question: Why do scuba masks fog up? Follow-up, how does anti-fog gel work to keep the mask from fogging up?
comment: Antifog get is a surfactant. It decreases the water tension just like soap does. When water droplets form they immediately flatten out and disperse instead of beading up and making the mask opaque.
passages: Masks tend to fog when warm humid exhaled air condenses on the cold inside of the faceplate. To prevent fogging many divers spit into the dry mask before use, spread the saliva around the inside of the glass and rinse it out with a little water. The saliva residue allows condensation to wet the glass and form a continuous film, rather than tiny droplets. There are several commercial products that can be used as an alternative to saliva, some of which are more effective and last longer, but there is a risk of getting the anti-fog agent in the eyes.
   BULLET::::- Titanium dioxide, becomes highly hydrophilic under UV light.
  
  Section::::Application.:Home recipes.
  
  One method to prevent fogging is to apply a thin film of detergent, but this method is criticized because detergents are designed to be water-soluble and they cause smearing. Divers often use saliva, which is a commonly known and effective anti-fogging agent.
  
  Section::::Uses.
  
  Section::::Uses.:Underwater diving.
   Double-layered or "thermal" lenses are also available. These lenses are much less prone to fogging. These work by separating an inside and an outside lens with an air chamber, that allows for the difference in temperature between the inside and the outside of the mask without forming condensation. However, if any moisture whatsoever somehow gets in between the two lenses, the inner faces of both lenses will fog, and it will take a very long time to dry out, if it does at all. 
   A demister is a substance applied to transparent surfaces to stop them from becoming fogged with mist deposit, often referred to as fog. Scuba divers and Underwater Hockey players often spit into their masks and then wash the surface quickly with water to prevent mist buildup that can impair vision. Several products are commercially available such as Sea Drops that are generally more effective. New masks lenses still have silicone on them from the manufacturing process, so it is recommended to clean the lenses with toothpaste, rinse the mask and then apply a demister solution.
  
  Section::::See also.
   Section::::Operation.:Defogging.
  
  Most full-face masks do not have fresh airflow directly over the interior of the faceplate, and are to some extent susceptible to fogging by condensation during a dive. This may be minimized by application of a demister surfactant before the dive, by rinsing during the dive, or in those cases where the system allows, by purging the mask through a free-flow/defog valve, which directs a flow of dry air over the inner surface of the faceplate. This facility uses a lot of air, and is usually only available on models intended primarily for surface supplied applications.
   Section::::Development.
  
  Anti-fog agents were initially developed by NASA during the Project Gemini, for use on helmet visors. During Gemini 9A, in June 1966, Astronaut Eugene A. Cernan tested NASA's first space suit, and discovered during the space walk that his helmet visor fogged, among other issues. Cernan's suit was tested using the Spacecraft 9 life support system after the flight, when it was discovered that a small patch of the visor treated with an anti-fog solution remained clear of condensation. Later Gemini flights all included the anti-fog solution, for application prior to the space walk occurring.
  
  Section::::Application.
   To prevent a mask from fogging up due to condensation on the glass many divers spit into the dry mask before use, spread the saliva around the inside of the plate and rinse it out with a little water. The saliva residue allows condensation to wet the glass and form a continuous film, rather than form droplets. There are commercial products that can be used as an alternative to the saliva method, some of which are more effective and last longer.
   In 2014, scientists demonstrated good cloaking performance in murky water, demonstrating that an object shrouded in fog can disappear completely when appropriately coated with metamaterial. This is due to the random scattering of light, such as that which occurs in clouds, fog, milk, frosted glass, etc., combined with the properties of the metamaterial coating. When light is diffused, a thin coat of metamaterial around an object can make it essentially invisible under a range of lighting conditions.
  
  Section::::Scientific experimentation.:Active camouflage.
   A convenient method is to use a cheap small spray bottle to spray water onto the lens rather than pouring it on. Another good lens-cleaning agent is a 50-50 mixture of rubbing alcohol and water. After it is mixed it should be put into a spray bottle for use. Use only a clean cloth on the mask; paper towels will scratch the lenses. Anti-fog spray is also available, which coats the lens in a temporary fog-resistant film. Some new lenses will come "pre-treated" by anti-fog, or the lens will say "fog-resistant" — with these lenses, it is advised that anti-fog chemicals are never used as the chemicals can damage the lens beyond further use.
   BULLET::::- Demisting the viewports: Some helmets directed inlet air flow over the inside face of the viewports, which was reasonably effective, but if this was not sufficient, the diver could open the spitcock and suck seawater into his mouth, then spit it onto the inside of a fogged viewport. This would wash off the condensation droplets, and the saliva may have helped defogging, as it is known to be effective as a surfactant for this purpose.
   ULV fogging machines are the most preferred misting machines as the size of the droplets can be controlled by means of machine calibration.
  
