Fluoride Sources, Toxicity and Its Amelioration: A Review

In recent scenario, fl uorosis is now going to be a severe problem throughout the globe due to toxic effects of fl uoride (F) on both plants and animals. F presents in the halogenated group of the periodic table and has the characteristics of electronegativity. Natural geological sources and increased industrialization have contributed greatly to the increasing incidence of fl uoride-induced human and animal health issues. In animals and human beings, it exerts adverse effects mainly through the attenuation of antioxidant defense mechanism and chelation of enzymatic cofactors. Thereafter, it causes metabolic disorders through interacting with various cellular processes such as gene expression, cell cycle, metabolism, ion transport, hormonal secretion, endocytosis, apoptosis, necrosis, and oxidative stress. These effects lead to dental mottling, skeletal dysfunctions including crippling deformities, osteoporosis, and other vital organs dysfunction. It was found that, water is the main source of fl uoride intake to plants and animals, which further may go into food chain of human beings through consumption of high fl uoride content plant and animal origin food. Several preventive and control measures have been developed to ameliorate the fl uoride toxicity, like application of synthetic chemicals, plants bioactive molecules, and plant products like fruit pulp, seed mixture, and plant buckle products. Therefore, this article presents up-to-date information on the fl uoride sources, toxicity and different amelioration measures to reduce fl uoride level directly from water as well as application of different natural/synthetic products/molecules to ameliorate the toxic effects of fl uoride in in-vivo models. Review Article Fluoride Sources, Toxicity and Its Amelioration: A Review Vijay K Bharti*, Arup Giri and Krishna Kumar Defence Institute of High Altitude Research (DIHAR), DRDO, Leh-Ladakh, Jammu & Kashmir, India Dates: Received: 30 December, 2016; Accepted: 17 January, 2017; Published: 21 January, 2017 *Corresponding author: Dr. Vijay K Bharti, Scientist D, Defence Institute of High Altitude Research (DIHAR), DRDO, Ministry of Defence, Leh-Ladakh-194101, India, Tel: +91-9779618799; Fax: +91-1722638500; E-mail:


Introduction
In the halides group of the periodic table, fl uoride (F) has great importancy due to its smallest size and most electro negativity. Although the mechanisms of F in biological forms are remains unclear but it has the unique chemical and biochemical properties for the size and reactivity [1][2][3]. It is ubiquitously present in soil, water, plants and air. In the animal body, F makes its presence through water and food. But, some of the recent studies indicate that, most of the F comes from pharmaceutical drugs (20%) and through agrochemicals (30-40%) [4,5]. The variability and presence of fl uoride depends upon the location. It was found that F is present in the soil within the range of 10-1000 parts per million (ppm). However, in water it ranges from 0.5 to 2000 ppm. This incident depends upon the sources of water [6,7]. According to World Health Organization (WHO), F exposure to animals above the 1.5 ppm, set at chronic fl uoride toxicity. Through water exposure, this type of toxicity is going to endemic in most of the countries across the world [8]. In USA, the normal level of F in drinking water is 4 mg/L [9]. But, in the European country, it is 0.8 ppm [10]. In India, most of the states are showing the greater level of F in drinking water [11]. Fluoride exerts its effects on plants also [12]. It attenuates all the cells and tissues, impaired the stomatal conductance. Simultaneously, it acts as the metabolic and reproductive inhibitor, impaired photosynthesis and respiration pathways. Ultimately, F caused even to plants death [13][14][15][16][17][18]. In animals, fl uoride intoxication causing skeletal impairment, called as skeletal fl uorosis. Recently, high fl uoride intake has been associated with dental cancer and tumors of other organs. First clinical symptoms appeared like reduced in food intake and loss of body weight gain. After attenuating the antioxidant defence mechanism, F also affect to the gastrointestinal tract, brain, muscle etc.. [19][20][21][22]. To ameliorate these effects, several types of synthetic chemicals, herbal drugs, plant bioactive molecule, and plant natural products have been incorporated in the medicinal documentary. For example, melatonin, pineal proteins (epiphyseal proteins), quercetin, curcumin, ascorbic acid, lipoic acid, fl avonoids, polyphenols have been found great role against the F toxicity [23][24][25][26]. The present review critically discusses on the fl uoride sources, worldwide levels and its toxic effects on plants and animals.
Furthermore, the article discusses the recent ameliorative steps developed through synthetic chemicals, plant bioactive molecules, and plant natural products.

