Just as drops of water condense on the outside of a glass containing ice water, so the cold lenses cause the air trapped in the space of the glasses to condense into tiny, fog-like droplets on the inner surface of the lenses.
Steam accumulates on the inside of the glasses' lenses due to the temperature differences between the cold water outside and the inside, whose temperature is 37°C. Just as drops of water condense on the outside of a glass containing ice water, so the cold lenses cause the air trapped in the space of the glasses to condense into tiny, fog-like drops on the inner surface of the lenses - and scatter the light rays that reach the lenses of the glasses.
Except when swimming in water at body temperature (or warmer water), the best way to prevent condensation is to not allow steam to build up on the lenses of the glasses. Most anti-fog coatings are based on "water-loving" materials that attach the water droplets well, in a flat layer, to the lens and ensure that the vapor will accumulate in a kind of thin sheet that almost does not cause the light rays to break - explains Chris Reiser, president of Opto Chemicals, a manufacturer of Zero Fog sprays and pastes.
The mucus in the brook works in a similar way - says Dennis Loftin from the University of Michigan. Scientists don't know for sure, but apparently the saliva also reduces the surface tension of the lens, making it even more difficult for water vapor to accumulate on it. Unfortunately, the saliva coating is not long-lasting, unlike the synthetic material - so it must be reapplied frequently.
By Melinda Wenner, published in the April 2008 issue of Popular Science.
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But disgust builds up.
I was told that it has to do with the enzymes in the saliva