Beyond therapy: elemental risks and benefits of spa waters in Ciechocinek, Poland

The spa waters of Ciechocinek (Poland) are internationally recognized for their healing properties and their distinctive Na–Cl–I composition of Mesozoic origin. This study provides comprehensive modern assessments of these waters, integrating analytical chemistry, toxicology and spa medicine. It delivers a detailed physicochemical and microbiological characterization of the four springs, clearly distinguishing the drinking mineral water source S19A (bottled as Krystynka) from the highly mineralized therapeutic brines S11, S14 and S16. In the case of S19A, a direct comparison with the parametric limits and labelling requirements of Commission Directive 2003/40/EC shows that all constituents regulated by the Directive (e.g. F⁻, Mn, NO₃⁻) remain below the maximum limits. However, the elevated sodium and selected trace metal content limits its suitability for long-term consumption in large quantities, emphasizing the importance of appropriate labelling, medical advice for sodium-sensitive individuals, and systematic monitoring. In contrast, for therapeutic brines an intake-based assessment is not appropriate; instead, a dermal exposure model adapted to spa practice is used. The hazard ratios for six trace elements (Cr, Mn, Fe, Co, Cu, Li) are all below unity, with values of approximately 0.05–0.06, confirming that the systemic toxicological risk from dermal absorption remains negligible under realistic balneotherapy conditions. By adapting the assessment criteria to the intended use, this study offers a reliable and clinically relevant assessment of the Ciechocinek waters: S19A is considered a microbiologically and chemically safe natural mineral water within the current European framework, provided its high salinity is taken into account in clinical recommendations and consumer information, while S11, S14 and S16 are safe for therapeutic use in spa treatment.