  Section::::Ultra low volume fogging machines.:Advantages and disadvantages.
  
  The chemicals used in this type of machine are more concentrated than the chemicals used in other spraying equipment, which also increases the killing efficiency. Other advantages of ULV misting machines includes lower risks of injury due to the fog cloud being nearly invisible, low volumes of carrier chemicals, lower application cost and low noise levels. 
   High range superplasticizers (dispersants) have generally been manufactured from sulfonated naphthalene condensate, although polycarboxylic ethers represent more modern alternatives. Both of these high range water reducers are used at 1/2 to 1/3 of the lignosulfonate types.
   Faze machines, or fazers, can also be used to produce a haze-like effect. These machines are typically fog machines with a fan built in front to disperse the output, creating a haze-like effect. These machines are typically considered more cost effective, with prices ranging from $100–$400 USD as compared to haze machines with prices ranging from $400–$1,400 USD.
  
  Section::::Fire detection.
   Waterlogging commonly occurs over a long period of time in polystyrene foams that are constantly exposed to high humidity or are continuously immersed in water, such as in hot tub covers, in floating docks, as supplemental flotation under boat seats, and for below-grade exterior building insulation constantly exposed to groundwater. Typically an exterior vapor barrier such as impermeable plastic sheeting or a sprayed-on coating is necessary to prevent saturation.
  
  Section::::Forms produced.:Copolymers.
   Fog produced with liquid air does not present the hazards of carbon dioxide or liquid nitrogen because the liquid air has the same composition as ordinary atmospheric air (including the same level of breathable oxygen).
  
  Fog machines which generate their effects solely using water, such as ultrasonic or pressurized technology, pose no special health risks, however "water based" fog fluid does contain glycol.
  
  A number of studies have been published on the potential health effects presented by exposure to glycol based theatrical fogs and artificial mists.
   Section::::Modern methods.:International use.
   Liquid nitrogen (N) is used to create low lying fog effects in a manner similar to dry ice. A machine heats water to at or near the boiling point, creating steam and increasing the humidity in a closed container. When liquid nitrogen is pumped into the container, the moisture rapidly condenses, creating a thick white fog. A fan placed at the output of the container directs the fog where it is needed, creating a rolling fog that lies low to the ground.
   Dry Ice (solid carbon dioxide) effects are produced by heating water to or near boiling in a suitable container (for example: a 55-gallon drum with water heater coils in it), and then dropping in one or more pieces of dry ice. Because at standard temperature and pressure carbon dioxide is a gas, the carbon dioxide sublimates and instantly produces a gas, condensing water vapor and creating a thick white fog. A fan placed at the top of the container directs the fog where it is needed.
   Military divers need equipment which hides their position and prevents explosives from being set-off, and to this end, they use rebreathers which produce less noise due to bubbles emitted from the equipment, and few or no bubbles on the surface, and which contain no magnetic components. This continues down to the design of their diving suit, which will normally have a non-magnetic zipper, and the face-mask may be fitted with special anti-reflective glass. Some navies have gone further and given their divers special contact lenses instead of large face-masks to cut down on the risk of a reflection.
   When used with a full-face mask or helmet, oral inflation becomes impracticable or impossible.
  
  Section::::Hazards.
  
  Although a correctly fitted and competently operated buoyancy compensator is one of the most important items of equipment for diver safety, convenience and comfort, particularly for scuba divers, it is also a significant hazard if used wrongly or in case of some kinds of malfunction:
   Water which enters the air spaces of the mask may be expelled either under the lower edge of the skirt, or through the exhaust port of the demand valve, or through a drain valve fitted for this purpose.
  
  Section::::Operation.:Equalising.
   The mask faceplate is a high impact strength polymer which is securely attached to the skirt or frame. A failure of the faceplate can be a life-threatening incident with some full face masks, as the water will flood into the oro-nasal inner mask and compromise the air supply. This problem is avoided by the masks which include a mouthpiece.
  
  Section::::History.
  
  A full face diving mask was invented by Yves le Prieur in 1933.
   Section::::Haze machines.
  
  Haze machines, also called haze generators or hazers, produce atmospheric effects (haze) which tend to be less dense than that of fog machines, allowing them to be used in creating more subtle looks. Depending on a venue's or location's ventilation, this haze can stay in the air for hours.
  
  Because of the small particle size of the output from hazers, under 5 micrometres, a hazer typically takes longer to fill the same volume of space with a visible effect relative to a fog machine.
  
  Section::::See also.
  
  BULLET::::- Dry ice
  
  BULLET::::- Haze machine
  
  BULLET::::- Theatrical smoke and fog
   Section::::Variations.
   Section::::Ultra low volume fogging machines.:Parts.
  