An estimated 200 million people from 29 countries including India are severely affected due to fluoride pollution. Ingestion of fluoride beyond the WHO recommended permissible level (1.5 mg/l) is associated with dental and skeletal fluorosis and other toxic responses while inadequate intake of fluoride is associated with dental caries. Natural weathering of various minerals (like apatite, fluorite, topaz, fluorspar, and mica) provides fluoride to soils. In site-specific cases some industries (mainly phosphorous fertilizer plants; steel, aluminum, zinc, smelting industries; glass and ceramic industries, etc) are also responsible for fluoride contamination of soils. Fluoride gets transferred from contaminated irrigation water to cultivated crops, vegetables and fruits. This bioaccumulation of fluoride contributes further fluoride to the human food chain in addition to the drinking water pathway, and thus it is causing larger risk to the already fluoride-contamination affected population. Moreover, this new avenue of fluoride highly endangers the most susceptible infants and children towards dental fluorosis. The ‘Nalgonda’ and ‘activated alumina’ processes are the most commonly used defluoridation techniques of drinking water. But, a suitable, efficient, user-friendly and cost-effective technique for defluoridation is yet to be developed. Equal emphases are to be given on creation of awareness in people regarding fluorosis and restriction of usage of fluoride-contaminated groundwater for irrigation purpose.
Key words Fluorosis; Fluoride pollution; Fluoride bioaccumulation; Fluoride in food chain
Moderate to high concentration of fluoride in groundwater is reported to be one of the major environmental issues of Algeria, Brazil, Canada, China, Ethiopia, Ghana, India, Iran, Italy, Japan, Jordan, Kenya, Korea, Malawi, Mexico, Norway, Pakistan, Sri Lanka, Thailand, Turkey, and the USA (WHO 2006a; Brindha and Elango 2011) threatening an estimated 200 million people (Bhattacharya and Chakrabarti 2011). Fluoride can be found in different environmental components originated from mineral sources, atmospheric sources, and geothermal sources. After its dissolution from weathered rock it becomes mobile, and it enters soil, groundwater, cultivated crops, fruits and vegetables depending on the prevailing geological agents. The natural cause which leads to high fluoride concentration in groundwater is the dissolution of fluorite, apatite, and topaz from bedrocks (Suthar et al. 2008). Moreover, different anthropogenic influences like usage of phosphate fertilizers and pesticides, discharge of untreated or partially treated sewage and sludge in groundwater, overexploitation of groundwater for irrigation, etc., are also responsible for the increase of fluoride concentration in groundwater (EPA 1997; Ramanaiah et al. 2006). Fluoride in groundwater is mostly of geological origin. Waters with high levels of fluoride content are mostly found at the foot of high mountains and in areas where the sea has made geological deposits. Ingestion of excess fluoride, most commonly via drinking water (other sources are food, industrial gas and excessive use of toothpaste), can cause ‘fluorosis’ which affects the teeth and bones (WHO 2006b). Moderate amounts of fluoride lead to dental effects, but long-term ingestion of large amounts of fluoride can lead to potentially severe skeletal and other adverse problems. Paradoxically, low levels of fluoride intake help to prevent dental caries. The control of drinking water quality is therefore critical in preventing fluorosis. The associated human health risks from fluoride can be broadly categorized as: dental effects, skeletal effects, reproductive effects, developmental effects, renal effects, neurological effects, endocrine effects, and carcinogenic effects. Thus, the World Health Organization (WHO) has made 1.5 mg/l as the permissible limit for fluoride in drinking water (WHO 2006a).
Fluorosis is endemic in 20 out of total 29 states of India, spreading over 65% of the total rural habitations of the country (UNICEF 1999). More than 65 million Indians including six million children are at risk due to the presence of fluoride beyond the desirable 1.5 mg/l level in drinking water (Andezhath et al. 1999; UNICEF 1999). High concentration of fluoride in groundwater has been mainly reported from different regions of Assam, Andhra Pradesh, Bihar, Chhattisgarh, Gujarat, Karnataka, Madhya Pradesh, Maharashtra, Odisha, Rajasthan, Telangana, Uttar Pradesh and West Bengal (PHED Report 2007; Chatterjee et al. 2008; Salve et al. 2008; Sankararamakrishnan et al. 2008; Suthar et al. 2008; Mishra et al. 2009; Kundu and Mandal 2009a; Bhattacharya and Chakrabarti 2011; Brindha et al. 2011; Dar et al. 2011; Chakrabarti and Bhattacharya 2013; Samal et al. 2015; Bhattacharya 2016). The present review focuses on the assimilation of relevant data and information on past and present status of fluoride contamination in India, mobility of fluoride in ecosystems, and the related human health issues.
Fluoride contamination in water—sources, essentiality vs. toxicity
Fluoride is present in almost all groundwater, but the percentage of total fluoride that is leachable varies widely with rock types (Lirong et al. 2006). The types of minerals, residence time, and climate primarily control dissolution of fluoride from bedrock (Hem 1985). The pH, hardness, ionic strength, and other water quality parameters also have important roles in site-specific cases influencing mineral solubility, complexation, and sorption/exchange reactions (Apambire et al. 1997). The common fluoride-bearing minerals being sparingly soluble release fluoride to water slowly with the exception of villiaumite (Saxena and Ahmed 2003). Dissolution of fluorapatite gets enhanced in the presence of microbes which seek to separate phosphorus from the solid phase (Allen-Long 2001). Similar is the case of fast fluorite dissolution in sodium-bicarbonate waters (Apambire et a1. 1997) while the release of fluoride from clay minerals depends strongly on pH (Saxena and Ahmed 2003). The maximum concentration of fluoride in groundwater is usually controlled by the solubility of fluorite (Edmunds and Smedley 2005). Hence the groundwater fluoride concentration is not linearly correlated to the percentage of fluoride-bearing minerals in the geologic substrate. The influence of climate on fluoride concentrations in groundwater is largely dependent on the recharge of groundwater through rainfall and subsequent groundwater flow (IGRAC 2003; Edmunds and Smedley 2005). Humid tropical regions with high rainfall are less likely to have high fluoride concentrations in groundwater. This is because soluble ions such as fluoride are leached out and diluted due to heavy rainfall. On the opposite side arid environments normally have high fluoride contents due to the low rates of groundwater recharge. Low rainfall leads to prolonged water-mineral interaction and higher salinities in the arid environment, which further enhance mineral dissolution (Smedley et al. 2002). Temperature also regulates the solubility of fluorine-bearing minerals. The presence of high pH, and low calcium and magnesium contents in water had been reported to be the two major factors responsible for rapid leaching of fluoride and consequently resulting into elevated concentration of it in water (Rao et al. 1993).
