Unmasking fluoride: A global examination of health complexities and groundwater dynamics

In groundwater, excessive fluoride concentration has been evidenced in more than 20 developing and developed countries, including India, wherein 19 states are confronting serious problems owing to fluorosis. Essential for skeletal and dental development in small quantities, excessive fluoride intake leads to fluorosis, a debilitating disease impacting approximately 200 million people globally. India faces a severe crisis with high fluoride intake, leading to dental and skeletal health issues.

The harmless fluoride concentration in drinking water is around 1.5 mg/l. Geological factors, such as granite gneisses and slate stones, contribute to fluoride ion concentration, with variations in rock types and mineral breakdown. Additionally, climate, contact time with aquifer minerals, and arid regions significantly influence fluoride levels. Human activities, including burning fossil fuels and the production of fly ash, contribute to elevated fluoride content, posing health risks worldwide.

Numerous scientists have reported that a small amount of fluoride is beneficial for strengthening the bone and preventing dental caries, instead, its high dose causes severe effects on human health such as skeletal fluorosis, dental fluorosis, increase in bone fracture, decrease in birth rates, impaired thyroid functions, increased rates of urolithiasis and lower intelligence quotient (IQ) in children. Fluoride exposure may lead to high frequency of chronic irritative respiratory disease, chronic bronchitis. Due to high intake of sodium fluoride pesticides and dental products, severe fluoride toxicity and death have also been witnessed.

Hydro-geochemistry of fluoride in groundwater

Fluoride ion exhibits the lowest solubility within the pH range of 5.0–6.5 (Adriqno, 1986). At higher pH levels, ionic exchange between fluoride and hydroxyl ions (found in minerals like mica, illite, and amphiboles) occurs, leading to increased fluoride absorption in groundwater (Sunkari et al., 2022). The presence of elevated bicarbonate and sodium levels, coupled with higher pH, facilitates fluoride leaching from minerals into groundwater, enhancing the solubility of fluoride-containing minerals due to the alkaline nature of the water. In alkaline environments, fluoride ions are desorbed, contributing to the suspension of fluoride minerals.

Excessive fluoride in groundwater is often linked to higher concentrations of bicarbonate ions and, in certain locations, elevated nitrate ion levels (Handa, 1975). Higher Total Dissolved Solids (TDS) concentrations elevate ionic strength, further increasing the solubility of fluoride ions in groundwater (Perelman, 1967). Prominent fluoride-containing minerals include muscovite, lepidolite, biotite, fluorapatite, fluorite (fluospare), cryolite, and topaz. The dissolution of fluoride (CaF2) contributes significantly to high fluoride concentrations in groundwater globally, as reported by Smedley and Edmunds (2013).

Health impacts of fluoride

Fluorine, with its high electronegativity, has a unique affinity for positively charged cations similar to calcium. Modest fluoride consumption has been associated with reduced dental fluorosis and strengthened bones under specific conditions (Rao, 2003; Kaminsky et al., 1990; Harrison, 2005; Heller et al., 1997; Doull et al., 2006). Free fluoride ions in the fluid phase also contribute to the growth of apatite minerals (Aoba and Fejerskova, 2002).

The recognition of fluoride's benefits dates back to the 1930s, when researchers aimed to determine an optimal fluoride content in drinking water to reduce dental deformities and promote bone development. Adequate fluoride intake has been linked to preventing osteoporosis, as demonstrated in studies conducted in North Dakota (Bernstein et al., 1996).

• Dental effects:

  Fluoride is crucial for strengthening teeth and preventing dental fluorosis, a condition marked by tooth surface discoloration or enamel blotching. Dental fluorosis severity is linked to fluoride ion consumption during enamel development, often manifested in opaque white areas or yellowish-brown staining (Choubisa and Choubisa, 2021). Regions like Bathinda, Mansa, Faridkot, and Muktsar in Punjab, India, are highly vulnerable to dental and skeletal fluorosis (Duggal and Sharma, 2022).

• Skeletal effects:

  Skeletal fluorosis, characterised by increased bone mass and density, presents challenges in understanding its progression. Initial stages involve joint pain and increased bone density, leading to conditions like "Pokar Back." Skeletal fluorosis depends on water quality, consumption, and nutritional factors (Raja Reddy et al., 1985; Jolly et al., 1969).

• Developmental effects:

  Fluoride exposure during pregnancy can lead to adverse developmental effects. Studies link high fluoride intake to decreased IQ levels in children and potential interference with iodine status, affecting thyroid function during critical intrauterine brain development (Griebel-Thompson et al., 2023).

• Reproductive effects:

  Increased fluoride consumption has been associated with fertility issues, affecting hormone levels and disrupting spermatogenesis in males. Female reproductive health is also impacted, with higher levels of follicle-stimulating and luteinizing hormones and reduced estrogen levels (Ortiz-Perez et al., 2003; Jiang et al., 2005).

• Renal effects:

  The kidneys play a crucial role in eliminating fluoride, and chronic exposure may lead to non-carcinogenic consequences and kidney stones. Regions with elevated fluoride concentrations in groundwater show increased hospital admissions for urolithiasis (Singh et al., 2001).

