Effect of Humidity on Antibacterial Activity of Copper and Its Alloy Surfaces

Copper and its alloys are well known to have bactericidal activity and applied to touch surface to suppress the transmission of pathogens via material surface. The touch surface is exposed to real life conditions, which are generally lower in humidity than that in laboratory testing conditions. The influence of humidity on the antibacterial activity of copper and other materials was reported as contradictory results due to different inoculation methods and contact periods. In this study, the antibacterial activity of copper (C1020) and its alloys (C7060, C7150, MONEL400, and antibacterial stainless steel) was systematically investigated by a method simulating droplet transmission under controlled humidity (60 and 35%RH) with Escherichia coli or Staphylococcus aureus. Corrosion rate of C1020 was estimated by electrochemical impedance spectroscopy under the controlled humidity. As a parameter of antibacterial activity, the contact time to reduce the viable bacteria to be 1% of the inoculated ones, T0.01, was decided based on the test results. The all materials increased their T0.01 with reduction in humidity, indicating the decrease in their antibacterial activity. In case of C1020, the smallest T0.01 for E. coli was observed at 90% RH as 6.96 min, which increased to 67.4 min at 35%RH. At both high and low humidity, the materials with high copper contents (C1020, C7060, and C7150) had the smaller T0.01 than those with lower copper contents. S. aureus, which has higher tolerance to the environment with low water activity than E. coli, resulted in greater increase in T0.01 with reduction in humidity (In case of C1020, T0.01 at 90 and 35%RH for S. aureus were 8.50 and 230 min, respectively). The corrosion rate of C1020 decreased with reduction in humidity, suggesting the importance of copper ion release at the surface for their antibacterial activity.