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Sweat disables copper’s antibacterial properties, with implications for doorknobs and handles everywhere.

Transcript

Bacteria, sweat, and copper. I’m Bob Hirshon and this is Science Update.

Copper-containing metals like brass have antibacterial properties—a selling point for brass doorknobs, sink handles, and other fixtures in hospitals and schools. But now, British scientists have found that handling those fixtures can actually disable their germ-killing powers.

According to University of Leicester forensic scientist John Bond, human sweat forms a corrosive layer on the metal surface. That blocks an exchange of charged particles that would otherwise kill harmful bacteria. And the fixtures that are handled the most would be the most affected.

Bond:

The more often it’s touched, you’re replenishing and indeed concentrating the active ingredient in sweat that corrodes the metal, which is just salt.

So even if an institution has germ-resistant copper fixtures, they still need to clean them and encouraging frequent hand-washing. I’m Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

You may have heard that doorknobs and sink handles are big germ-carriers. With so many people handling them all day long, often one right after another, it isn't hard to see how germs from one person's hand could end up on another's by way of a doorknob.

You probably didn't know that copper had antibacterial properties. But if it were your job to design the interiors of large buildings, or buy fixtures for schools, hospitals, or other big institutions, you'd probably have heard this. Manufacturers often promote copper's proven bacteria-killing properties to sell fixtures made of brass, bronze, or other metals with a high copper content.

This experiment shows the importance of considering all factors before reaching a conclusion. Copper helps kill germs; therefore, it makes sense that copper-based doorknobs would pass along fewer germs than knobs made of glass, plastic, steel, or other materials. However, there's a crucial factor that may be left out of this scenario: the sweat from the hands that touch the doorknobs.

It was known that sweat can corrode copper alloys (metal mixtures) like brass in the long term. But in this study, the researchers studied the effects of sweat on copper surfaces within a few hours of contact. They found that within as little as an hour, the salt in sweat can form a corrosive layer on the surface of the metal, which would prevent the electrochemical reaction that kills microorganisms.

Now, a full hour after someone touches a doorknob may sound like a long time for this effect to take hold. But consider how many people handle doorknobs or similar fixtures in a day: more than enough to neutralize the metal's germ-killing powers as long as the building stays busy. What's more, Bond points out that frequently touched items collect salts from the sweat of many people, which makes the corrosive layer tougher and longer lasting.

It would be difficult to study this in a real-life setting, for a number of reasons. In order to control the study properly, researchers would have to study bacterial colonies on brass fixtures handled all day long, and compare them to other brass fixtures that were somehow protected from sweat but exposed to the same people's skin microorganisms. However, the research suggests that institutions using copper-based fixtures shouldn't slack off on cleaning them, nor should they make any less of an effort to make sure people keep washing their hands.

Now try and answer these questions:

Why have copper-based doorknobs, sink handles, and other fixtures been promoted as healthy choices for schools, hospitals, and surgical wards? What did this study find out about the relationship between sweat and the usual properties of copper? What does this say, in your opinion, about the challenges of coming up with practical uses for scientific discoveries?

You may want to check out these related resources:



In the Science Update Triclosan and Staph, learn how some antibacterial chemicals may actually backfire and actively help a kind of bacteria they're designed to kill. The Science Update UV Disinfectant describes another approach to helping keep hospitals germ-free. For more about alloys, see the video Shape Memory Alloys, in which Dr. Ainissa Ramirez, associate professor of mechanical engineering and materials science at Yale, demonstrates "metals with a memory" used in space, in robots, and even in your mouth!

Going Further

For Educators In the Science Update lesson Triclosan and Staph, students can learn how some antibacterial chemicals may actually backfire and actively help a kind of bacteria they're designed to kill. The Science Update lesson UV Disinfectant describes another approach to helping keep hospitals germ-free. For more about alloys, see the video Shape Memory Alloys, in which Dr. Ainissa Ramirez, associate professor of mechanical engineering and materials science at Yale, demonstrates "metals with a memory" used in space, in robots, and even in your mouth! Related Resources Appendix Purpose

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