Gory Details

The bizarre side of science

Erika Engelhaupt
Gory Details

Why you should be only a little afraid to pee

When pouring clean water into a cup filled with tea leaves, some leaves glide upstream into the kettle thanks to differences in the water’s surface tension.

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You probably won’t die of a horrible infection brought on by pathogens traveling up your urine stream. Most likely. But one of our dear readers has spoiled lunch for me, and now maybe for you, by raising that and other nasty possibilities based on a recent Science News story.

The story was about a surprising discovery that particles can travel upstream, against the flow of moving water. A scientist in Cuba noticed the phenomenon when he poured hot water into a cup of tea leaves, then looked in the kettle and found tea leaves that had apparently traveled up the stream of water as he poured. After heaps of experiments, physicists published a paper showing that particles of chalk and mate tea can travel upstream. The key is surface tension. Small particles disturb the network of hydrogen bonds that creates surface tension, and as the bonds pull apart particles are pushed upstream toward pure water with higher surface tension.

This prompted a reader to send in the following comment:

The article reminded me of when I once made the mistake of expectorating into a toilet which contained bowl-cleaning chemicals. The result was an immediate, acrid taste in my mouth. Apparently the chemicals had traveled several feet up the stream of saliva.

This led me to ponder whether viruses and bacteria might take advantage of the same phenomenon to ride a stream of urine back to the source to infect an unsuspecting host.

Skirting past the obvious question of how this reader produced a several-feet-long stream of saliva, it was an interesting idea. But could particles really travel so far upstream? And does it matter what the liquid is? What about particle size?

I posed this question to the physicists who reported the tea leaf study, Ernesto Altshuler of the University of Havana and Troy Shinbrot of Rutgers, and they had some thoughts.

Altshuler begins by noting that expectoration (the thesaurus word for spitting) may involve different phenomena than urination. “Saliva can be quite elastic, like a rubber ribbon,” he says, “which may help the ‘several feet’ contamination observed by the reader.” 

Shinbrot raised a caveat, though. “We calculated that the upstream acceleration is tens of times faster than gravity, but the only contamination that we actually observed was up a 1- or maybe 2-centimeter waterfall.” They just haven’t done the experiments to know how far a particle could travel under various circumstances.

So if you’re a bit paranoid, perhaps standing would provide a better distance buffer than squatting (bad news, ladies!). Altshuler says, “In my talks I use a joke which is not perhaps a complete joke: I always use Norwegian urinals (conceived for very tall people) with a bit of precaution,” he says, referring not only to Very Important Urinal Physics governing vertical distance and splash radius, but also to his newly discovered physics. “In that case, they may be some ‘authentic’ upstream contamination!”

Before you get too concerned, though, consider that there are plenty of more probable ways for pathogens to enter your body when you’re urinating (or spitting) in contaminated areas. Incidental contact, aerosolized spray or splashback could easily transfer pathogens without involving your urine stream at all. Don’t you feel better?

Shinbrot adds that he actually looked into the urine question a bit before writing the research paper because of an urban myth about candiru fish. If you haven’t heard of them, Google will provide you with many horrific tales of this Amazonian fish that supposedly swims up men’s urine streams to lodge itself in their penises. These tales thankfully tend to fall apart on further scrutiny, and in any case the fish is much too large for surface tension physics to apply to upstream travel. 

“I am highly dubious about the idea of candiru fish swimming upstream,” Shinbrot says, “but I can't say whether it could occur with bacteria or shmutz: The smaller the particle, the more likely it seems to me that it could travel upstream. Certainly gives me the heebie-jeebies.”

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