Handshakes: Sharing the Love, Sharing the Microbes

But could you ever seriously fist bump your physician?

The first official world record for handshaking was set by President Theodore Roosevelt on January 1, 1907, after he shook hands with a finger-crushing 8,510 people in the White House.

One hopes these 8,510 had thoroughly washed their hands, as engaging in intimate palm-on-palm contact is a well-known cause of spreading microbes from person-to-person, a process known scientifically as transmission.

Handshakes as social greetings date back to at least 5th century BCE Greece, which we know from depictions in carved slabs from around that time.

One theory is that the custom possibly originated as a peace gesture, since it demonstrated that the hand held no weapon.

But what of the handshake’s potential for spreading bacteria between individuals?

Would an official at, say, a graduation ceremony be placed at risk when shaking hands with dozens – perhaps hundreds – of people?

As ever, there’s a study on this, coming from Johns Hopkins Bloomberg School of Public Health.

The project’s leader had heard that some officials at Johns Hopkins graduations were “sneaking squirts of hand sanitizer behind the podium,” so his team set out to swab officials’ hands after ceremonies.

Actually they found a very low prevalence of pathogens, estimating that the rate of contamination was around 100 times lower than might be expected in those who were shaking hands in a healthcare setting.

Now this, of course, is excellent news for those officiating at graduation ceremonies.

But it’s perhaps less comforting for anyone in a healthcare environment, and it brings us to two other studies that sought to investigate whether something other than a conventional handshake might be more hygienic.

In 2013, researchers at West Virginia University noted that, partly thanks to President Obama’s use of the technique, fist bumping was becoming a popular form of physical greeting.

Could a fist bump transmit less bacteria from bumper to bumper than is transferred from shaker to shaker in a more conventional greeting?

The researchers engaged two participants, both healthcare workers, to shake hands with 20 colleagues, then got them to press their palms on specially prepared agar culture plates.

After thoroughly washing their hands, they went through a similar process, only this time using fist bumps.

Fist bumping resulted in a reduction in colony-forming units of around a quarter, compared to handshaking, a phenomenon the researchers said was mainly due to smaller areas of skin coming into contact.

The next year, a team of scientists at Aberystwyth University in the UK picked up the baton, and stepped up a gear, to continue where their American predecessors had left off.

Curiously, their paper referred to fist bumps as being a “dap greeting,” a term we’d never come across before.

As it turns out, neither had the experts in the Stanford University library, but we have them to thank for digging up that the phrase “giving dap,” describing fist bumps, high-fives, and other elaborate forms of handshaking, appears to date back to black soldiers fighting in the Vietnam War in the early 1970s.

Some suggest that “dap” may be an acronym for “dignity and pride,” associated with black power, but in all honesty, the jury’s out on its etymology.

Anyway, the British researchers approached the handshake vs. fist bump vs. high-five issue with commendable thoroughness, asking a participant they called a “greeting donor” to put on a sterile glove, then immerse their hand into a bucket containing a dense soup of E. coli.

On removing their hand, the glove was allowed to dry, then it was time for the greeting donor to interact with a sterile-gloved recipient.

After either shaking hands, fist bumping, or high-fiving, the recipient’s glove was checked to find out how much bacteria had been transmitted.

In fact, twice as many bacteria were transferred in a handshake than in a high-five, but a fist bump resulted in the lowest transfer of all.

You know we said they were thorough?

Well, the scientists determined the area of contact by copiously spraying a donor’s glove with acrylic paint before an interaction, in order to see how much was transferred to the recipient.

They also investigated whether the short duration of a fist bump led to less bacterial transfer, by having participants do awkward three second fist bumps.

And, yup, lengthy fist bumps do indeed transmit more bacteria.

They even looked at pressure, to see if firm handshakes are somehow “dirtier.”

Sure enough, a firm handshake does indeed increase the amount of bacteria transferred.

We really can’t leave the subject of handshakes without reporting briefly on an utterly riveting 2015 study from Israel, in which scientists covertly observed 271 participants after they’d met a researcher who greeted them either with or without a handshake.

It may sound hard to believe, but having first observed that we humans sniff our hands a lot, generally without realising it, the research showed that this behavior increases significantly after a handshake.

What’s more, there’s a remarkable gender difference.

After handshakes within gender (male-to-male, or female-to-female) sniffing of the right-hand – the one that did the shaking – increased by 100%.

But handshakes across gender led to a 100% increase in sniffing of the non-shaking hand.

While it’s unclear why this might happen, the researchers hypothesized overall that some kind of subliminal social “chemosignaling” is going on.

Rather like animals sniffing one another, humans may be investigating others – or checking in on themselves – after a handshaking encounter.

It’s always fun to research and write these newsletters, and we’re always happy to hear that people enjoy reading them, too.

High-five!

(Well, perhaps a fist bump.)


Further reading

A handy guide to passing on pathogens

A social chemosignaling function for human handshaking

Giving dap

Graduation contamination through handshaking

Handshake

Reducing pathogen transmission in a hospital setting. Handshake verses fist bump: a pilot study

Routes of transmission

The fist bump: A more hygienic alternative to the handshake

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