, inhalation, or dermal contact can cause damage to several human systems, the monitoring and baseline determination of TEs in volcanic environments assume particular importance for the inhabitants of these areas.

mining and metallurgical activities, commercial fertilizers, biosolids, irrigation water, coal combustion residues, and auto emissions [34, 35]. Mining and metallurgical activities are recognized as the most important producers of waste and environmental pollution; the metalliferous mines, processing plants, and smelters generate huge amounts of mine tailings that can be transported mainly in the form of wastewater and airborne dust particles [36–38]. Using fertilizers in intensive agriculture can increase the TEs in the soil given that fertilizers contain trace amounts of several elements [15]. Sewage sludges and effluents also contain variable amounts of trace elements of various nature and various anthropogenic origins, although they can be an interesting way to enrich the soil with organic matter [39]. The direct use of sewage sludge can lead to heavy metal phytotoxicity problems due to the lack of stability of organic matter that could be obtained through an appropriate composting process. The TEs also enter the soil trough the water used for irrigation or even by the atmospheric deposition from industrial and urban combustion emissions. The TEs can travel long distances in either gaseous form or in particle phase before deposition; therefore, the contaminated zones can extend up to a great distance from the contamination site. This is easily observed with the automobile exhaust emissions that contaminate not only the roadside soils but, depending on the location, traffic intensity and predominant wind conditions (direction and frequency); the contamination can be observed in soils hundred

*DOI: http://dx.doi.org/10.5772/intechopen.90786*

meters apart from the road [40].

volcanic soils [15, 43].

uptake or absorption by soil-dwelling organisms.

#### *Trace Elements in Volcanic Environments and Human Health Effects DOI: http://dx.doi.org/10.5772/intechopen.90786*

*Trace Metals in the Environment - New Approaches and Recent Advances*

followed by geostatistical interpolation of the data to obtain the spatial distributions of soil heavy metal content. For the assessment of TE in agricultural areas, the protocol of geochemical mapping of agricultural soils and grazing land of Europe (GEMAS) is the most used as the aim of the project is to provide harmonized geochemical data of arable land and of land under permanent grass cover at the continental European scale. The application of this protocol for meadows requires that all samples will be taken as rather large (2–2.5 kg) composite samples from one extensive field; the minimum size of field should be about 25 × 50 m [24]. The sampling stage is critical and it must take into account what we want to measure and the geological attributes of the site. Also, in order to avoid cross-contamination in the sampling of TEs, metal tools should not be used in the field or in the lab. The sample preparation and storage in the lab often require that the soil samples are air-dried and sieved to less than 2 mm [25]. Afterward, the total TE contents or the extractable fraction can be determined. Given that the application of these methods has some disadvantages, since they are time-consuming and costly and cannot provide accurate estimates of soil heavy metal content over large areas, new approaches such as remote sensing are starting to be widely used as they can rapidly lead to spatially explicit estimates of soil heavy metal content and monitor their dynamics at a regional scale with low cost [26]. By capturing electromagnetic radiation reflected from the target, remote sensing can be used in the detection of heavy metals in soil and vegetation [27]. However, soil's properties cannot be easily assessed using hyperspectral sensing so the monitoring heavy metal contamination in soils has not been assessed comprehensively and it

The assessment of the total concentration of trace elements is required to: (i) determine the background (natural) TE levels in the soil; (ii) assess the total metal content; and (iii) evaluate if there has been TE accumulation over time [29, 30]. To assess the total concentration of TE, soils need to be digested to break down the primary silicate structures of the more resistant quartz and feldspar soil minerals and release the TE into solution. The most common types of digestion are carried with concentrated nitric acid and hydrogen peroxide or with a mixture of

