## 1 Introduction  
Fluoride easily penetrates cell membranes due to their properties and thus affects cell functioning. Among the toxic results of fluoride action at the cellular level, oxidative stress induction, organelle damage, apoptosis, essential protein inhibition, and changes in enzyme activity can be mentioned [1; 2; 3]. As has been shown in previous reports, fluoride also affects melatonin production, which can have various consequences for human health. Melatonin is a pineal hormone that has a strong antioxidant and immunomodulatory effect. It acts in cells through different signaling pathways by binding to the appropriate receptors, which are located in the central nervous system and also in the immune system, skin, and organs, such as the kidneys, liver and gastrointestinal tract [4]. It has been implied that the cutaneous melatoninergic system protect skin cells against various stress conditions, mainly by scavenging free radicals and stimulating the antioxidant defense system [5]. The protective role of melatonin also results from its neuroprotective and anti-apoptotic effect, modulation of autophagy, and permeability transition pore [6]. The pleiotropic effect of melatonin comes from the possibility of its synthesis by other cells [7] and its receptor-independent action as well [8].  
Due to the inhibition of melatonin production by fluoride, these ions weaken its protective role in reducing cellular damage [9]. However, it is not tested how fluoride influence the gene expression involved in melatonin-dependent signaling pathways. On the other hand, it should be emphasized that fluoride is an element that influences the proper development of bones and teeth. The nutritional requirements for its consumption have been established at the level of adequate intake (AI) and the AI for women is 3 mg/day and for men, it is 4 mg/day of fluoride for the Polish population [10]. However, the effects of fluoride in the body are also influenced by systemic, metabolic and genetic factors [11]. Therefore, factors that can reduce or weaken its toxic effect on cells are being sought [12].  
Recently, more and more attention has been paid to the possibility of using a static magnetic field (SMF) in medicine. The static magnetic field that is created by permanent magnets has found applications in the treatment of arthritis, insomnia, musculoskeletal injuries, pain, or wound healing [13,14]. On the other hand, many of the in vitro and in vivo studies that have been conducted to date have not produced uniform results on the biological and health effects of the impact of a static magnetic field [15]. However, it has been shown that the mechanism of its action at the cellular level is associated with a reduction in the oxidative stress and the inhibition of apoptosis [16,17]. Furthermore, it has also been suggested that a physical factor such as a static magnetic field may influence melatonin biosynthesis [18]. Since the beneficial action of a static magnetic field on cells has already been proven, there has been increased interest in it as a physical factor that might counteract the negative effect of fluoride on cells.  
Therefore, in order to understand the molecular mechanism of a static magnetic field and fluoride interactions, the main objective of the study was to analyze the expression of the genes that are associated with the activity of melatonin in human fibroblasts that had been co-exposed to fluoride and a moderate-strength static magnetic field.