Why Does a Mask Fog?
First, what is happening when a mask “fogs”? Simply put the “fogging” effect you see on the inside of the mask is condensation. That condensation is present due to the water vapor in the air (humidity) meeting with the cooler glass (or plastic) lens and forming microscopic water droplets on the lens. This is similar to you pouring a tall glass of iced tea inside your kitchen. With our modern home air conditioning units our cool dry air conditioned house may have 40% to 60% humidity. When you look at your glass of tea, there may be a little condensation or wetness on the outside of the glass. But, as soon as you go outside (especially in a tropical environment), you’ll notice your glass looks like it’s sweating. Big water droplets appear, coalesce (join together) and run down the glass where a big water ring forms on the table. In this case, the temperature of the glass and contents did not change, but the outside air has now changed to about 80% to 90% humidity. So, the more water vapor in the air, the more condensation on the outside of the glass.

How these microscopic water droplets form inside a mask is factored by the surface tension on the lens, the humidity of the air inside the mask, and the temperature of the water. The use of a surfactant like commercial anti-fog or defogger solutions or even spit reduces the surface tension smoothing out the droplets on the lens. When condensation takes place on a surface that has surfactant applied it is less visible since the droplets coalesce creating a uniform moisture layer. That uniform moisture layer is easier to see through than the “fog” effect of microscopic water droplets.
So, what surfactants are available for de-fogging your mask? Well, there are a few considerations. But first we must understand the importance of “treating a mask” that is brand new prior to its first use.

How To Pre-Treat a Mask
First, many times when a diver experiences consistent mask fogging it is because there is something on the lens which enhances the fogging effect.  This substance could be the original residue from the manufacturing process as seen with new masks, or it could be as simple as a little sunscreen was on ones finger when they applied defog to his or her mask.  In these cases we need to “prep” or “pre-treat” the mask prior to its use.  This is a special process, so to learn how to do it see my article and two-part video How to Pre-Treat a New Dive Mask.
Now, treating the mask won’t prevent your mask from fogging on each dive, so we have to defog our mask prior to each use. Here are a few methods.

Methods of Defogging a Mask
1. Commercial Defog Products – AKA “mask defog” is commercially available in just about every dive center. A couple products I have used and like are the McNett/AquaSeal Sea Gold® Defog and 500PSI® Mask Defogger. Priced each around $6-$10 dollars per bottle their value comes from how good and efficient they work. Both come in small two ounce sizes, and since small drops are used at time a two ounce bottle can last the typical diver quite a while with its advertised hundreds of applications per bottle. Plus, one application may last two to three dives.
• Simply put one drop on the inside lens of a DRY mask and rub evenly with clean fingers (no sunscreen residue – sunscreen will remove the surfactant).
• Rinse briefly in either fresh or salt water.
• NOTE: DO NOT rub the lens or touch the lens during or after rinsing.
2. Spit – There’s nothing more fun than welling up a big one and planting it in your mask to be rubbed around! For some, this is downright disgusting. Whatever your take on the method, it actually works and remains one of the more common methods for defogging a mask. Saliva makes for an okay surfactant. It’s easy and cheap albeit gross. However, it does not last as long commercial defog products and usually needs to be applied just prior to every dive. The method of application is similar to the commercial defog:
• Simply work up some saliva in your mouth and spit on the inside lens of a DRY mask and rub evenly with clean fingers (no sunscreen residue– sunscreen will remove the surfactant).
• Rinse briefly in either fresh or salt water.
• NOTE: DO NOT rub the lens or touch the lens during or after rinsing.
A couple points worth noting about spit. 1. Some divers and dive professionals strongly discourage the spit method for defogging the mask. They claim that the high bacterial content in saliva is bad for eyes. But consider this, reports of eye infections associated with diving are rare. And, we have to remember that a slightly coated lens that has been rinsed simply does not have a high likelihood of getting into eyes – even when doing partial and full-flooded mask clears. Compare that to the plenty of waterborne bacteria in the ocean, lakes, freshwater springs and even swimming pools. Doing full-flooded mask clears or mask removal and replacement will expose the eyes to far greater amounts of bacteria, than that of the fine saliva coating on a mask lens. 2. Under NO circumstances should you spit in a mask and then rinse it in the rinse bucket of a dive boat! This is an almost certain way to spread viruses! The rinse bucket on a charter dive boat should be reserved exclusively for those using commercial defog or the dive boats’ preferred defog solutions. All it takes is one sick person spitting in a mask and briefly dunking it in the rinse bucket to contaminate the bucket.
3. Baby Shampoo – Here is a defog solution that is catching on in popularity. Baby shampoo is inexpensive, comes in travel size containers, is biodegradable, and won’t sting the eyes. It can be applied directly in small amounts or mixed with water and delivered through a spray bottle. Many charter boat operators are going with the spray bottle solution next to the rinse bucket. The steps, you guessed it, are pretty much the same as above:
• Simply dab or squirt the baby shampoo on the inside lens of a DRY mask and rub evenly with clean fingers (no sunscreen residue– sunscreen will remove the surfactant).
• Rinse briefly in either fresh or salt water.
• NOTE: DO NOT rub the lens or touch the lens during or after rinsing.

Be cautious of swishing the solution during the rinse and dumping it out on the deck of the boat. The soapy nature of the shampoo can create a very slippery deck.

Another NOTE: You may have noticed I keep saying use on a DRY lens. Go back to the first paragraph about surface tension and surfactants. If water is present on a lens before a surfactant is used, it will not work as well. Sometimes you notice just one or two spots on a lens is continuously fogging. Many times that is a result of defog or spit being applied to an already wet mask where water spots were already present. As an instructor trainer I often play a student during an instructor or divemaster class. I will intentionally wet my mask before putting whatever method of defog the instructor candidate tells me to use. Then, underwater my mask fogs over and over again totally confusing and frustrating the candidate. Ah… that’s good fun! Okay, in fairness I am also employing another instructor trainer problem solving challenge – exhaling through the nose increases the likelihood of mask fogging – which leads us to our final point.

Avoid Exhaling Through Your Nose – The air between your face and mask lens remains warm and moist. And with an occasional puff out your nose to equalize your mask as you descend, the airborne water vapor collects on the lens and disperses if there is a proper surfactant in place. But, if one were to continuously exhale through their nose, now there is a continuous supply of warmer and highly humid air. The stagnant air temperature warmed by the face is also cooled by the water temperature affecting the glass lens and the skirt. However, air that is exhaled from the lungs is substantially warmer and has a relative humidity of 100%. This means an ever increasing amount of condensation will form, eventually washing away inadequate surfactant like spit or inadequately applied surfactant. There are many reasons why divers exhale through their nose: a poor fit or leak lets water enter the mask and the diver regularly has to clear the mask by exhaling. Most divers do not efficiently clear their mask with just enough air to get the water out. Instead, it is a dramatic production complete with large exhalations of air into the mask. Another issue is for divers that have a sinus condition. Mucous production discomfort forces them to exhale into the mask. For others, a mask that is too tight is temporally relieved on face when exhaled air through the nose pushes the mask off the face ever so slightly. Another issue is overexertion, the harder a diver breathes the more likely for the nasal tracts to engage exhaling air into the mask. And, of course, we are all nose breathers primarily. For many, shutting off the nose breathing is difficult and they incorrectly inhale through their mouth and exhale through their nose. So focus just on inhalations and exhalations through the mouth only.

Whew… I know this was more information than you were expecting. But knowing why your mask is fogging will really help you understand what changes you need to make and what tools you can now use to prevent your mask from fogging.