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Fluorine in water

Fluors ūdenī

Fluorine is one of the most common element in the Earth’s crust. In nature (including water) it is found in the form of minerals called fluorides. Fluorides at a certain concentration in the human body strengthen the bones and teeth. However, their excessive content can be harmful to health and cause disease called fluorosis.

In this article we will explain whether fluorides in drinking water are harmful, what is their effect on bones and teeth as well as consider what methods are used today for the water fluoridation and defluoridation.

Fluorine and fluorides

Fluorine (F) as a chemical element belongs to the group of halogens. It is the thirteenth most common element in the Earth’s crust. As a pure substance it is a pale yellow colored gas that can only be observed in the laboratory because fluorine (when interacting with water or hydrogen) is explosive. Fluorine occurs naturally in the form of salts, the main fluorine minerals are fluorides (CaF2).

Fluorides are important for human body, they strengthen bones and teeth. In some countries they are added to the drinking water to prevent tooth decay. Fluorides are also added to toothpaste, agrochemicals and are also used in the production of medicines.

Where does fluoride in water come from?

Fluors ūdenī

Fluorine compounds or fluorides are found in all natural water sources. Sea water on average contains 1 mg/l fluoride, river water 0.5 mg/l, while higher concentrations are found in underground waters (up to 30 mg/l) where the fluoride concentration is determined by the rocks that form the aquifers.

Fluorides are found in both sedimentary and igneous rocks. At increased concentration they can be found:

  • in places where seaweed settles;
  • near volcanic activity;
  • in places where granite and gneiss rocks are common.

In some countries fluorides are deliberately added to the water. The United States was the first country to begin fluoridating water to prevent dental diseases. Today water fluoridation is taking place in several countries: USA, Argentina, Australia, Brazil, Canada, Chile, Colombia, Hong Kong, Ireland, Israel, Korea, Malaysia, New Zealand, Philippines, Serbia, Singapore, Spain, Great Britain and Vietnam. In addition, approximately 40 million people worldwide use naturally fluoridated water every day.

How much fluoride should be in water?

According to the recommendations of the World Health Organization the fluoride norm in drinking water is 0.8 – 1 mg/l. According to the regulation of safety and quality requirements for drinking water the maximum allowable fluoride concentration (MPC) in water is 1.5 mg/l.

Are fluorides in drinking water harmful?

Depending on the fluoride amount in the water fluorides can be both beneficial and harmful.

Fluorides in the water at a concentration of 1 mg/l show high efficiency for caries prevention and do not affect the bone structure. On the other hand, increased fluoride content in water can cause disease called fluorosis. Fluoride excess displaces calcium from bone and teeth causing pigmentation and erosion of dental tissue. The next stage of the disease is bone damage when joint pain and movement difficulties occur. In the case of a severe disease also changes occur in the bone structure, ligaments become calcified, muscle damage and acute pain occur.

Fluorides effect on teeth

Fluors ūdenī

Dental caries is caused by the growth of bacteria (e.g. Streptococcus mutans and Lactobacillus) which absorb carbohydrates and produce organic acids. Bacterial activity is especially active at high sugar consumption.

When the pH level drops below 5.5, acids dissolve hydroxyapatite (the main component of tooth enamel) and a process called demineralization occurs. When a person does not consume sugar the restoration of tooth enamel or a process called remineralization takes place.

Fluorides disrupt the demineralization mechanism. During remineralization they are able to form fluorapatites which are more resistant to acidic environments and protect teeth from decay.

Fluorosis and caries development risk (%) at a certain fluoride concentration in water

Fluoride concentration in water, mg/l Caries Fluorosis
Up to 0,3 mg/l 200–300% 3%
0,3 – 0,7 mg/l 20–200 % 5-7%
0,7 – 1 mg/l The incidence rate is relatively low 7-10%
1,0 – 1,5 mg/l The incidence rate is relatively low 1. stage – 7-10%
2. stage – 3%
1,5 – 2,0 mg/l The incidence rate is relatively low 1. un 2. stage – 30-40%
3. stage – 3%

Fluorides effect on bones

Regular use of water with a fluoride content of 1-2 mg/l reduces the frequency of ossification (the transformation of soft tissue into bone tissue) for children and the frequency of osteoporosis (a disease that results in a decrease in bone mass) for the elderly.

On the other hand, when using water containing 2.5 – 6.0 mg/l fluorides, in some cases osteosclerosis is observed at the initial stage (hardening of the bones due to excessive formation of bone tissue or excessive precipitation of calcium salts). At a fluoride concentration of 8 mg/l osteosclerosis in a mild form is observed in 10-15% of cases, which progresses within 20-30 years. But at a fluoride concentration of 10-20 mg/l and above, a severe course of the disease can be observed already after 10-15 years.

Exceeding the maximum permissible concentration of fluorides (MPC) in water has much more negative consequences for the body than their lack.

How is water fluoridated?

The most common fluorination reagents today are:

  • Sodium Fluoride (NaF): white odorless powder or crystals.
  • Hexafluorosilicic acid (H2SiF6): reagent in liquid form.
  • Sodium fluorosilicate (Na2SiF6): powder or crystals. Bottled water with fluoride usually contains this substance.

How is water defluoridated?

Water defluoridation is a relatively expensive process, even more expensive than water fluoridation.

Traditional defluoridation methods:

  • Sorption on carbon, activated alumina and clay, which is a relatively efficient method. The principle of operation of these substances is based on ion exchange, when the OH – ions in these compounds are exchanged with F – ions.
  • Sedimentation, which includes the addition of calcium, magnesium, phosphate, iron, aluminum and other compounds.
  • Reverse osmosis, which is the best solution for purifying water from fluoride at home.


  1. In nature fluorine does not occur in pure form, it is available in the form of fluorides.
  2. The maximum permissible fluoride concentration (MPC) in water is 1.5 mg/l.
  3. Depending on the content of fluorides in the water, they can be useful (in the prevention of dental caries) as well as harmful (can cause the disease called  fluorosis).
  4. Exceeding the MPC in water has much more negative effects on the human body than its lack.
  5. Water defluoridation is a more expensive process than fluoridation.
  6. Water fluoridation in low-fluoride regions is recommended but it is not cheap. Alternatives available: using fluoride toothpaste or fluoridated salt.
  7. Reverse osmosis is the optimal solution for water defluoridation both at home and in centralized water treatment plants.

Read also: Magnesium water.

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