# Identification of Factors Influencing Fluoride Content in Tea Infusions: A Systematic Review
## 1 Introduction  
Fluoride is an element commonly found in the natural environment and is classified as a microelement. It enters the human body through the digestive and respiratory tracts and, in small amounts, through the skin. The main sources are water, food, and everyday hygiene products such as toothpastes and mouthwashes [1, 2, 3, 4]. The recommended daily intake of fluoride varies according to age and gender. It is 0.7 mg for children and 3-4 mg foradults. These values are considered to be the adequate daily intake of fluoride. The tolerable upper intake level (UL) for adults, beyond which the risk of fluorosis increases, is 10 mg per day [2,5]. Fluoride is an element that performs important systemic and local functions in the human body [6]. It is a component of enzymes involved in the metabolism of proteins, carbohydrates, and lipids [3]. Ninety-nine percent of the fluoride absorbed by the human body is found in bones and teeth, where hydroxyapatite have been converted by the presence of fluoride into more valuable fluoroapatites and/or hydroxyfluoroapatites [1,2]. In addition, fluoride plays a very important role in dentistry as a component of an anti-caries substance [7,8]. In addition to being added to drinking water and milk, fluoride is found in everyday hygiene products such as toothpastes and mouthwashes. It is also used professionally in the form of varnishes, foams, and gels [9,10]. Its cariostatic action is based on the prevention of enamel demineralization, its remineralization (stopping caries at an early stage), and its antibacterial action. These properties have contributed significantly to the worldwide reduction in dental caries [8,9,11]. It is also worth mentioning the risks associated with overdosing on fluoride. Excess fluoride leads to enamel fluorosis, which is a disorder of the quality and/or quantity of enamel. Studies show that exceeding the daily intake of fluoride has a systemic toxic effect and, in extremely high doses, can lead to death [12,13]. Tea infusion is a beverage commonly consumed by humans. It is produced worldwide from the leaves of the _Camellia sinensis_ tree. Apart from water and some foods such as fish and seafood, tea is the main source of fluoride in the human diet [3,14]. The amount of tea consumed varies depending on the region of the world and cultural habits. A 250 mL infusion can provide up to 1.5 mg of fluoride, which is a significant part of the recommended daily intake of this element [15]. Therefore, tea should be considered as a significant source of fluoride in the human diet, and it should be borne in mind that the accumulation of fluoride from all sources creates a risk of dental fluorosis, skeletal fluorosis, and chronic poisoning with this element [16]. Studies show that there is considerable variation in the fluoride content of teas. This is affected by many variables. The amount of fluoride depends on the type of tea. For example, black teas release the most fluoride into the infusion, followed by white and green teas [17,18]. Herbal products release the least fluoride [14,19]. The same type of tea may also have different fluoride levels depending on the region of origin [20]. The age of the plant has a major effect on the fluoride content of tea trees. Older specimens have been shown to have higher levels of fluoride due to the accumulation of fluoride over time. The pH of the soil in which the plant is grown is also important, as the lower the pH of the environment, the greater the deposition of fluoride in the stems and leaves [14,21]. In some regions of the world, frequent consumption of tea because of cultural behavior creates a situation in which a given population is exposed to exceeding the daily limit of fluoride intake [15,22]. This can result in dental and/or skeletal fluorosis and chronic poisoning leading to cardiac, neuro, and nephrotoxicity [13,15].