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More than you ever wanted to know about mask fogging

This is photoshop, lest anyone think Jo would dive with a mask this way ;-)

Having a fogging dive mask is one of the most annoying things a diver can ever experience. It’s a subtle thing that can really take away from your dive and create anxiety, particularly in new divers. 

When working as a dive professional, you see many different ways that people try to eliminate mask fogging. Some divers rinse and soak their masks endlessly. Some put up with stinging eyes because they’ve put way too much defog in their mask. Some divers just put up with the fog.

But it doesn’t have to be this way. Over the years I’ve seen a number of articles discuss mask fogging. Even a quick internet search reveals hundreds of articles on the topic. But many of these are just lists of commercial defog products in order of ‘tester reviews.” There’s little meaningful information about what causes the problem. In this article, we’re going to take a really good look at what causes a fogging mask, and what we can do about it. 

Why does a mask fog? 

To begin, we need to understand a few basic concepts in physics. Very simply, fog occurs when the temperature of the glass on your mask falls below the dew point of the air inside the mask. When this happens, the air can no longer hold the water vapor, which will begin to condense on the surface of the glass. The measurement of the dew point is related to humidity. A higher dew point means there is more moisture in the air. 

In the simplest possible terms, this means when the humid air that is trapped in our mask skirt cools as we go underwater, it will begin to condense on the surface of the glass. Provided the water we are diving in is cooler, this condensation will always occur and there is nothing we can do about it. 

So what is the difference between fog and no fog? The answer is the surface of the glass itself, and how water sticks to it. 

To help understand why droplets form in various sizes, we need to explain a few things -  the first is a concept called surface tension. Wikipedia defines surface tension as “the tendency of liquid surfaces to shrink into the minimum surface area possible.” Well that explanation is not terribly useful in this context, so let’s break this down a bit more. 

Surface tension basically means that the water is more attracted to itself than it is to its surrounding material it is touching. This surrounding material can be the air around the water, or dive mask glass. 

An example of surface tension - a water droplet on a leaf. The water sticks to itself more than it sticks to the waxy surface of the leaf, making a tiny sphere droplet. (Wikipedia)

A good example of surface tensions in water is to examine a water droplet on a leaf. Cohesive forces between the water molecules make them stick to each other more than they stick to the waxy surface of the leaf. This is why the water forms into a large droplet. When the water vapor condenses into many very tiny droplets, they appear as fog. 

Well what makes the droplets form in larger or small sizes? To answer that, the second thing we need to understand is the surface of the glass in the mask. Any reputable manufacturer of dive masks will use tempered glass. On a microscopic level, tempered glass tends to be very smooth. On this surface, water will form into large droplets and not fog. Fog occurs when the surface of the glass is textured. Because of the texture, the water tends to form into these tiny droplets of fog we mentioned earlier. 

So if tempered glass in a dive mask is normally smooth, what causes it to have a texture? The answer to this question lies in how the mask is manufactured. 

A sample of an injection mold. These 2 steel halves are a negative of the product to be made. The halves are clamped together and material is injected under high pressure, which fills the voids. After a period of curing, the halves are separated to reveal the final product.

The skirt of your dive mask is made from a synthetic rubber or silicone. The rubber is formed into the shape of the mask skirt in a process known as injection molding. That is, the rubber is forced into a metal mold of the skirt using heat and high pressure. After the mask skirt is formed, there is a process of curing, which is essentially the process of allowing the rubber to harden. Curing occurs over time. There will be a period of time where the rubber is hard enough that the mold shaping it can be removed, however the curing process does not stop at this point. The rubber will continue to cure over a period of hours to days, depending on a number of factors. 

During curing, a process of outgassing will occur. Outgassing (sometimes called off-gassing) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material - in this case, the rubber the mask skirt was made from. 

Basically outgassing creates a vapor, which will collect onto the nearby glass surface of the mask, creating a thin film deposit. It is this film on the glass that creates a microscopic textured surface which ultimately leads to our mask fogging. 

What do we do about mask fogging? 

So there are a few things we can do to prevent mask fogging, many of which are commonly known but sometimes misunderstood. The first is removing the outgassing film from the surface of the glass. 

It’s likely when you bought a dive mask, the shop salesperson, or online shop tried to up-sell you an abrasive agent to scrub it with. They may or may not have explained the importance of scrubbing it, perhaps with toothpaste, or maybe you've read this on the internet. 