Finally, the elemental concentrations of the digest solutions can be determined by spectroscopic methods, such as atomic absorption spectroscopy (AAS), inductively coupled plasma optical emission spectroscopy (ICP-OES), or by plasma mass spectrometry (ICP-MS) [31]. ICP-OES and ICP-MS have more advantages when compared with the AAS, as they allow one to obtain numerous data from running the sample just once and have very low detection limits [32]. While ICP-OES is based on the measurement of excited atoms and ions at the wavelength characteristic for the specific element being measured, ICP-MS measures an atom's mass by mass spectrometry (MS). These distinct approaches result in different lower detection limits; the lower detection limit in ICP-OES is in parts per billion (ppb) while in ICP-MS can be extended to parts per trillion (ppt) [33]. On the study of trace elements in environmental samples, ICP-OES is more commonly used since it may be applied for samples with high total dissolved solids or suspended solids and is, therefore, more robust for analyzing groundwater, wastewater, soil, and solid waste. It is therefore, usually used to measure contaminants for environmental safety assessment and elements with a higher regulatory limit [31]; if the trace elements in

study have very low regulatory limits, ICP-MS is adequate for quantification.

their behavior and fate in soils differ according to their source and species.

Trace elements can enter the soil by natural or anthropogenic sources [34], and

*aqua regia* concentrated nitric and hydrochloric acids.

**104**

**2.2 Sources of trace elements**

needs further studies [28].

The most important anthropogenic sources of trace elements for soils include mining and metallurgical activities, commercial fertilizers, biosolids, irrigation water, coal combustion residues, and auto emissions [34, 35]. Mining and metallurgical activities are recognized as the most important producers of waste and environmental pollution; the metalliferous mines, processing plants, and smelters generate huge amounts of mine tailings that can be transported mainly in the form of wastewater and airborne dust particles [36–38]. Using fertilizers in intensive agriculture can increase the TEs in the soil given that fertilizers contain trace amounts of several elements [15]. Sewage sludges and effluents also contain variable amounts of trace elements of various nature and various anthropogenic origins, although they can be an interesting way to enrich the soil with organic matter [39]. The direct use of sewage sludge can lead to heavy metal phytotoxicity problems due to the lack of stability of organic matter that could be obtained through an appropriate composting process. The TEs also enter the soil trough the water used for irrigation or even by the atmospheric deposition from industrial and urban combustion emissions. The TEs can travel long distances in either gaseous form or in particle phase before deposition; therefore, the contaminated zones can extend up to a great distance from the contamination site. This is easily observed with the automobile exhaust emissions that contaminate not only the roadside soils but, depending on the location, traffic intensity and predominant wind conditions (direction and frequency); the contamination can be observed in soils hundred meters apart from the road [40].

There are also several natural sources (and processes) that contribute to TEs' deposition in soils. The main natural source is the soil parent material as the weathering of rocks and mineral deposits produces metals in dust, sediments, groundwaters, and surface waters [17]. Forest fires with the release of metal and mineral particulate matter in ash, gas, and aerosols or even sea spray with metals in the water droplets can also contribute to the deposition of TEs in soils, not only in the vicinity of their sources but also in areas far away. Another natural factor of major importance is geological activities; they are rather volcanic activities, earthquakes, landslides, debris flows, etc. all of which introduce major and trace elements into the environment [41]. The concentration of metallic TEs is the most affected by volcanic activity [42], resulting in high baseline concentrations of some metals in volcanic soils [15, 43].

The excess of metals in soils may affect the surrounding ecology and human health as these elements are non-biodegradable and therefore environmentally persistent [44]. While some of these trace elements are essential for plant growth and development, the monitoring of TE baseline values in soils is fundamental since at elevated levels they can become toxic. The toxicity of TEs will depend on the dose, exposure pathway, and duration of exposure [45, 46]. Regarding the bioavailability of TE in soils, the context is much wider as it includes chemical availability to a variety of biota [47]. Even with no universal definition, it is assumed that for the study of the environmental risk assessment of TEs in soils, the bioavailability assessment should include their soluble and solid-phase-associated labile fractions. Although the dissolution-desorption, transport, and uptake are very complex processes, the Committee on Bioavailability of Contaminants in Soils and Sediments [48] considered that bioavailability is the fraction of the total concentration of contaminants present in soil (solid and solution phases) which is potentially available for plant uptake or absorption by soil-dwelling organisms.

Considering that the excess of TEs in soil due to volcanic activity cannot be controlled, and that TE ingestion, inhalation, or dermal contact can cause damage to several human systems, the monitoring and baseline determination of TEs in volcanic environments assume particular importance for the inhabitants of these areas.

