# Identification of Factors Influencing Fluoride Content in Tea Infusions: A Systematic Review
## 3 Results
### Quality Assessment and Risk of Bias of the Included Studies  
For all of the 9 questions, 8 papers received a positive answer to 8 of them [31; 32; 48; 50; 52; 54; 64; 67], 10 papers received a positive answer to 7 of them [68; 69; 70; 22; 49; 51; 53; 49; 55; 62; 66; 68; 70], 9 papers received a positive answer to 6 of them [19; 28; 47; 55; 58; 59; 63; 65; 69; 63; 65; 69; 70], and 3 remaining papers received a positive answer to 6 of them [57; 60; 61] (see Supplementary Table S2.)  
Among the thirty studies included in this systematic review, thirteen were classified as having a low risk of bias, with total scores ranging from 7 to 9 points [64; 67; 50; 51; 52; 54; 64; 22; 28; 53; 54; 63; 22; 29; 27; 28; 29; 30; 31; 32; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 65; 66; 68; 69; 70]. The remaining seventeen studies were rated as having a moderate risk of bias, scoring 5 or 6 points [19; 47; 49; 55; 56; 57; 58; 59; 60; 61; 62; 63; 65; 66; 68; 69; 70]. None of the studies were classified as high risk, as no score fell below the threshold of five points (see Figure 5).  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Author** & **Parameter(s)** & **Effect on Fluoride** & **Results** \\ \hline [PERSON] et al. [68] & Temperature (60–95 \({}^{\circ}\)C), repeated brewing & \(\uparrow\) with temperature, \(\downarrow\) with each brew & Detailed temperature profile and brewing ratios studied \\ \hline [PERSON] et al. [51] & Water type (DI vs. drinking), repeated brewing & \(\uparrow\) with time, variation by water type & Fluoride release higher in drinking water \\ \hline [PERSON] et al. [54] & Water fluoride content (0.25–30.2 mg/L) & Complex—sometimes \(\downarrow\) due to absorption & Tea leaves absorb fluoride from highly fluoridated water \\ \hline [PERSON] et al. [63] & Repeated brewing, fermentation & \(\downarrow\) with each brew, \(\uparrow\) with fermentation & Loose vs. packed tea also compared \\ \hline [PERSON] et al. [56] & Water source (different pH/fluoride levels) & \(\downarrow\) fluoride release with \(\uparrow\) pH & Water origin matters \\ \hline [PERSON] et al. [65] & Water type (distilled vs. natural) & \(\uparrow\) in natural water & Higher fluoride release in mineralized water \\ \hline [PERSON] et al. [32] & Water hardness (CaCO\({}_{3}\) 0–350 mg/L) & \(\uparrow\) hardness = \(\downarrow\) fluoride release & Significant reduction in extraction with hard water \\ \hline [PERSON] et al. [48] & Calcium presence & \(\downarrow\) fluoride extraction & Tea with calcium-rich solution released less fluoride \\ \hline [PERSON] et al. [67] & pH, Al, and Fe\({}^{3+}\) ion concentration & \(\uparrow\) pH = \(\uparrow\) fluoride; concentration & Ion competition affects free fluoride \\ \hline [PERSON] et al. [60] & Brewing time (10 vs. 30 min) & \(\uparrow\) fluoride with longer time & Limited temperature data \\ \hline [PERSON] et al. [69] & Repeated infusion & \(\downarrow\) fluoride with more brews & Region of origin also noted \\ \hline \hline \end{tabular}
\end{table}
Table 3: Impact of Temperature and Water Composition on Fluoride Release; \(\uparrow\)- increase, \(\downarrow\)-decrease.