Drinking water has been established to be the major contributor for entry of fluoride in the human food chain (Susheela 1999). According to Wood (1974) the intake of fluoride within the range of 0.5–1 mg/l in water is beneficial for human health. This permitted range of fluoride helps in the production and maintenance of healthy teeth and bones. Later, the United States Environment Protection Agency (USEPA) revised the recommended range of fluoride as 0.7–1.2 mg/l (USEPA 2010). Thus, any water sample with fluoride contents <0.7 or > 1.5 mg/l should not to be utilized as a source of potable water. Fluoride may be added at low levels during treatment of drinking water in places which are devoid of adequate quantity of fluoride to aid in dental and skeletal health of the residents, especially children. Again, excessive intake of fluoride causes chronic dental and skeletal fluorosis, which is manifested by mottling of teeth in mild cases, softening of bones and neurological damages in severe cases (Steinberg et al. 1955). In India the Bureau of Indian Standards (BIS) and Indian Council of Medical Research (ICMR) have prescribed 0.6–1.2 mg/l and 1.5 mg/l as the desirable range and permissible limit for fluoride, respectively (ICMR 1975; BIS 2003). Consumption of fluoride in excess of 2 mg/l was reported to cause dense and brittle bone, and also dental problems (Chatterjee et al. 2008). However, the United States Public Health Service (USPHS) has advocated following a range of allowable concentrations for fluoride in drinking water for different regions depending on prevailing climatic conditions (USPHS 1962). The USPHS argued that the amount of water consumed and consequently the amount of fluoride ingested being influenced primarily by the air temperature of that region. Thus, presence of fluoride in drinking water is often metaphoric as two-edged sword−prolonged ingestion in excess of the permissible fluoride level is associated with dental, and skeletal fluorosis and other toxic responses while inadequate intake of fluoride is associated with dental caries.
Fluoride contamination in soil−sources and significances
Fluoride is the 13th most abundant element of the earth’s crust representing ~0.3 g/kg of the earth’s crust. It is mainly present as NaF or HF, which can be found in minerals fluorspar, fluorapatite, topaz and cryolite. In most of the rocks fluorine is present normally in the range of 100−1,300 mg/kg (Faure 1991), but in soils fluoride contents generally vary in the range 20−500 mg/kg (Edmunds and Smedley 2005). Exceptionally higher presence of fluoride (>1,000 mg/kg) was observed in soils derived from rocks with high fluorine contents (Cronin et al. 2003) or in cultivated soils affected by anthropogenic inputs, like application of phosphate fertilizers (Kabata-Pendias and Pendias 2001), intrusion of sewage sludge, and industrial contamination (Cronin et al. 2003). According to Bhat et al. (2015) fluoride contamination of soil is fundamentally because of phosphorous fertilizers containing <1% to >1.5% fluorine. The sorption capacity of soil (which varies with soil pH), types of sorbents present in soil, and soil salinity largely regulate the mobility of fluoride in soil (Pickering 1985; Cronin et al. 2003). In comparison to sandy soils, fine-grained soils have higher clay and oxyhydroxide contents, and thus retain more fluoride. High salinity (ionic strength) affects fluoride mobility by enhancing the potential for fluoride complexes to form and by increasing the number of ions that compete for soil sorption sites. Edmunds and Smedley (2005) suggested that evapotranspiration was able to raise the salinity of soil solutions leading to higher content of fluoride ultimately reaching groundwater. The primary anthropogenic sources of fluoride pollution in soil are different chemical industries (like phosphorous fertilizer plants, steel, aluminum, zinc, smelting industries, etc), glass and ceramic industries, and power plants. The soil of the areas in vicinity of the above said industrial units contain medium to high content of fluoride (Bhat et al. 2015).
Agriculture is also responsible for disposal of fluoride in soil. This is due to the facts that irrigation and application of fertilizers normally increase the sodicity of soils which ultimately result into high fluoride content in soils (Brindha et al. 2011). A significant correlation (r = 0.56) was detected by Samal et al. (2015) between the fluoride content in deep tube wells and that in agriculture field soils. In a fluoride affected area if fluoride-contaminated groundwater is used for irrigation then there is a high possibility of bioaccumulation of fluoride into irrigated crops. This would further endanger human health due to augmented fluoride consumption through cultivated crops, fruits and vegetables in addition to drinking water pathway. Soils polluted with fluoride negatively impact human wellbeing through direct contact with soil, through inhalation of vaporized soil contaminants, through ingestion of contaminated foods, and through intake of contaminated groundwater (Blagojevic et al. 2002; Begum 2012; Bhat et al. 2015). Fluorine can penetrate deep soil layers of <80 cm, and when present in high concentrations it can oppress biological activities operating in soil medium (Polonskij 2013).
Transfer of fluoride from contaminated irrigation water to cultivated crops
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