naturally introduce fluoride and are also considered the largest fluoride reserve. Figure 2.1 represents the expected sources of fluoride in environment.
2.2.1 Natural Sources
2.2.1.1 Rocks and Minerals
One of the most abundant trace elements present in the Earth’s crust is fluoride, having 625 mg/kg average concentration in different kinds of rock (Edmunds and Smedley 2005; Tavener and Clark 2006). The rocks containing fluoride‐rich minerals are the largest fluoride reserve. According to some studies, the highest fluoride concentrations are combined with quartz, felsic, gneisses, syenites, granites, and alkaline volcanic (Robinson Jr and Kapo 2003; Rosi et al. 2003; Moore 2004; Chae et al. 2006, 2007; Ozsvath 2006). Rocks found in the Coimbatore district of Tamil Nadu contain 180–2600 mg/kg fluoride (Table 2.1). Among various fluoride‐rich minerals, some of them are fluorite (CaF2), micas, amphiboles, villiaumite (NaF), and topaz (Al2[SiO4]F2). These chemicals are abundantly found minerals in sediments and rocks (Cronin et al. 2000; Saxena and Ahmed 2003; Edmunds and Smedley 2005; Chae et al. 2007).
2.2.1.2 Groundwater
There are several factors affecting the fluoride presence like granite, gneissic rocks, and volcanic as well as mountainous area sediments of marine origin. The above‐mentioned rocks are rich in fluoride and are often found beneath the Earth's surface, ultimately leading to groundwater contamination. Alarming high fluoride ion concentrations together with various other toxic and infectious substances present in the groundwater of South and Southeastern Asia are of utmost concern (Ghosh et al. 2013).
Figure 2.1 Sources of fluoride contamination in the environment.
Table 2.1 Fluoride levels reported in different regions.
| Country | Location | Fluoride concentration (mg/L) | References |
|---|---|---|---|
| India | Hyderabad, Andhra Pradesh | 0.38–4.0 | Sreedevi et al. (2006) |
| Ranga Reddy, Andhra Pradesh | 0.4–4.8 | Sujatha (2003) | |
| Karbi Anglong, Assam | 0.4–20.6 | Chakraborti et al. (2000) | |
| Bihar Shallow | 0.1–2.5 | Ray et al. (2000) | |
| Delhi | 0.2–32.5 | Raju et al. (2009) | |
| Gujarat | 0.1–40 | Raju et al. (2009) | |
| Palghat, Kerala | 0.2–5.75 | Shaji et al. (2007) | |
| Chandidongri, Madhya Pradesh | 1.5–4.0 | Chatterjee and Mohabey (1998) | |
| Shivpuri, Madhya Pradesh | 0.2–6.4 | Ayoob and Gupta (2006) | |
| Orissa | 0.1–10.1 | Kundu et al. (2001) | |
| Churu/Dungarpur, Rajasthan | 0.1–14 | Muralidharan et al. (2002); Choubisa (2001) | |
| Kancheepuram, Tamil Nadu | 1–3.24 | Dar et al. (2011) | |
| Tamil Nadu | 0.5–4.0 | Raju et al. (2009) | |
| Cambay, North Gujarat | 0–10 | Gupta et al. (2005) | |
| Varanasi, Uttar Pradesh | 0.2–2.1 | Raju et al. (2009) | |
| Sonbhadra, Uttar Pradesh | 0.48–6.7 | Raju et al. (2009) | |
| Mathura, Uttar Pradesh | 0.6–2.5 | Misra et al. (2006) | |
| Canada | Gaspe, Quebec | 0.05–10.9 | Boyle and Chagnon (1995) |
| Ghana | Nathenje and Lilongwe | 0.5–7.02 | Msonda et al. (2007) |
| Pakistan | Nagar Parkar | 1.13–7.85 | Naseem et al. (2010) |
| Sri Lanka | Dry Zone | 0.02–5.30 | Chandrajith et al. (2011) |
| Iran | Posht‐e‐Kooh‐e‐Dashtestan | 0.7–6.6 | Battaleb‐Looie and Moore (2010) |
| China | Taiyuan Basin | 0.4–2.4 | Li et al. (2011) |
| Germany | Muenster Region | 0.01–8.8 | Queste et al. (2001) |
| Mexico | Hermosillo city, Sonara | 0–7.59 | Valenzuela‐Vasquez et al. (2006) |
2.2.2 Anthropogenic Sources
In many developing countries as well as developed countries, the fluoride concentration present in the environment has been majorly altered by industrial