Q. How good are copper doorknobs at protecting patients from germs in hospitals?

A. Copper surfaces are valuable in killing disease-causing microbes left on them, but are not infallible in preventing the spread of disease, according to a 2011 review article in the journal Applied and Environmental Microbiology. A significant percentage of many pathogens die on copper surfaces, but those are only an adjunct to normal hospital sanitation procedures of all kinds.

The killing mechanism is not fully understood. A newer study from England in the journal Applied Surface Science suggests that sweaty hands on doorknobs can interfere with the germ-killing effect. The salt in sweat causes corrosion that blocks the deposited microbes from the copper, so frequent cleaning is called for.

The phenomenon of so-called contact killing was known in ancient times, when it was observed, for example, that copper vessels could purify drinking water. Copper compounds were often used as antimicrobial agents in medicines until the discovery of antibiotics.