The issue of tarnishing is often brought up as it is familiar to all who have observed copper surfaces. Escherichia coli O157:H7 was exposed to several copper alloys in a tarnished and untarnished state to determine whether tarnishing has an impact on the antimicrobial properties of copper. The results clearly show tarnishing is not an issue. In fact, copper surfaces with an oxide layer (tarnish) actually kill organisms faster than untarnished surfaces. The data below clearly dispels the notion that tarnished copper surfaces will no longer be antimicrobial.

chart showing E. coli viability on 5 different copper alloysNote: C197, C220 and C770 are copper alloys. The "B" after the alloy indicates untarnished, and the "T" after the alloy indicates a tarnished surface.