In air, the mechanisms of amorphous calcium carbonate (ACC) transformation into crystalline polymorphs of CaCO3 and whether the atomic ordering is attributable to a solid-state transformation or a dissolution and reprecipitation process are still under debate. While some studies observed a significant influence of relative humidity on ACC transformation, other studies suggested a dehydration process of ACC prior to crystallization. In the present study, we focus on the metastability of additive-free ACC in air and in particular on its interaction with relative humidity. Our findings indicate that the transformation of ACC into crystalline CaCO3 is triggered only after the physisorption of a critical H2O level. Consequently, ACC metastability was prolonged by retarding H2O uptake and by keeping the physisorbed H2O below the critical level, ACC remained in its metastable state. Therefore, the conceptual formation of a "thin film" of about four monolayers of physisorbed H2O is considered to govern the transformation of ∼90 nm sized ACC particles via partial dissolution and reprecipitation. Furthermore, we observed simultaneous formation of calcite, vaterite, and aragonite from ACC, where distinct proportions correspond to different H2O exposure conditions. Thus, polymorph formation from ACC depends also on physicochemical boundary conditions during transformation rather than on prestructural formation within ACC alone.