# Identification of Factors Influencing Fluoride Content in Tea Infusions: A Systematic Review
## 5 Conclusions  
This systematic review of thirty studies demonstrated that the fluoride content in tea infusions is influenced by a wide range of factors. A consistent finding across the reviewed studies was that fluoride concentration increases with longer brewing times, regardless of the temperature used. Additionally, a positive correlation between higher brewing temperatures and elevated fluoride concentrations was frequently observed. The presence of other ions, such as aluminum, iron, calcium, and fluoride itself, in the brewing water significantly affected the amount of free fluoride released into the infusion, highlighting the importance of considering both water hardness and fluoride content when preparing tea. It is also noteworthy that repeated brewing of the same tea leaves results in progressively lower fluoride concentrations in subsequent infusions. The global diversity and variety of teas available on the market contribute to the observed variability in fluoride concentrations among tea infusions. Nevertheless, irrespective of the country of origin or specific tea variety, black tea infusions consistently exhibited the highest fluoride levels. Tea bags and loose tea leaves were the most frequently studied forms, with tea bag infusions consistently releasing higher fluoride concentrations than loose-leaf counterparts. Regarding analytical techniques, fluoride determination was most commonly performed using potentiometry with an ion-selective electrode. However, more precise and reliable measurements can be achieved using methods such as ion chromatography. The findings of this systematic review demonstrate that the fluoride content in tea infusions ranges from 0.008 to over 8 mg/L, while the recommended daily intake for adults is 3-4 mg. Based on these results, regular consumption of teas with high fluoride content (particularly black and green teas in bagged or powdered form) could potentially contribute significantly to exceeding the daily fluoride requirements for adults, especially when prepared with longer brewing times and higher water temperatures.Several studies, including [PERSON] et al. (2019), indicated that drinking 1 L of tea could provide up to 150% of the daily fluoride requirement, while [PERSON] et al. (2019) reported that populations consuming up to 3.2 L of tea daily were at risk of excessive fluoride intake. Conversely, herbal teas, white teas, and loose-leaf preparations generally contain lower fluoride levels and are less likely to cause overconsumption. Based on these findings, the recommendation are for the tea consumers, particularly those who drink multiple cups daily, to choose herbal teas, white teas, or loose-leaf preparations, which generally contain lower fluoride levels; limit consumption of bagged or powdered black and green teas, which consistently show the highest fluoride concentrations; consider shorter brewing times (under 5 min) to reduce fluoride extraction; and be aware that tea brewing in areas with naturally high-fluoride water compounds the total exposure. Given the widespread consumption of tea worldwide, understanding these factors is critical for assessing potential fluoride exposure through diet. Further standardized research is necessary to refine safe consumption guidelines, especially for populations at higher risk of fluoride overexposure. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15115974/s1](https://www.mdpi.com/article/10.3390/app15115974/s1)]([https://www.mdpi.com/article/10.3390/app15115974/s1](https://www.mdpi.com/article/10.3390/app15115974/s1)), Table S1: Detailed characteristics of the included studies; Table S2: Quality assessment--JBI checklist for quasi-experimental studies (nonrandomized experimental studies). Conceptualization, A.M., J.M. and M.D. ; methodology, A.M. and J.M. ; software, A.M. and M.J.; validation, J.M. and M.D. ; formal analysis, M.D. and J.M. ; investigation, A.M., A.K., [PERSON] ([PERSON]), [PERSON] ([PERSON]), S.K. and M.M.