Silver versus Copper

Because silver is frequently mentioned as an antimicrobial material, a study was conducted to compare  the antimicrobial efficacy silver and copper.  The anti-bacterial efficacy of copper is typically measured after 2 to 6 hours at room temperature at room temperature (approximately 20 degrees C)  and typical indoor humidity levels (RH) found in laboratories and hospitals.  In contrast, the efficacy of silver is tested under a Japanese Standard JIS Z 2801 that monitors survival after 24 hours of exposure at temperatures significantly higher than normal room temperature (35 degrees C) and very high humidity levels (>90% RH). To maintain this high RH level, a plastic film was used to seal the inoculum on the samples. 


As illustrated in the table below, when tested under high humidity condition (>90% RH) and the high temperature (~35 degrees C) required by JIS Z 2801, a commercial silver ion-containing material, showed a >6.4 log drop, as did 4 of the 5 copper alloys: C110(100% Cu), C510(95% Cu), C706(90% Cu) and C752(65% Cu). The fifth copper alloy, C260(70% Cu) exhibited a >6.3 log drop, which is very close to the other 4 copper alloys.  When tested at the high humidity of >90% RH and room temperature of about 20 degrees C, the silver ion-containing material had a 5 log drop in viable bacteria, while the five copper alloys showed a >6.1 log drop.


When the test was run at low humidity, ~20% RH and a high temperature of about 35 degrees C, the silver ion-containing material showed no significant killing while all five copper alloys had a >5.5 log reduction in viable bacteria. 


Finally, when the test was run at an ordinary indoor level of humidity, about 24%RH, and a typical indoor temperature of about 20 degrees C, the silver ion-containing material showed <0.2 log drop, while the five copper alloys recorded a >5.9 log drop.  This indicates that the silver ion-containing material is only effective at high humidity condition (>90% RH) and the high temperature (about 35 degrees C) after 24 hours.  Thus, silver ion-containing material is not appropriate for use in public spaces.

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Michels et al. (2009) ran laboratory killing tests of 360 minute duration on C110(100% Cu) and the silver ion-containing  materials, with stainless steel as an experimental control.  The differences in efficacy are even more apparent, as seen in the figure below. This test, which was carried out at about 50% RH and 22 degrees C, shows that C110(100% Cu) completely kills the inoculated bacteria, exhibiting a 7 log drop in 75 minutes.  The silver ion-containing materials shows no meaningful effect and is indistinguishable from the stainless steel experimental control.  In summary, the silver ion-containing materials do not work under the indoor humidity levels in hospitals and our homes.

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H. T. Michels, J. D. Noyce and C. W. Keevil, Effects of temperature and humidity on the efficacy of methicillin-resistant Staphylococcus aureus challenged antimicrobial materials containing silver and copper, Letters in Applied Microbiology 49, pp. 191-195, 2009 

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