#### **2.3 Trace elements in volcanic soils**

There are several hazards that result from a volcanic eruption; the most immediate and threatening hazards are pyroclastic falls, pyroclastic density currents, lava (flows and domes), lahars and flooding, debris avalanches, and volcanic gases. Nevertheless, volcanic emissions also occur in the post-eruptive phase and in quiescent volcanoes, continuing to affect the ecosystem and consequently human health.

Volcanic regions step-up important scenarios for the study of TEs in soils because: (i) they are densely inhabited in some areas of the Earth and (ii) due to the physicochemical properties, volcanic soils retain TEs, acting as a reservoir and affecting agriculture [15].

#### **2.4 Human health effects**

The concern regarding the health effects of environmental exposure to TEs from natural sources has driven the development of tools and methods for assessing the impact of emissions in water, soil, and air. Biological monitoring or biomonitoring is the most commonly applied method to measure human exposure to xenobiotics [49]. There are several studies worldwide that establish the association between concentrations of TE in volcanic soils and its effects on human health.

One of the most well-detailed health problem associated with volcanic activity is fluorosis, which results in high fluoride (F) concentrations in groundwater. The problem was first recognized in Japan and was called "Aso volcano disease" [50], but during the course of the year, high fluoride concentrations (greater than the WHO guideline value of 1.5 mg/l) were also found in Africa, where the crater lakes of western Uganda often have high F concentrations (e.g.4.5 mg/l F in Lake Kikorongo) [51]. High concentrations of F were also found in eastern Turkey, near the Tendurek Volcano, where the natural waters contained fluoride levels between 2.5 and 12.5 ppm [52], and in oceanic islands, such as Tenerife Island (Canary-Spain) [53] and São Miguel Island (Azores-Portugal) [54–56].

Another health problem that has been proven to be strongly associated with the exposure to volcanic environments is thyroid cancer, in particular papillary thyroid cancer (PTC). In 2009, Pellegriti et al. [57] in a register-based epidemiologic study showed that the residents of Catania, a province in the vicinity of Mt. Etna, presented a higher incidence of papillary thyroid cancer than elsewhere in Sicily. More recently, these results were reinforced in the study conducted by Malandrino et al*.* [58]. In this study, the authors evaluated the environmental pollution and biocontamination in a volcanic area of Sicily and compared the data with the thyroid cancer epidemiology data obtained from the Sicilian Regional Registry for Thyroid Cancer. Their results indicated that the residents in Mt. Etna volcanic area had significantly higher levels of several TEs in their urine when compared to the control area [Cd (×2.1), Hg (×2.6), Mn (×3.0), Pa (×9.0), Th (×2.0), V (×8.0)] and that thyroid cancer incidence was 18.5 and 9.6 per 105 inhabitants in the volcanic and the control areas, respectively; the observed thyroid cancer incidence was exclusively from the papillary histotype.

Besides these, that pose as the most studied TE and linkages to human health effects, there are several other health issues that have an increased risk due to environmental exposure. In Sicily, Italy, various soil types developed from different parent materials were analyzed to compare heavy metal distribution under different geopedological conditions, evidencing that the former depended on the parent rocks [59]. In Turkey, an association was established between the volcanic soil and the high prevalence of upper gastrointestinal cancer rates in the Van region as the fruit and vegetable samples produced in those soils contained potentially

**107**

*Trace Elements in Volcanic Environments and Human Health Effects*

of not only dental fluorosis but also of skeletal fluorosis.

The Azores archipelago is located in the North Atlantic Ocean, in the triple junction of the North American, African, and Eurasian plates [71–73]. The archipelago is formed by nine islands of volcanic origin that represent the emerged part of the Azores Plateau, a thick and irregular area of the oceanic crust roughly limited by the 2000-m bathymetric curve [74]. As a result of the Azores archipelago's location on an active plate boundary, frequent seismic and volcanic activity occurs, including volcanic eruptions and secondary manifestations of volcanism, such as hydrother-

São Miguel, the largest island of the archipelago, is formed by five active volcanic systems, including three central active volcanoes (Sete Cidades, Fogo and Furnas), separated by two fissure systems (Picos and Congro), and two extinct

