Why Do Divers Spit in Their Masks?

Last Updated: February 21, 2023

Scuba divers spit into their masks to prevent fog from building up in the mask.

When warm air inside a dive mask meets the cold glass of the lenses, tiny water droplets condense on the glass and make the mask foggy. Saliva changes the surface tension of the water droplets, which causes them to join into larger drops that roll off the glass, preventing condensation.

You may be surprised to see divers all suited up and ready to go, and then spitting in their masks. But did you know that saliva is actually a free, portable, and environmentally-friendly surfactant? What’s a surfactant? Read on.

Why Do Scuba Masks Fog Up?

how spit works in defogging a scuba mask

Just like glasses, windshields, and cold drinks, scuba masks fog up with condensation. This happens when the temperature inside is higher than the temperature of the glass lenses. When this happens, droplets of moisture in the air condense against the cold glass.

Condensation occurs because glass isn’t as smooth as it looks. If you looked at it under a microscope, you would see all sorts of pits and grooves. These little pockets hold onto the tiny water droplets, creating fog.

A foggy mask impairs visibility and interferes with the enjoyment of a scuba trip. In addition, it creates a safety hazard, as divers cannot see obstructions, boats, or dive buddies. Divers who are distracted by impaired vision may not be as focused on safety or surroundings.

How Does Spit Work in Defogging a Scuba Mask?

Spit keeps a mask from fogging up because it contains proteins that act as a surfactant. A surfactant is a chemical agent that reduces the surface tension of a liquid, which makes the liquid spread more easily.

If the condensation in your dive mask spreads more easily, that means it will join up with more water droplets and make a film of water rather than tiny droplets.

When this happens, droplets won’t fit into those tiny grooves in the glass and will just roll right off it in large drops. In other words, water droplets won’t seem to “stick” to the glass as much, and your mask will not get foggy.

Other Methods of Treating Your Mask Before a Dive

Obviously you may have reasons for not wanting to spit in your diving mask. Maybe you are borrowing your buddy’s mask and spitting in it might seem weird. Or maybe you are renting one and someone else will be using it later that afternoon.

From a public health perspective, spitting can spread illness and should be avoided when diving or snorkeling with a group. Even more so in present times, when we have to deal with pandemic and various Covid rules, spitting in a mask just isn’t an option.

So what else can we do if spitting isn’t the right thing?

Other types of surfactants will work like spit. Baby shampoo is probably the best alternative, since it is non-irritating and more environmentally friendly than regular shampoo. Spread the shampoo around the inside of the mask and give it a quick rinse.

You can also purchase defogging sprays like this, that are travel-ready and reef safe. Spray the entire glass of the mask and then rinse if recommended.

How Do I Treat a Brand New Dive Mask?

New masks are often treated with silicon or other compounds in the manufacturing process. It is generally a good idea to treat a mask before a first dive.

Some divers will treat a new mask with non-abrasive, non-gel toothpaste. Cover the glass with toothpaste and then rinse with water. You may need to treat it a few times with toothpaste before it works optimally.

If you are using toothpaste, you may want to take your mask on a test run to make sure it isn’t irritating to your skin.

Other divers use heat to treat a new mask. Make sure you are being safe and protecting the rest of the mask from damage when treating it with heat or fire.

More Tips For Defogging Your Scuba Mask

If you are using any type of surfactant, you will need to start with a dry mask. If you don’t, water droplets may stick in those tiny pits and grooves and may not react to the surfactant. This may result in tiny pockets of fog.

Make sure your face is clean before you put on your mask. If you are very sweaty, more humidity will form in your mask and may create more fog.

Sunscreen may interfere with the function of the surfactant, so make sure you don’t get sunscreen on the lenses. If you do, make sure you wash it thoroughly.

Applying your surfactant should be one of the last things you do before you dive. You don’t want to take your mask off or prop it on your head after you have applied your chosen defogger.

Closing Thoughts

While spit is an effective way to keep your dive mask (or swim goggles) from fogging up, there are other options to consider as well.

The decision whether to spit or not is up to you, you do not have to rely on this old-school method. But if you ever forget your defogging spray, you have an alternative that is available at all times.

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