Where the explanation and instructions tend to get vague is how you should do this. There’s a tremendous amount of variables in this process, and here are the 3 biggest: 

1. The amount of abrasive material you are scrubbing with

If you use toothpaste to scrub the mask, the amount of abrasive material in the toothpaste is important. Abrasive elements in a toothpaste mechanically scrape away plaque and tartar from your teeth. Abrasives however, also also remove some of the outer layer of your teeth. Your tooth’s enamel is therefore at risk of being permanently damaged, so the American Dental Association rates the abrasive properties of toothpaste in what’s known as the Relative Dentin Abrasivity (RDA) scale. Between brands of toothpaste, there can be a huge difference. The value depends on the size, quantity and surface structure of abrasive used in toothpastes, so when it comes to scrubbing your mask, our recommendation would be to use a product with a very high RDA value. Also you could use one of the commercially available products made for this purpose. The point to remember here, is just because you are scrubbing with toothpaste doesn't mean it's effective. If the toothpaste contains little to no abrasive compounds, all you are doing is making your mask smell minty. 

Products sold by dive companies for the purpose of scrubbing your mask don’t have RDA values, however since they were made expressly for this purpose, suffice to say that they have a reasonable amount of abrasive material for the purpose of mask scrubbing. 

2. The amount of force applied and time spent on scrubbing

Scrub it like you stole it.

Remember you are trying to clean off a film of silicone byproducts that has adhered itself to the glass. A cursory scrubbing won’t do. You need to really apply some pressure, and work at it with some elbow grease. I once worked with an engineer, who removed the lenses from his dive mask, and cleaned them with jewelers rouge and a polishing wheel. Although his mask never ever fogged, (and I mean never,) this may be a bit overkill for most people. But do put a bit of effort into the cleaning process. Lightly using your finger for a minute won’t cut it. 

3. The manufacturing process of the mask itself

A quality mask manufacturing company will have some sort of post-curing procedure to minimize the effect of outgassing materials. Post curing methods can vary from simple ventilation, to a heated process, to the addition of other materials or chemicals into the rubber. 

The better the post-cure process, the less material there will be to create a film on the surface of the glass. Which means the less scrubbing you will have to do (or the less material there will be for water vapor to adhere to.)

This is a tough one to determine, because dive mask manufacturers generally don’t include the post-cure methods in their marketing materials. I’d wager that most of the sales staff doesn’t even know what this means. That said the post-cure process will add expense to the mask making process, so you can bet that low cost, economy diving and snorkeling masks are minimizing or skipping this process. 

Fortunately, the amount of material only defines the amount of time you'll need to scrub and the amount of force you need to apply. A proper scrubbing will remove a thick or thin layer.

There’s another way to remove off gassing film, and that is to burn it off. If you buy a new mask at Divetech, there’s a 99% chance that the person who sold it to you will pull out a cigarette lighter and apply the flame to the inside surface of the mask for a few minutes. If you watch carefully, you’ll see a small black edge “peel” away from the center of the flame. 

Burning, or scrubbing the film off only needs to be done to the inside surface of the glass. The outside surface will of course have water on it when you are diving, and cannot fog. Please be sure to only do this to dive masks with glass lenses! There are a few dive masks out there that have acrylic or plastic lenses, and scrubbing or burning a plastic lens will not have good results. 


No matter how carefully you scrub or burn your mask lenses, it’s likely an impure surface remains to some degree, and this is why we use defog. There are 2 types of defog: 

Surfactants that minimize the surface tension of the water, such as soap or baby shampoo and hydrophobic coatings, that repel water, such as waxes, polymers, and gelatins. 

Surfactants will reduce the surface tension in the water droplets. This means they are less likely to form little spherical droplets of fog, and more likely to smooth out across the surface of the textured glass. 

Hydrophobic coatings are like our leaf example earlier. The Hydrophobic coating will serve to smooth the textured surface of the glass, as well as reduce the cohesive force between the water and the glass. The water will form into larger droplets which will roll away. Anyone who has ever used the commercial windshield product known as Rain-X has seen an example of hydrophobic coatings in action. 

Some myths

Mask over rinsing - Rinsing your mask is a perfectly normal thing to want to do, but a quick rinse is all you need. Many times I see customers over-rinse their mask - essentially all you’ve done is rinse your defog right out of the mask. Remember when you are using a soap or any other surfactants-type defog on your mask, some of it needs to remain in the mask to be effective. Too much rinsing will rinse the product out of the mask, rendering it useless. 

Soaking their mask is another one that has perplexed me. I often ask customers why they are soaking their mask in the bucket. The most common answer I’ve been given is, it’s what their dive instructor told them to do. One time I did get an answer that he was getting his mask the same temperature as the water, which doesn’t really make any sense either. 

Our recommendations

Divetech recommends the following when you buy a new mask, provided it has glass lenses: 

  • Use a lighter to burn off the film from the inside of the lens

  • Use an abrasive product to scrub the inside of the lens. Be sure to do this with adequate abrasive material, with a proper amount of force, and for enough time. Some of the commercial products I looked at did not have an amount of time you should scrub listed in their directions. So ,my advice is, when in doubt - scrub longer. 

  • Spray a moderate amount of your defog agent into a dry mask just before diving

  • Rinse the mask quickly and lightly in water

  • Go diving

Stick with us and you'll never have to suffer through fog again! Thanks for reading and we hope to see you diving with us soon!


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