Cannonball!!! It's the telltale sound of summer — and the potential warning sign that someone's about to get a painful jet of water right up their nose. There's nothing quite like the summer discomfort of a nose full of pool water. But wait: people use neti pots to clear their sinuses with water when they're sick, and that actually feels pretty good. Why does pool water up your nose feel so awful by comparison? The answer comes down to chemistry.
Your body is full of water — 78 to 47 percent, depending on your age, build, and gender. But that water isn't pure. As anyone who has drank an electrolyte-infused sports drink after a sweaty workout knows, there's plenty of salt in your body's moisture reserves, too. The water that fills your cells contains just under one percent sodium chloride (0.9 percent, if you want to get technical). That's not as salty as seawater, which has an average salinity of 3.5 percent, but it's saltier than pool water. And that's important.
If you think back to chemistry class, you may remember the principle of osmosis: the way that molecules will move through a semipermeable membrane from a less concentrated solution to a more concentrated solution in order to balance out the concentration on each side. Well, the cells of your nose are protected by a semipermeable membrane. If the water that enters your nose has a lower salt concentration than the one percent in your body's cells, guess what happens? Water rushes through the cell walls to try to balance out that concentration. The result is that uncomfortable, often painful sensation you only seem to get with a nostril full of pool or lake water.
This shock to your cells is also why pool water tends to make your nose run. (All this talk about pee in swimming pools is a convenient diversion from what is surely way too much snot.) "The cells inside of your nose are designed to secrete mucus," Stacey Gray, associate professor of otolaryngology at Harvard, tells STAT. "That's actually their normal function, and they do that also in response to an insult." By insult, she means a shock like pool water.
Nothin' But Neti
At this point, you may be figuring out why pool water up the nose is so different than water from a neti pot. The water in a neti pot — or a nasal spray, for that matter — is formulated with a certain salt concentration. Sometimes it's roughly equal to the concentration in your body's cells, other times it's saltier. While you might think that too-salty water would be just as much of a shock to your system as fresh water, that's not always the case.
Remember how osmosis makes water travel from a less concentrated solution to a more concentrated solution? Well, filling your nose with extra-salty water can remove water from your nasal cells. That helps because, contrary to popular belief, a stuffy nose is usually due to swelling, not mucus. The fact that warm, salty water helps wash away excess mucus is another great benefit, of course.
In the end, water up your nose isn't always bad: just when the water isn't salty enough. If it's a constant problem for you at the pool, you may want to practice breathing out through your nose as you dive in, or invest in a nose clip. That water wants to invade your cells — don't let it in!
This article first appeared on Curiosity.com.