Biochemistry of fl uoride
In the halides group of the periodic tables (group VII), among all other molecules, fl uoride has the great importancy due to it's smallest size and most electro negativity. Although, the mechanisms of F in biological forms are remains unclear but it has the unique chemical and biochemical properties for the size and reactivity [1][2][3]. It is 13 th most abundant element and distributed widely throughout the earth in soil, water, and food. F, a pale yellow colored gas, has atomic number 9 and atomic weight of 18.9984 at standard temperature and pressure [27]. The brief about the F, have been mentioned in the Figure   1 [28]. It has the tendency to exist in the Free State as diatomic molecules. Due to electromotivity characteristics, these can react with less electromotive elements or chemical groups.
Fluoride compounds are formed when the element fl uoride combines with other chemical elements. It does not occur in a free state in nature [28]. Fluoride however has many unique chemical properties. These properties had a great impact on the special biochemical physiological effects. For these reason, F can affect the metabolism and mechanisms of action within the living system [29]. In addition to the chemical properties and isotopic nature of fl uorine has had an important impact on our understanding of the metabolism, toxicity, and therapeutic effects of fl uoride. 19-F is one of the isotopes of F and occurs naturally. This isotope has the extremely short half-life.

Sources of fl uoride
Natural and anthropogenic sources are the two main ways through which F entered in the environment [30].

Natural sources
Soil: The normal total fl uoride content of soil ranges from 150-400 mg/kg. F level in the clay soil is 1000 mg/kg [31]. F contamination to soil is because of the utilization of phosphorus fertilizers which have total 1-1.5% fl uorine [32].
Contaminated soil with F, show it's toxicity after the inhalation of soil contaminants which have vapourized or through the contaminated ground water after the F leaching from the soil [33][34][35].
Water: Water containing the F concentration up to 1.0 mg/L is safe. Whereas, the F levels in between 1.1 and 2.5 mg/L are marginally contaminated. However, above 2.6 mg/L F level is determined as the highly contaminated [31]. It was found that the level of F in ground water is higher than the surface water as the F percolates from the soil to ground water through leaching process. There are several factors which are responsible for the presence of F in natural ground water from the soil. Among them, geological factors, consistency of the soil, nature of rocks, pH and temperature of the soil, chelating action of other elements, depth of wells, leakage of shallow groundwater, and chemical and physical characteristics of water [36]. Water is an important source of F exposure to human beings and animals.
Forage, grasses and grains: At the vicinity of industrialized area, it was found that forages and grasses contain the higher level of F than the other area. Some studies also found that, grasses and forages has the higher level of F than the industrialized area. It is due to the fl uoride rich dust, ash, raining factors for which plants could be affected far from the industry. Plants contamination depends upon several factors like the amount of F released in to the atmosphere, distance between the F source and contaminated area, type of vegetation, height of plants, atmospheric condition, and seasons etc. [37][38][39]. It has been established the relationship between the F level in soil and plants of F will be increased by 3 ppm for each 100 ppm increase in soil F up to the 2200 ppm [39].
Volcanic activities: Due to volcanic eruption, animals and plants kingdom have been affected throughout the globe ( Table   1). Volcanic ash contains high level of F and contaminations of F to the geochemical cycle are frequent. From the volcanic eruption, F has been released in the form of hydrogen fl uoride.
Erupted F may covered several places and stay for many years.
After decaying and leaching, F caused severe casualty to domestic and wild animals [6,40,41].

