What Exactly Is Sea Foam?

By: Kristen Hall-Geisler  | 
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, 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 those are the sea foam bubbles to watch out for. Let's take a look at how sea foam forms and whether it has any impacts on human health.


What's Inside Thick Sea Foams

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 inorganic and organic material. When these particles get agitated by wind and waves, they froth. You can create the same effect by shaking ocean water vigorously in a bottle.

The sea foam bubbles happen because of molecules called surfactants, which Popular Science explains are "sticky molecules that cling to the surface between water and air." This sea surface microlayer 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!

Think Twice Before Popping Sea Foam Bubbles

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 algal toxins can irritate eyes and ears, and even cause respiratory conditions.

The white foam may not be toxic, but that doesn't mean it's harmless. When weather conditions get completely out of control, the churning of the seawater 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.


Beach Goes Beware!

You may not sense any health risk connected to those small bubbles along the coastline, but it's best to keep your distance. While most sea foam contains only tiny particles of dissolved salts and other pollutants, it can be detrimental to human health.

When assessing the safety of an area where sea foam forms, it's a good idea to research the body of water in question. Are high concentrations of algal toxins present? Is there a lot of pesticide use in the area? These questions can help you decide whether the sea foam is part of a productive ocean ecosystem or a symptom of something unnatural.