Plasma‐activated media (PAM) can be as effective as direct plasma treatment in killing cancer cells. PAM is produced by exposing liquid cell culture media to low temperature plasma. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the aqueous state play key role in the anti‐tumor effects of PAM. The effectiveness of PAM is highly dependent upon the dose of reactive species. The concentrations of reactive species and consequently the effectiveness of PAM decreases over time after plasma exposure. In this paper, the effect of PAM on the viability of SCaBER cells, originally obtained from a bladder squamous cell carcinoma, is shown and its efficiency at different aging times is evaluated. To investigate the selective effect of plasma on normal epithelial cells, MDCK (Madin‐Darby Canine Kidney) cells from normal epithelial tissue of a dog kidney were treated by PAM. The concentrations of hydrogen peroxide at different times after plasma exposure were measured. Our findings indicate that there is a correlation between PAM efficiency and H2O2 concentration, as both decrease over time.