Anthropogenic sources
Anthropogenic fl uoride contamination happens by human activities like industrialization, motorization, fl uoride  containing pesticides, fl uoridation of drinking water supplies, dental products, refrigerants, and fi re extinguishers [46,47]. F contamination due to airborne sources also occurred. The mean F concentration in normal areas (unpolluted/nonindustrialized) is generally less than 0.1 μg/m 3 . The levels may be slightly higher in the vicinity of industries, but should not exceed 2-3 μg/m 3 [6]. In many countries, coal burning for household purposes was documented as the main source of F causing endemic fl uorosis [48,49]. Industrial release fl uoriderich fumes and effl uents into the environment also caused casualty in livestock sector like cattle, buffaloes, sheep, goats, camels etc. [50][51][52][53][54][55]. There are several reports documenting mineral mixture supplements as a major source of fl uoride toxicity in livestock [56]. Moreover, incorporation of modern creation and utilization of chemicals in different sectors like hydrogen fl uoride (HF), calcium fl uoride (CaF), sodium fl uoride (NaF), fl uorosilicic corrosive (H SiF), sodium hexafl uorosilicate (Na SiF), sulfur hexafl uoride (SF), and phosphate manures are the main sources of fl uoride.

Global scenario of fl uoride levels
Around the globe, twenty three nations are belongs to the critical region regarding the fl uoride level. Among them India is also present. Billions of people are affected due to fl uoride exposure. In India, twenty million people are severely affected by fl uorosis and 40 million people are exposed to risk of endemic fl uorosis [57]. Level of fl uoride in drinking water throughout the globe has been tabulated in the Table 2.

Fluoride toxicity
In Animals: Chronic exposure to F induces an array of deleterious impacts in livestock animals, experimental animals, as well as humans also [6,97,98]. First symptoms of chronic F toxicity in animals are reduced feed intake and body weight gain (BWG) loss [19,22]. Prolonged exposure to F causes fl uorosis, leading to a progressive degenerative disease, dental mottling and several types of skeletal dysfunctions [4]. Main mechanism of these deformities, after exposure of F is mainly the generation of different types of ROS production (Table  3). Experimental evidence (Tables 4,5) has indicated that exposure to fl uoride results in oxidative stress both in vitro and in vivo in soft tissues such as liver, kidney, brain, lungs etc. Fluoride inhibits the activities of antioxidant enzymes like superoxide dismutase, glutathione peroxidase and catalase and reduces levels of glutathione. Glutathione reduction leads to overproduction of reactive oxygen species at the mitochondrial level, resulting in damage of cellular components. Besides, production of excessive reactive oxygen species results in oxidation of macromolecules, membrane phospholipid breakdown, lipid peroxidation, mitochondrial membrane depolarization and apoptosis (Tables 4,5). Neurodegeneration also occurred due to the F exposure. Several studies indicated that hippocampus of rat brain can lead to the degenerate due to the imbalance between oxidant-antioxidant balance. F crossed the blood brain barrier (BBB) easily and induces neural cell degeneration [24,[99][100][101]. All the effects of fl uoride are summarized in the Tables 4,5.   Murine hepatocytes at 100mM for 1 h Generation of ROS, level of GSH, GSH:GSSG ratio, activities of SOD, and catalase, lipid peroxidation, and oxidation of proteins [104] In-vitro (Human Cells) Hepatocellular carcinoma (HepG2) cells at 3mM for 6 and 24 h GSH/GSSG ratio, gene expression of Mn-SOD [105] Neuroblastoma (SH-SY5Y) cells exposed at 0.05-5mM for 24 h Lipid peroxidation, and protein oxidation [106] In vivo (Animals)

