## 3 Discussion
The resulting variability in the fluoride release models may be related to the composition of the varnishes, i.e., the type of resin used as a matrix for the components contained in it, the presence of additional substances, and the consistency, homogeneity and viscosity of the varnish or the particle size [22; 43; 45; 46; 67; 70; 72; 73]. The significantly higher cumulative fluoride release at each measuring point in MI Varnish may be due to several factors. [PERSON] et al. [17], [PERSON] et al. [59] and [PERSON] et al. [74] emphasize very good solubility of the CPP-ACP complex itself in aqueous solutions, which can directly accelerate the release of fluoride. Some researchers pointed to the behavior of polyvinyl acetate polymer, i.e., the so-called PVA, present in MI Varnish. In the study, it was noted that it also showed great ease and speed of dissolution in water [75], and at the same time, the maximum release took place within the first hour after the application [76]. In addition, its solubility increased in the presence of organic solvents (in this case, ethanol), which facilitated its rapid evaporation, thus leaving free spaces for easier and faster contact with water [76]. In addition, based on recent reports, the constitution of CPP-ACP varnish molecules--which are smaller and more homogeneous than those of other varnishes--can cause their faster dissolution and easier ion release [64]. The pH of the environment turned out to be an additional factor having a highly significant (\(p<0.001\)) effect on the rate of fluoride ion release. The greatest cumulative release of fluoride was achieved at pH = 4 for all tested varnishes. Slightly lower amounts were measured for pH = 5, but when comparing acidic environments, these differences were not statistically significant. The level of released fluoride in the neutral environment was significantly lower than in environments with pH = 4 and pH = 5, as confirmed by post hoc tests. In addition, between the 48 th and 168 th hour, the decrease in fluoride release in the environment with pH = 4 was greater than in the environment with pH = 5, while the decrease in fluoride release in the environment with pH = 7 was insignificant. In the literature--although there is relatively little research comparing the fluoride release depending on the pH of the environment--attention is paid to the effect of low pH on both the release of fluoride ions and on the formation of calcium fluoride [3; 24]. [PERSON] et al. [24], when comparing the formation of calcium fluoride after application of 2% neutral NaF with acidified fluoride preparations, found the presence of significantly larger amounts of CaF\({}_{2}\) deposited on the surface of the enamel. Interesting conclusions were also drawn by [PERSON] et al. [77], who showed that Fluor Protector, containing the lowest concentration of fluoride ions in relation to other varnishes but low in pH, initiates the formation of the largest amount of CaF\({}_{2}\) on the surface of the enamel. This was explained by the affinity of the fluoride released in the acidic environment to the enamel that was higher than in neutral conditions. This was confirmed by [PERSON] et al. [38], who showed that in an acidic environment, the concentration of calcium ions and phosphates in curious lesions is higher than in the fluids surrounding the tooth, which promotes increased absorption of fluoride by the tooth structure. However, it should be emphasized that the behaviour of different varnishes in an acidic environment is not the same, and that it would be wrong to assume that it always causes faster release of ions. This was demonstrated by [PERSON] [78] and [PERSON] et al. [22] in their research, where only a portion of fluoride-containing preparations released more fluoride at low pH. As for MI Varnish, the pH value an additional factor influencing the balance between free and loosely bound calcium, phosphorus and fluoride ions, affecting their bioavailability, and, according to [PERSON] et al. [17] and [PERSON] et al.