Fluoride in water

Fluorides are characteristic impurities found in groundwater, and fluoride itself is an essential component. At low concentrations, it helps prevent cavities, while in high concentrations, it accumulates in the body, leading to a bone disease known as fluorosis.

As a chemical element, fluoride belongs to the halogen group, which includes chlorine, bromine, and iodine. In its pure form, it is a pale-yellow gas that can only be observed under laboratory conditions, as fluoride is the most reactive non-metal and interacts with nearly all substances, often releasing intense heat, sometimes explosively. This is why it exists in the environment only as salts—fluorides.

Where Does Fluoride in Water Come From?

‍Fluoride compounds are found in all natural water sources. For example, seawater typically contains about 1 mg/L, rivers less than 0.5 mg/L, and underground sources up to 30 mg/L (where concentration depends on the composition of aquifer rocks).

Fluorides are present in a wide range of minerals, including fluorite, rock phosphates, cryolite, apatite, mica, and others. CaF₂ is one of the minerals with low solubility, limited to 40 mg/L.

Fluorides are found in both sedimentary and igneous rocks. Elevated levels are typical of areas where:

• Marine algae are deposited in foothills,
• Volcanic activity occurs,
• Granite and gneiss rocks are common.

According to WHO recommendations, the optimal fluoride concentration in drinking water is 0.8 - 1 mg/L.

In some countries, fluoride is intentionally added to the water supply to prevent tooth decay. The U.S. was the first country to introduce water fluoridation to prevent dental diseases in both children and adults.

Today, water fluoridation occurs in several countries: the U.S., Argentina, Australia, Brazil, Canada, Chile, Colombia, Hong Kong, Ireland, Israel, Korea, Malaysia, New Zealand, the Philippines, Serbia, Singapore, Spain, the United Kingdom, and Vietnam.

Additionally, approximately 40 million people worldwide consume naturally fluoridated water daily.

If fluoride levels in water are high, removal is necessary.

How Is Water Fluoridated?

‍How is tap water fluoridated?
The most commonly used reagents for fluoridation are:

• Sodium fluoride (NaF) - a white powder or crystals, odorless.
• Hexafluorosilicic acid (H₂SiF₆) - a liquid reagent, a by-product of fertilizer production. Its drawback is the high transportation cost due to high water content.
• Sodium fluorosilicate (Na₂SiF₆) - also a powder or crystals, less toxic for water treatment personnel than sodium fluoride and more cost-effective in transportation than hexafluorosilicic acid. Bottled fluoridated water often contains this substance.

Is Fluoride in Drinking Water Harmful?

‍As mentioned earlier, depending on its concentration, fluorides can be both harmful and beneficial.

Excessive fluoride in water leads to a condition known as fluorosis. Its mechanism involves fluoride replacing calcium in bones and teeth.

The first signs are dental fluorosis, characterized by pigmentation and erosion of dental tissue. After the age of six, the risk of dental fluorosis decreases in children.

Following dental damage, skeletal effects occur. Over time, fluoride accumulates in bones. Primary symptoms include joint stiffness and aches. Severe cases lead to structural changes in bones and ligament calcification, causing muscle pain and acute discomfort.

Regarding acute fluoride exposure, a single dose of 20 mg may cause nausea or vomiting, while more than 40 mg per kg of body weight can be lethal. Only half of people can detect a taste difference in water with fluoride at 100 mg/L.

For caries prevention, a concentration of 1 mg/L shows high effectiveness without affecting bone structure, though it may cause slight tooth discoloration. This has been confirmed through long-term studies in countries that practice water fluoridation.

How Fluoride Works on Teeth

‍The cause of cavities is bacterial growth in dental plaque, particularly Streptococcus mutans and Lactobacillus. They consume carbohydrates and produce organic acids, with activity intensifying when sugar intake is high.

Fluorides disrupt the caries demineralization mechanism. When pH drops below 5.5, acids dissolve hydroxyapatite, the base component of tooth enamel—a process called demineralization. When sugar is absent, enamel recovery or remineralization occurs. Tooth damage happens when remineralization rate is lower than demineralization rate.

Fluorides can form fluorapatite, which is more acid-resistant, thus protecting teeth from decay during remineralization.

Dental Fluorosis vs. Cavities
The table below shows fluoride concentration in daily water consumption and its effects.

The Impact of Fluoride on the Skeletal System

At fluoride levels of 1-2 mg/L in water, cases of ossification in children and osteoporosis in the elderly decrease. However, with regular consumption of water containing 2.5 - 6.0 mg/L of fluoride, initial stages of osteosclerosis are observed in some individuals. At a concentration of 8 mg/L, mild osteosclerosis is observed in 10-15% of the population, with severe forms developing over 20-30 years. At concentrations of 10-20 mg/L, severe forms appear within 10-15 years, and growth delays are seen in children.

At higher concentrations, acute fluorosis can occur within 10-15 years.

As you can see, the optimal fluoride content in water has already been determined. Exceeding the maximum allowable concentration (MAC) leads to significantly more negative consequences than a deficiency.

How is Water Defluoridated?

Defluoridation is a fairly expensive process, even more costly than fluoridation, so finding a safe water source is always preferable to treatment.

Traditional methods include:

1. Adsorption on Bone Char, Activated Alumina, and Clays
   This method, using minerals such as magnesite, clinoptilolite, vermiculite, zeolite, diatomite, kaolinite, and others, is quite effective. The process is typically conducted in columns in either a continuous or batch mode. The operation of these substances is based on ion exchange, where OH- ions in these compounds are exchanged for F- ions. These materials often require increased adsorption properties through activation (acid treatment, drying, calcination, etc.). Their application is justified when there are local deposits of such minerals in the region where water defluoridation is needed.
2. Contact Precipitation
   This method involves adding compounds of calcium, magnesium, phosphates, iron, aluminum, etc. Fluoride precipitation occurs either by forming poorly soluble compounds that settle by gravity (e.g., calcium and magnesium fluorides) or by coagulation of fluoride ions on the surface of specific coagulant particles (such as iron and aluminum hydroxides).
3. The Nalgonda method—a coagulation technique widely used in India for defluoridating water with fluoride concentrations up to 30 mg/L—uses a solution that contacts coagulants (aluminum or iron salts), lime or soda, and bleaching powder (to increase pH). The water then settles for 2-4 hours and is filtered through sand filters. In addition to reducing fluoride content, this process also reduces turbidity, color, and organic pollutants.
4. Reverse Osmosis
   This technology provides complete fluoride removal without producing large amounts of toxic sludge and does not require bulky equipment.
5. Reverse osmosis can be used in POU systems installed directly under the sink at home and at centralized water treatment facilities. A membrane filter can be an effective solution if a water analysis shows excess fluoride levels, as it is an optimal method for home water defluoridation.

The most common methods include activated alumina filtration and reverse osmosis.

Fluorine as a Water Disinfectant

Questions often arise about using fluorine for water disinfection. Elemental fluorine gas or fluorine compounds are not used for disinfection due to the explosive nature of gaseous fluorine, which is rarely used in gaseous form, and because fluorides lack disinfectant properties.

Conclusions

Overall, elevated fluoride levels in water do not present a critical issue in Ukraine. In eastern regions, some fluoride in tap water is removed by existing coagulation technologies.

Water fluoridation is recommended in areas with low fluoride content, but due to associated costs, it is not widely implemented. Alternatives include fluoride-enriched toothpaste and fluoridated salt.

Reverse osmosis is the optimal solution for water defluoridation, both for home use and at centralized water treatment facilities.