Since the volcanic activity on the island contributes to a distinct soil elemental profile, resulting in a higher baseline for elements, the study of the baseline levels of TE is fundamental. When bioavailable to plants, animals, and humans, TEs can cause several diseases due to their elevated concentrations in soils. In addition, the high concentrations of some TEs in soils can inhibit the bioavailability of other elements, promoting the deficiency in elements that can be essential for plants,

**3. Azores as a volcanic scenario**

mal vents and soil degassing processes.

animals, and human health.

volcanic systems (Povoação and Nordeste) [75, 76].

carcinogenic levels of heavy metals [60]. More recently, Rodriguez-Espinosa and co-authors [61] analyzed the elemental composition of 25 soils and ash samples after the eruptions of the Popocatépetl Volcano in México, observing that the concentrations of TEs such as Zn, Pb, Ni, Hg, Cr, Cd, Cu, and As were significantly higher than those observed in older samples from eruptions in 1997, suggesting that the naturally highly volatile and mobile metals leach into nearby freshwater sources. In the Azorean volcanic islands, there are also some studies focusing in the concentrations of TEs in soils [15, 62] and its effects on local organisms [39, 63, 64] and human health [50, 53], the latter being particularly focused on fluoride [65–67]. In 2016, Linhares and co-authors [65] verified that there are areas in São Miguel Island-Azores that even with modern water treatment systems present fluoride concentrations slightly above the WHO recommendations [68]. Considering that the main sources of human exposure to fluoride are diet, especially through the ingestion of water and, that in volcanic regions fluoride is continuously released into the environment, these authors developed a biomonitoring study to investigate the feasibility of urine and nail clippings as biomarkers of exposure. Nail clippings revealed to be a more reliable biomarker of chronic exposure to fluoride than urine for populations of different age classes (children vs. adults), with a positive correlation between the fluoride daily intake and fluoride content in nail clippings in children (rs = 0.475; p < 0.001), and in adults (rs = 0.495, p < 0.001). More recently, Linhares et al*.* [66] assessed the risk of skeletal fluorosis from environmental exposure to fluoride in hydrothermal areas, using wild mice (*Mus musculus*) as bioindicator species. *Mus musculus* were collected in Furnas village (a village located inside the caldera of Furnas volcano), an area where volcanic activity is marked by active fumarolic fields, hot and cold CO2-rich springs, and soil diffuse degassing phenomena [69, 70]. The results demonstrated that mice from Furnas village had higher concentrations of fluoride in bones when compared with mice from an area without volcanic activity (616.5 ± 129.3 mg F/g vs. 253.8 ± 10.5 mg F/g, respectively), reinforcing that chronic exposure to fluoride may lead to the development

*DOI: http://dx.doi.org/10.5772/intechopen.90786*

#### *Trace Elements in Volcanic Environments and Human Health Effects DOI: http://dx.doi.org/10.5772/intechopen.90786*

*Trace Metals in the Environment - New Approaches and Recent Advances*

There are several hazards that result from a volcanic eruption; the most immediate and threatening hazards are pyroclastic falls, pyroclastic density currents, lava (flows and domes), lahars and flooding, debris avalanches, and volcanic gases. Nevertheless, volcanic emissions also occur in the post-eruptive phase and in quiescent volcanoes, continuing to affect the ecosystem and consequently human health. Volcanic regions step-up important scenarios for the study of TEs in soils because: (i) they are densely inhabited in some areas of the Earth and (ii) due to the physicochemical properties, volcanic soils retain TEs, acting as a reservoir and

The concern regarding the health effects of environmental exposure to TEs from natural sources has driven the development of tools and methods for assessing the impact of emissions in water, soil, and air. Biological monitoring or biomonitoring is the most commonly applied method to measure human exposure to xenobiotics [49]. There are several studies worldwide that establish the association between

One of the most well-detailed health problem associated with volcanic activity is fluorosis, which results in high fluoride (F) concentrations in groundwater. The problem was first recognized in Japan and was called "Aso volcano disease" [50], but during the course of the year, high fluoride concentrations (greater than the WHO guideline value of 1.5 mg/l) were also found in Africa, where the crater lakes of western Uganda often have high F concentrations (e.g.4.5 mg/l F in Lake Kikorongo) [51]. High concentrations of F were also found in eastern Turkey, near the Tendurek Volcano, where the natural waters contained fluoride levels between 2.5 and 12.5 ppm [52], and in oceanic islands, such as Tenerife Island (Canary-