• Neuronal effects:

  Excessive fluoride ingestion may lead to neurotoxicity, impacting learning and memory. Studies in China link fluoride consumption to lower IQ levels in children, and similar observations have been made in India (Trivedi et al., 2007).

• Endocrine effects:

  Fluoride exposure has been associated with adverse effects on the endocrine system, including increased parathyroid activity, reduced thyroid function, and altered glucose tolerance (Doull et al., 2006).

• Gastrointestinal effects:

  Animal studies reveal that fluoride exposure can affect gastrointestinal functions, leading to symptoms like diarrhoea, abdominal pain, and nausea. Effects are generally mild for concentrations below 4.0 mg/l in drinking water (Doull et al., 2006).

• Carcinogenic effects:

  The carcinogenic potential of chronic fluoride exposure remains uncertain. Some studies suggest a link between occupational fluoride exposure and bladder cancer. However, evidence regarding fluoride's role in cancer, including osteosarcoma, is inconclusive (Grandjean et al., 1990 & 1992; Takahashik et al., 2001).

• Genetic erythrocyte disorder:

  Chronic exposure to fluoride ions has been associated with genetic blood disorders in certain tribal communities in Rajasthan, India, such as β-thalassaemia, glucose-6-phosphate dehydrogenase (G-6-PD) enzyme deficiency, and sickle cell anaemia (Choubisa and Choubisa, 2021).

Drinking water standards

Several factors, including weathering of rocks, evaporation, climate, diet, anthropogenic activities, and population characteristics, determine fluoride content standards in drinking water. Fluoride deficiency in drinking water can lead to tooth decay, while controlled addition up to 1.0 ppm through water fluoridation helps prevent dental issues. However, excess fluoride causes tooth mottling and bone defects, prompting adherence to safe limits.

Establishing drinking water standards for fluoride involves complex considerations such as climate, diet, and population characteristics. The U.S. Environmental Protection Agency (EPA) sets a maximum contaminant level (MCL) of 4 mg/l and a secondary MCL (SMCL) of 2 mg/l. The World Health Organisation (WHO) recommends less than 1.5 mg/l, while India and China adopt a 1 mg/l standard due to naturally higher fluoride levels. Groundwater quality studies, such as in Dausa district, Rajasthan, reveal varying fluoride concentrations, emphasising the need for tailored assessments in different regions (Wu et al., 2015; Chen et al., 2017; Tiwari et al., 2020).

Fluoride removal techniques

Addressing elevated fluoride levels in water is crucial for ensuring safe drinking water. Various methods, categorised as in-situ and ex-situ treatments, aim to reduce fluoride concentrations.

In-situ treatment methods

• Check dams:

  Constructing check dams not only lowers fluoride concentrations but also improves groundwater quality for domestic and agricultural purposes. Instances in Tamil Nadu and Andhra Pradesh demonstrate successful in-situ fluoride reduction near dams.

• Percolation tanks and recharge pits:

  Artificial recharging structures, such as percolation tanks and recharge pits, strategically placed for rainwater harvesting, prove effective in reducing fluoride levels in groundwater.

Ex-situ treatment methods

• Reverse osmosis:

  A significant method, removing over 95% of fluoride ions, involves applying pressure to force water through a permeable membrane, leaving impurities behind.

• Flocculation method:

  Employing aluminum sulphate as a coagulant, this method removes over 90% of fluoride ions, making it suitable for both household and community applications.

• Adsorption using activated alumina:

  Activated alumina acts as an adsorbent to extract fluoride ions through a solid media column, offering a durable and efficient solution.

• Distillation:

  While a slightly slower process, distillation relies on evaporation and condensation to produce pure water free from contaminants.

• Bone charcoal:

  Using finely crushed animal bones or activated alumina, this technique removes up to 90% of fluoride ions from water.

In 2019, the Indian government launched the Jal Jeevan Mission to provide piped drinking water to every rural household by 2024, addressing water management and contamination issues. This initiative aligns with the broader goal of mitigating skeletal fluorosis and enhancing water quality across the country.

Conclusion: The complex reality of fluoride ion exposure

Groundwater, a major source of fluoride accumulation, results from mineral decomposition, dissociation, and dissolution, particularly in areas with weathering rocks, volcanic activities, or fluoride-rich sedimentary formations. Anthropogenic sources, such as pesticide use and industrial runoff, contribute to elevated fluoride levels.

Various techniques, including reverse osmosis, distillation, adsorption with activated alumina, flocculation, and bone charcoal, offer solutions for removing fluoride from water. The global challenge involves addressing fluoride-related health problems in third-world countries due to insufficient access to clean water and knowledge gaps. Developed nations also grapple with health issues, often linked to dietary choices. Thus, establishing a clear link between fluoride intake and its impact on human health, coupled with increased awareness, is crucial for ensuring a healthier future.

Future perspectives in fluoride research

Environmental ramifications of fluoride are under scrutiny, evaluating potential toxicity to wildlife and ecosystems. Researchers explore alternative fluoride sources beyond traditional toothpaste and water fluoridation, introducing new compounds and delivery methods.

In developing countries, studies assess the effectiveness of fluoride programs, aiming to enhance fluoride access and dental care. As technology and research methods progress, the landscape of fluoride research will likely expand, driven by growing concerns for health and the environment.

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