The interactions between plasma‐generated excited particles and water play an integral role in sustainable degradation of pharmaceutical compounds, improving aerobic respiration of activated sludge, and efficient removal of microorganisms from water, and are fundamental to the intentional transfer of reactivity from plasmas to biological solutions for such medical applications as cancer treatment and wound healing. The physical and chemical mechanisms that govern this transfer of reactivity are complex, and include concomitant generation and consumption of species in the gas and liquid phases, and at the interface. As such, it is challenging to predict the quantities of biologically‐active radicals and molecules in liquid phase from gas phase measurements alone. Rapid and accurate quantification of reactive species, such as OH radicals and H2O2 molecules within the liquid phase and their link to specific biological effects is therefore critical for medical applications of plasma‐activated solutions. Using a simple, low‐cost method for trapping and stabilization of OH radicals by means of salicylic acid, this work seeks to provide further insights into the physics and chemistry of generation of OH radicals within the liquid phase, and integrate these findings with decontamination outcomes for four commonly used processing gases.