of fluoride input into the environment via anthropogenic activities are the aluminum and zinc industry, coal‐burning, brick/clay burning, steel production, oil refining, chemical production, uranium trifluoride, magnesium smelting, ceramic glass and uranium hexafluoride production, enamel manufacturing, and fluoride‐containing fertilizers or pesticide industries (Sujatha 2003). Discharge from industries heavily pollutes the soil and water, as well as vegetation cover around the industry and far away from it. Other than industrial emissions, agriculture runoffs having fluoride‐containing fumigants, fertilizers, and pesticides are some of the other predominant causes of fluoride pollution (Kundu and Mandal 2009; Borah and Saikia 2011). Industries involving coal burning pollute the atmosphere in a small area, and the extent of the pollution depends on the origin and type of coal. It has been estimated that burning of biomass releases 76 Gg fluoride into the air annually (Jayarathne et al. 2014).
2.3 Occurrence of Fluoride in the World and India
2.3.1 World
The quantity of fluoride in drinking water varies around the world as well as region to region depending on the geographical location. Contamination of fluoride ions has been largely illustrated in the groundwater of mainly humid, tropical parts of the world. The countries having this type of climatic condition include China, countries in South Asia, and countries in Africa (Ayoob and Gupta 2006). According to WHO 2002, the recommended fluoride concentration of drinking water is 1.5 mg/L. However, estimations show that 200 million or more people are consuming fluoride‐contaminated drinking water worldwide. According to some studies, it has been revealed that over 5 million people are exposed to fluoride‐contaminated groundwater in Mexico (Ayoob and Gupta 2006).
2.3.1.1 America
A high level of fluoride in groundwater has been reported from the USA, mainly in facility wells of industries in Pennsylvania, having 3.2 and 6.5 mg/L of fluoride; Lakeland, Southern California, having 3.6–5.3 mg/L of fluoride; and deep aquifers present in the western US comprising 5–15 mg/L of fluoride (Cohen and Conrad 1998). The presence of fluorosis has also been reported in different states of the USA, such as Oklahoma, Nevada, South Carolina, North Dakota, Texas, Oregon, California, Utah, Colorado, New Mexico, Virginia, and North Carolina. Five million people in Mexico (around 6% of the total population of the country) have been affected by pollution of fluoride in groundwater. In Canada, there are several communities whose natural drinking water sources contain higher levels of fluoride (as high as 4.3 mg/L). Industrial discharges are the main reason behind fluoride contamination reported in the USA and Canada (Rose and Marier 1977). A few parts of Argentina contain fluoride concentrations of about 5 mg/L in groundwater (Kruse and Ainchil 2003).
2.3.1.2 Indian Scenario
Twelve million tons of fluoride deposits are found in India from the Earth crust out of 85 million tons of total deposition (Teotia and Teotia 1984). Therefore, the contamination of fluoride has broadly spread at an alarming rate in the Indian scenario. In the capital city of India, Delhi, the percentage of groundwater crossing the maximum permissible limit of fluoride present in drinking water is around 50% (Datta et al. 1996). According to the report of Jacks et al. (2005), the main reason behind the higher concentration of fluoride ion found in the groundwater of many parts of India was due to the evapotranspiration of groundwater with residual alkalinity. In the southern parts of India, major proportion of fluoride contamination in groundwater is due to fluoride enriched rocks for instance, groundwater of Andhra Pradesh, precisely Nalgonda district has high fluoride level, i.e. 320–3100 mg/kg due to presence of fluoride‐rich granitic rocks. The average fluoride level found in the granites of Hyderabad is 910–920 mg/kg (Ramamohana Rao et al. 1993).
According to the investigation the two sites of Uttar Pradesh and Madhya Pradesh have considerably higher concentrations of fluoride, i.e. 0.1–0.3 mg/L (Das et al. 1981; Satsangi et al. 1998; Singh et al. 2001). The main cause of the increasing fluoride contamination in this region was predicted as deposition of soil dust. According to the study of Jain et al. (2000), in Haryana, wet deposition of crustal material increases the fluoride load. Thirteen sites in Madhya Pradesh show about 0.05–0.22 mg/L concentration of fluoride as reported by Chandrawanshi and Patel (1999) and the area is considerably close to the industrial aluminum plant. A recently reported evaluation of dry deposition near Agra, reported by Satsangi et al. (2002) shows higher amounts of fluoride due to atmospheric deposition. Several authors have claimed that the atmospheric deposition is mainly from the crustal source. Concentrations of fluoride in different regions are presented in Table 2.1.
2.4 Effects of Fluoride on Human Health
The effect of fluoride contamination on human health has been studied by researchers from all over the world for more than a century. Fluoride causes both good and bad effects on the human body depending on the level of exposure. According to the study of Ozsvath (2009), ingestion of a moderate amount of fluoride can actively decrease the risk of occurrence of dental caries as well as promote the growth of strong bones under certain conditions. Chronic exposure to fluoride can cause various ill effects on human health such as dental fluorosis and skeletal fluorosis; it can also increase the rate of urolithias, and decrease natality and IQ level of children. In some cases, chronic exposure might also lead to a number of defects such as genetic mutations, birth defects, and Alzheimer's disease; the scientific data at present are inconclusive (Ozsvath 2009). According to the World Health Organization (WHO), the maximum intake of fluoride in drinking water is recommended as 1.5 mg/L (Edition 2011). Among various other adverse health impacts on the body, fluorosis remains the major problem in affected populations and is categorized as dental fluorosis and skeletal fluorosis, as discussed in Sections 2.4.1 and 2.4.2.
2.4.1 Dental Fluorosis
Dental fluorosis is a developmental disturbance of tooth enamel or tooth surface that occurs due to systemic overexposure of fluoride during enamel formation (Kanduti et al. 2016). During the first six years of life, the development of enamel occurs over the tooth and increased mineralization is accompanied by reduction of matrix protein. Exposure to fluoride causes dose relationship disruption in the process of amelogenesis and dentinogenesis that ultimately results in deformity in the crystalline structure of teeth (Swarup and Dwivedi 2002). Dental fluorosis is characterized by yellow to brownish mottling of enamel with narrow white horizontal striations (DenBesten and Li 2011). The dental fluorosis severity depends upon the degree of exposure of fluoride on humans. In India, 70% of adolescents consuming fluoride‐contaminated drinking water with more than the recommended value of the World Health Organization are affected by dental fluorosis (Chaudhry et al. 2017). Enamel opacities also occur due to malnutrition and deficiency of Vitamin A and D as well as due to low protein‐energy intake. Therefore, fluoride is not the only cause of dental enamel defects (Zohoori and Duckworth 2017).
2.4.2 Skeletal Fluorosis
Skeletal fluorosis is a somewhat more severe adverse health impact, characterized by increased bone mass and bone density (osteosclerosis) that occurs due to a prolonged period of exposure to fluoride at the time of bone modeling and/or remodeling. Fluoride exposure of more than 4 mg/L concentration may lead to skeletal fluorosis; the exposure may be either direct (ingestion) or indirect (inhalation) (Yadav et al. 2019). Skeletal fluorosis occurs in three stages: initial, intermediate, and final:
1 The initial stage – The initial stage shows various mild symptoms like joint pain, stiffness of bones and joints, muscle weakness, periodic pain, and chronic fatigue.
2 The intermediate stage – The intermediate stage is characterized by calcification of bone followed by hardening and stiffening of joints as well as calcification of ligaments in