Another health problem that has been proven to be strongly associated with the exposure to volcanic environments is thyroid cancer, in particular papillary thyroid cancer (PTC). In 2009, Pellegriti et al. [57] in a register-based epidemiologic study showed that the residents of Catania, a province in the vicinity of Mt. Etna, presented a higher incidence of papillary thyroid cancer than elsewhere in Sicily. More recently, these results were reinforced in the study conducted by Malandrino et al*.* [58]. In this study, the authors evaluated the environmental pollution and biocontamination in a volcanic area of Sicily and compared the data with the thyroid cancer epidemiology data obtained from the Sicilian Regional Registry for Thyroid Cancer. Their results indicated that the residents in Mt. Etna volcanic area had significantly higher levels of several TEs in their urine when compared to the control area [Cd (×2.1), Hg (×2.6), Mn (×3.0), Pa (×9.0), Th (×2.0), V (×8.0)] and that thyroid cancer incidence was 18.5 and 9.6 per 105 inhabitants in the volcanic and the control areas, respectively; the observed thyroid cancer incidence was exclusively

Besides these, that pose as the most studied TE and linkages to human health effects, there are several other health issues that have an increased risk due to environmental exposure. In Sicily, Italy, various soil types developed from different parent materials were analyzed to compare heavy metal distribution under different geopedological conditions, evidencing that the former depended on the parent rocks [59]. In Turkey, an association was established between the volcanic soil and the high prevalence of upper gastrointestinal cancer rates in the Van region as the fruit and vegetable samples produced in those soils contained potentially

concentrations of TE in volcanic soils and its effects on human health.

Spain) [53] and São Miguel Island (Azores-Portugal) [54–56].

**2.3 Trace elements in volcanic soils**

affecting agriculture [15].

**2.4 Human health effects**

**106**

from the papillary histotype.

carcinogenic levels of heavy metals [60]. More recently, Rodriguez-Espinosa and co-authors [61] analyzed the elemental composition of 25 soils and ash samples after the eruptions of the Popocatépetl Volcano in México, observing that the concentrations of TEs such as Zn, Pb, Ni, Hg, Cr, Cd, Cu, and As were significantly higher than those observed in older samples from eruptions in 1997, suggesting that the naturally highly volatile and mobile metals leach into nearby freshwater sources. In the Azorean volcanic islands, there are also some studies focusing in the concentrations of TEs in soils [15, 62] and its effects on local organisms [39, 63, 64] and human health [50, 53], the latter being particularly focused on fluoride [65–67].

In 2016, Linhares and co-authors [65] verified that there are areas in São Miguel Island-Azores that even with modern water treatment systems present fluoride concentrations slightly above the WHO recommendations [68]. Considering that the main sources of human exposure to fluoride are diet, especially through the ingestion of water and, that in volcanic regions fluoride is continuously released into the environment, these authors developed a biomonitoring study to investigate the feasibility of urine and nail clippings as biomarkers of exposure. Nail clippings revealed to be a more reliable biomarker of chronic exposure to fluoride than urine for populations of different age classes (children vs. adults), with a positive correlation between the fluoride daily intake and fluoride content in nail clippings in children (rs = 0.475; p < 0.001), and in adults (rs = 0.495, p < 0.001). More recently, Linhares et al*.* [66] assessed the risk of skeletal fluorosis from environmental exposure to fluoride in hydrothermal areas, using wild mice (*Mus musculus*) as bioindicator species. *Mus musculus* were collected in Furnas village (a village located inside the caldera of Furnas volcano), an area where volcanic activity is marked by active fumarolic fields, hot and cold CO2-rich springs, and soil diffuse degassing phenomena [69, 70]. The results demonstrated that mice from Furnas village had higher concentrations of fluoride in bones when compared with mice from an area without volcanic activity (616.5 ± 129.3 mg F/g vs. 253.8 ± 10.5 mg F/g, respectively), reinforcing that chronic exposure to fluoride may lead to the development of not only dental fluorosis but also of skeletal fluorosis.
