Valence: The number of electrons which an atom can give or take up when reacting with other atoms to form a chemical compound.

van der Waals Forces: The forces of attraction between molecules.

Vapors: Gases formed by the evaporation of materials which are normally liquids or the sublimation of materials which are normally solids.

Velocity: The distance travelled in a given time. Air velocity is measured in feet per minute (fpm) or meters per second (m/s).

Velocity Pressure: The kinetic pressure in the direction of flow necessary to cause a fluid at rest to flow at a given velocity. See also "Pilot tube."

Ventilation: The introduction of outdoor air into a building by mechanical means.

Virus: A microscopic particle composed of DNA or RNA surrounded by a protein coat. Viruses replicate by entering a cell, releasing their own DNA or RNA and controlling the DNA of the cell to manufacture more of the virus DNA or RNA and the surrounding protein coating. In the process the cell is usually destroyed, its walls disrupted, and the virus released into the surrounding enviroment.

VOC: Volatile Organic Compound. An organic compound which evaporates into the surrounding enviroment.

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