When it comes to preventing the spread of germs, maybe US President Barack Obama is on to something with his fondness for fist bumps.
The knocking of knuckles spreads only one-twentieth the amount of bacteria that a handshake does, researchers report. That's better than a high five, which still passes along less than half the amount as a handshake.
So fist bumps - popularised by Obama and others - seem to be the wisest greeting, especially during cold and flu season, said researcher David Whitworth of Aberystwyth University in Wales.
The importance of hand hygiene is nothing new in medicine. But the researchers realised that while a lot of research focused on hands getting grimy from touching doorknobs and other surfaces, only a few studies had looked at handshakes.
"And there are alternatives to handshakes. You see them on telly all the time - the fist bump and high five and all that," Whitworth said.
He and a student, Sara Mela, shook hands, fist-bumped and high-fived each other dozens of times for the research. One wore a glove covered in bacteria, while the other had a clean sterilised glove. After each greeting, they measured how much bacteria had been transferred.
Their results were published online on Monday in the American Journal of Infection Control.
What makes the fist bump more sanitary? Mostly, it's the smaller amount of surface area in contact between the two hands, an analysis suggests.
"It's a novel study," though the results were not surprising, said Mary Lou Manning, president-elect of the Association for Professionals in Infection Control and Epidemiology. Her organisation publishes the journal. She said she hadn't seen much fist-bumping or high-fiving in hospitals. Handshakes were more common - but they had to be followed by good hand-washing, she added.