What's in your swimming pool?

'Welcome to our ool. Notice there is no p in it. Let’s keep it that way.'

When we were kids, that was the ubiquitous message nailed to nearby fences or on the toilet doors of those lucky enough to have a backyard pool (sorry—ool). A plea to keep the germs out, to keep their pristine, chlorinated water sparkling clean. But when it comes to the germs in our pools, it turns out that a yellow current isn’t the worst of our worries. Not by a long shot.

Like us, the germs which enjoy hanging out in swimming pools come in all shapes and sizes. They include various viruses (such as hepatitis A and norovirus), bacteria (Shigella species; E. coli) and parasites (Cryptosporidium and Giardia). Each can cause a range of symptoms, usually of the kind most of us would tend to call ‘gastro’: things like diarrhoea, nausea, vomiting and stomach cramps.

So how do they get into the water? Obviously not by leaping from the diving board or clambering down the poolside ladder. Rather, they get there by hitching a ride—on us. The way many of these germs are spread from person to person in swimming pool water is via faecal transmission—in other words (prepare for the eww factor), by people swallowing water that’s had poo in it.

These bugs live and multiply happily in an infected person’s gastrointestinal tract and are excreted in their poo. Not surprisingly when you think about it, there are actually small amounts of poo on most people’s bottoms—about 0.14 grams of faeces on average, according to the United States’ Centers for Disease Control and Prevention. Poo gets washed from their skin into the water, and the contaminated water is then accidentally swallowed by other swimmers, transferring the germs to their bodies.

More drastically, a baby or toddler might poo in the water (what the World Health Organization rather euphemistically calls an accidental faecal release, or AFR), small amounts of which may then be swallowed by other swimmers.

If you’re pretty sure you don’t swallow the water when you swim, a 2006 study found that, during a 45-minute swim, adults swallowed, on average, 37 millilitres—that’s almost two tablespoons. Children swallowed twice that amount. Once swallowed, germs live in their new host’s gastrointestinal tract until they’re pooed out, beginning the cycle again.