Amelioration of fl uoride toxicity
Male albino guinea pigs exposed at 250mg NaF/kg subcutaneously and sacrifi ced 8 h later Generation of NO in blood [107] Male Wistar rats exposed at 5mg/kg body mass/day, orally for 8 weeks Generation O 2 − , activity of SOD, ∆ψm, lipid peroxidation in spermatozoa [108] Male Swiss mice exposed at 50 mg/L in drinking water for 10 weeks Generation of ROS, lipid peroxidation, activities of SOD, and catalase, activities of GST, and GSH-Px, ratio GSH:GSSG in brain [109] Albino rats exposed at 100 mg/L in drinking water for 4 months Level of ascorbic acid  level of uric acid in plasma Lipid peroxidation, level of GSH, activity of GSH-Px, activity of SOD in erythrocytes Lipid peroxidation, activities of GSH-Px, and GST, GSH in brain and liver [109] Male albino Wistar rats exposed at 1, 10, 50, and 100 mg/L in drinking water for 12 weeks Generation ROS, changes in levels of GSH in blood, generation ROS in liver, kidney, and brain [110] Second generation of Male Albino adult Wistar rats exposed at 10, 50, and 100 mg/L in drinking water for 180 days Lipid peroxidation, activities of SOD, catalase, and GSH-Px in lung [111] Chicks exposed by diet to 100, 250, or 400 mg F/kg for 50 days Generation of NO, lipid peroxidation, activities of SOD, catalase, and GSH-Px in serum [112] Male albino rats exposed at 10.3 mg NaF/kg body weight/day, orally for 5 weeks Lipid peroxidation, generation NO, activities of SOD, and catalase, Total antioxidant capacity, and level of GSH in liver [113] Pig exposed to food supplemented with 250 mg F/kg for 50 days Expression of gen Cu/Zn SOD in liver [114] Male rats exposed at 20 mg/kg/day for 29 days by oral gavage Level of conjugated dienes in the testis, epididymis, and epididymal sperm pellet. activities of GDH-Px, and catalase in the sperm [115] Male Wistar rats exposed at 50 and 100 mg/L in drinking water during 4 months Activity of CuZn-SOD in pancreas [116] Male and female Wistar rats exposed at 50, 100, and 150 mg/L in drinking water during 3 months Lipid peroxidation, activities of SOD, and GSH-Px in liver [116] Barrows exposed at 250 and 400 mg/kg (from NaF) in their diets for 50 days Generation of NO, lipid peroxidation, activities of GSH-Px, and SOD in serum Lipid peroxidation, activities of GSH-Px, and SOD in thyroid, liver, and kidney [117] Male Swiss mice exposed at 5 mg/kg body mass/day, orally for 8 weeks ROS in erythrocytes, level of GSH in blood, activities of SOD, catalase, and GSH-Px, lipid peroxidation, in kidney and liver [118] Female rats exposed at 100 mg/L in drinking water for 60 days Lipid peroxidation, activities of SOD, catalase, and GSH-Px in endometrium [119] Citation: Bharti  Swiss albino male mice exposed at 50 mg/L in drinking water for 3 weeks Generation of ROS, GSH level, activity of SOD in  activity of catalase in liver [120] Male albino rats exposed at 10, 50 and 100 mg/L in drinking water for 10 weeks Generation ROS in blood, liver, kidney, and brain GSH/GSSG ratio in liver, kidney, and brain [121] Female Albino mice exposed 5 mg/kg body weight/day, orally for 30 days Activities of SOD, catalase, and GSH-Px, level of GSH, total, dehydro and reduced ascorbic acid, lipid peroxidation in ovary [122] Male Balb/c mice exposed at 200 mg/L, in drinking water for 7 days Activities of SOD, GSH-Px, and catalase, lipid peroxidation, in erythrocytes, and liver [123] Female Wistar rats exposed at 150 mg/L in drinking water for 28 days Level of GSH, activities of SOD, GPx, catalase and, glutathione reductase, lipid peroxidation in brain [101] Wistar albino pups placentally and lactationally exposed from mother rats at 50, and 150 mg/L in drinking water Lipid peroxidation, protein oxidation in developing central nervous system [124] In-vivo

Type of the study Model & Dosages End point References
In vitro

(Human cells)
Neuroblastoma (SH-SY5Y) cells at  (Table 6). On the other hand, use of leaves, seeds, fruit pulps, plant juices of Azadirachta indica, Ficus religiosa, Acacia catechu, Peltiphyllum peltatum and tamarind seeds etc. are also using to reduce the toxic effects of fl uoride and summarized in the Table 7. Additionally, some synthetic chemical molecules like melatonin, pineal protein, lycopene, and quercetin, etc. also have the great role to reduce the fl uoride induced toxicity. All are summarized in the Table 8.