## 1 Introduction  
For almost 70 years, fluoride compounds have played an important role in preventing dental caries as the most effective method of fighting this disease. Their action is based on two mechanisms: pre-eruptive (endogenous) and post-eruptive (exogenous), whereby exogenous methods are now considered more effective and safer [1; 2; 3; 4; 5; 6]. The post-eruptive effect, i.e., after the emergence of teeth in the mouth, consists in affecting the adhesion of the bacterial plaque and its metabolism, thus causing a bacteriostatic and/or bactericidal effect, inhibiting the demineralization process and supporting the remineralization of the enamel [1; 3; 7; 8; 9; 10; 11]. Fluoride varnishes used in the contact fluoridation method were approved in 1994 by the FDA (Food and Drug Administration), but as a medicine for tooth hypersensitivity. Over the years, they have become standard practice in dentistry, currently recommended by international dental organizations and associations, i.e., ADA (American Dental Association), AAPD (American Academy of Paediatric Dentistry), EAPD (EuropeanAcademy of Paediatric Dentistry) and the American Dental Association Council on Scientific Affairs [12; 13; 14; 15]. Numerous meta-analyses [9; 16; 17; 18] have shown their effectiveness in inhibiting caries, especially in children and adolescents, by reducing it on average by 43% for permanent teeth and 37% for milk teeth. The lack of continuation of caries prevention with the aid of administration of fluoride, according to [PERSON] et al. [19], resulted in an increase in caries activity.  
The exact chemical composition of varnishes is not fully known because, according to the statement developed by OSHA, the so-called Hazard Communication Standard 29 CFR 1910.1200, only those ingredients that can be dangerous and life-threatening require documentation; the others are a trade secret [20]. Typically, each fluoride varnish consists of three basic elements. Two of them are resin (mastic, shellac or/and rosin, also called collophony), which is responsible for viscosity, elasticity and fluidity and thus extending the contact time with the tooth surface; and alcohol (usually ethanol), used as a solvent which evaporates after exposure to air and active agents [7; 21]. In addition, in the so-called first generation of varnishes, there are fluoride salts, commonly known as fluorides, usually in the form of neutral or acidified 5% NaF containing 2.26% fluoride ions (22,600 ppm F) or 1% difluorosilane containing 0.1% fluoride ions (1000 ppm F). The second-generation varnishes are additionally enriched with calcium and phosphorus compounds in various forms, e.g., as so-called functionalized tricalcium phosphate (\(\beta\)-TCP), amorphous calcium phosphate (ACP), casein phosphopeptide amorphous calcium phosphate nanocomplex (CPP-ACP), calcium sodium phosphosilicate (Novamin) or xylitol-coated calcium phosphate (CXP) [22; 23]. According to many researchers [24; 25; 26; 27; 28; 29; 30] these increase the bioavailability of fluoride and the rate of its release as well as improve oral retention and enhance the antibacterial effect. The aim of this study was to evaluate the dynamics of in vitro fluoride ion release from first- and second-generation fluoride varnishes and the impact of the type of varnish, the time from its application and the pH of the environment on this process. The extent to which factors influence the durability of the applied varnish, the amount of released fluoride ions over time and the duration of maintaining an elevated level of fluoride may alter the accepted treatment standards and contribute to enhancing the effectiveness of therapy.