Mask fogging results from the same phenomenon that causes your bathroom mirror or the inside of your car's windshield to cloud over: warm, humid air meeting a cooler surface. Warmer air is capable of holding more water vapor (water in gas form) than cooler air. Therefore, when air is cooled, a portion of its water vapor condenses into tiny liquid droplets.
Underwater, the air inside a mask's face pocket is warmed by your face. While its glass faceplate is cooled by the water. The layer of warm air that comes in contact with the faceplate becomes cooler, and the resulting condensation on the inside of the glass forms that annoying fog. Mask fogging is so common that some scuba enthusiasts consider it an unavoidable consequence of diving. But you needn't suffer obscured vision during a dive if you handle your mask properly before you enter the water.
The first step in preventing your mask from fogging underwater takes place after you buy a new mask. During the manufacture of most masks, an invisible chemical residue is left on the glass. This makes the surface more prone to fogging. The residue must be removed by scrubbing the glass with a non-abrasive cleaning agent like toothpaste. Scrub both sides of the lens, as well as the skirt. A toothbrush is helpful. If new masks (except no-fog models) are not scrubbed before their first use, they will fog regardless of other preventive measures.
Chemicals may continue to leach from the mask for some time, so further scrubbing may be necessary. It's also a good idea to re- scrub your mask periodically to remove residues that build up from de-fogging solutions or the water in which you dive.
A cautionary note: Before scrubbing any diving mask, read the printed material that accompanies it. Some new masks should not be scrubbed, as they are treated with a fog retardant. Scrubbing these masks may damage this substance and cause the mask to fog.
Even after a mask has been scrubbed, an anti-fogging solution must be applied to the inside of the glass faceplate prior to diving. These de-foggers break the surface tension of the water on the inside of the glass. Instead of separating into thousands of tiny droplets that cloud your vision, condensation forms a transparent sheet of water which is usually un- noticeable. The original, always-available anti-fogging solution is saliva, but today many divers consider spitting on the inside of their mask unsanitary, or at least uncivilized. Alternatives range from dishwashing liquid and baby shampoo to commercial rinses, gels and drops specially developed to keep your mask from fogging. These formulations work quite well if you follow the directions carefully. Some are meant to be used shortly before entering the water, and others can be applied earlier and allowed to dry. Rubbing and rinsing may or may not be required, and re-application may be necessary before each dive or only once per diving day. If you use the wrong application technique, not only is the anti-fogging preparation likely to be less effective, but eye irritation may result.
You may still suffer from a clouded mask despite scrubbing the mask and applying an anti-fogging preparation. Remember that fogging is caused by a difference in temperature between the inside and outside of your mask pocket. Anything you do to exacerbate that temperature differential increases the likelihood of fogging.
One common practice that often causes mask fog is placing the mask on your forehead while gearing up. Your forehead is almost certain to be warmer dm the surrounding air, plus the heated mask pocket may cause drops of sweat and body oil to form. The same thermal situation applies when you rest the mask on your hair or on top of a hood, or leave it sitting in the sun while you gear up.
Protect your mask from extremes of temperature until it is placed on your face. It is best not to seat the mask on your face until you are ready to enter the water. Even a small amount of warm, moist air exhaled through your nose into the mask can cause instant fogging.
Once you're underwater, simply clearing your mask can lead to fogging. If you clear often, the de-fog will eventually be washed off. Another difficulty is that in order to clear your mask, you must exhale through your nose. Introducing this warm air from your lungs into the mask pocket encourages fogging. The most effective solution to this problem is to wear a mask that makes a good seal on your face. Acclimating yourself to small amounts of water against your face, so that excessive clearing is unnecessary, is also a good idea.
It follows that if you habitually exhale through your nose instead of your mouth underwater, you are likely to have problems with mask fogging. This is easily corrected by training yourself to breathe entirely through the regulator.
Avoid leaving your mask in the sun. The increased heat contributes to fogging.
So what do you do if, after your best efforts, your mask still fogs JUST while you're underwater? Must you complete your bottom time looking through a haze? No, it is possible to de-fog your mask during a dive. Simply let a little water inside by pulling the top of the skirt away from your face. This method allows you to break the seal just enough to control the amount of water you let in. Re-seal the mask on your face, then look down so the lens surface is horizontal and swish the water around to rinse away the fog. Clear the mask in the usual manner. This technique works temporarily, but will probably have to be repeated as the fog returns. Scrub, de-fog and keep your mask cool until you enter the water, then don't exhale through your nose or constantly clear your mask, and you won't have to view the underwater sights through a pea-soup fog.