Vapor The gaseous form of a solid or liquid substance as it evaporates. Vapor Density The weight of a vapor or gas compared to the weight of an equal volume of air is an expression of the density of the vapor or gas. Materials lighter than air have vapor densities less than 1.0 (examples: acetylene, methane, hydrogen). Materials heavier than air (examples: propane, hydrogen sulfide, ethane, butane, chlorine, sulfur dioxide) have vapor densities greater than 1.0. All vapors and gases will mix with air, but the lighter materials will tend to rise and dissipate (unless confined). Heavier vapors and gases are likely to concentrate in low places--along or under floors, in sumps, sewers, manholes, trenches and ditches--where they may create fire or health hazards. Vapor Pressure The pressure exerted by a saturated vapor above its own liquid in a closed container. When quality control tests are performed on products, the test temperature is usually 100°F and the vapor pressure is expressed as pounds per square inch (psig or psia), but vapor pressures reported on SDSs are in millimeters of mercury (mmHg) at 68°F (20°C), unless stated otherwise. Three facts are important to remember: ●● Vapor pressure of a substance at 100°F will always be higher than the vapor pressure of the substance at 68°F (20°C). ●● Vapor pressures reported on SDSs in mmHg are usually very low pressures; 760 mmHg is equivalent to 14.7 pounds per square inch. ●● The lower the boiling point of a substance, the higher its vapor pressure. Vasoconstriction Decrease in the cross-sectional area of blood vessels. This may result from contraction of a muscle layer within the walls of the vessels or may be the result of mechanical pressure. Reduction in blood flow results. Ventilation See “General Exhaust”, “Local Exhaust”, and “Mechanical Exhaust.” Vermiculite An expanded mica (hydrated magnesium-aluminum-iron silicate) used as sorbent for spill control and cleanup. Vertigo Dizziness; more specifically, the sensation that the environment is revolving around you. Vesicant Any substance capable of producing blisters on the skin. Vesicle A small blister on the skin. Viscera Internal organs of the abdomen. Viscosity The tendency of a fluid to resist internal flow without regard to its density. Visible Radiation (Light) Electromagnetic radiation that can be detected by the human eye. This term is commonly used to describe wavelengths of 400 nm to 700 nm. Volatility A measure of how quickly a substance forms a vapor at ordinary temperatures.