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What Exactly Is Sea Foam?

Sea foam is created by the agitation of seawater containing high concentrations of dissolved organic matter, which can come from the natural environment in the form of algal blooms or from man-made sources. Wikimedia Commons (CC BY-SA 3.0)

As the waves crash onto the shore or lap at the beach, depending on where you are, there's usually some leftover frothy bits that cling to the sand. On windy days, this foamy stuff can even be lifted off the ground and blown around. What is it? And is it dangerous to touch it or let the dog run around in it?

It's sea foam, and it is not nearly as green as the color we call sea foam. It's usually whitish, though it's probably a little dingy rather than being sparkling white. Sometimes it's more of a reddish-brown, though, and that's the sea foam to watch out for.

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Sea water is 96.5 percent water and 2.5 percent salt. That only adds up to 99 percent, so what's the other 1 percent? A lot of things. According to the National Oceanic and Atmospheric Administration (NOAA), it's "proteins, fats, dead algae, detergents and other pollutants," plus other bits of organic and inorganic matter. When these particles get agitated by wind and waves, they froth. You can create the same effect by putting some sea water in a bottle and shaking it.

The bubbles happen because of molecules called surfactants, which Popular Science explains are "sticky molecules that cling to the surface between water and air." These surfactants can come from natural sources, like algae and seaweed, or from human pollution, like fertilizers, detergents and sewage. Ew.

One end of the molecule is hydrophobic – it repels water. The other end is hydrophilic – it attracts water. The easiest shape for these molecules to form is a sphere, with the hydrophobic ends on the inside and the hydrophilic ends pointing outward. Spheres (even if they aren't perfect) make bubbles. And a lot of bubbles make sea foam!

Now, about that reddish-brown sea foam. That's often due to phytoplankton, also known as algae blooms, which is a natural phenomenon. These tiny organisms release toxins that aren't good for you, your dog, the birds, or anything, really. The toxins can irritate eyes and ears, and even the respiratory system.

The white foam may not be toxic, but that doesn't mean it's harmless. When weather gets completely out of control, the churning of the water creates a lot of sea foam. Like, a lot. In the winter of 2020, a king tide in Washington state with 25-foot (nearly 8-meter) waves created a "blender effect" that churned up sea foam as high as a man's chest. Later that spring and halfway around the world, so much sea foam appeared in the Netherlands that it killed five very experienced surfers.

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