Vapor pressure is the pressure a liquid's vapor exerts when it is in balance with the liquid at a given temperature, showing how readily a chemical evaporates into the air.
Vapor pressure is the pressure exerted by a vapor when it is in equilibrium with its liquid (or solid) form in a closed space at a given temperature. Put simply, it measures how easily a substance evaporates. A liquid with a high vapor pressure, such as acetone, gives off vapor quickly. A liquid with a low vapor pressure, such as motor oil, evaporates very slowly.
Vapor pressure always depends on temperature. As a liquid warms, more molecules escape into the air and the vapor pressure rises. When the vapor pressure equals the pressure of the surrounding air, the liquid boils. At sea level, that pressure is about 760 millimeters of mercury (mmHg), or 101.3 kPa or 14.7 psi.
For safety professionals, vapor pressure is one of the most useful properties on a safety data sheet (SDS). It helps predict how much vapor a chemical will release, which affects both inhalation exposure and fire risk.
Under OSHA's Hazard Communication Standard, 29 CFR 1910.1200, Appendix D requires Section 9 of every SDS to cover physical and chemical properties. The list includes vapor pressure (including evaporation rate), along with boiling point, flash point, upper and lower flammability limits and relative vapor density. Always note the temperature the vapor pressure was measured at, because values at different temperatures are not comparable.
The NIOSH Pocket Guide to Chemical Hazards lists vapor pressure in mmHg at 68°F unless another temperature is noted. Some examples:
For comparison, water has a vapor pressure of about 17.5 mmHg at the same temperature.
Vapor pressure is commonly reported in mmHg (also called torr), kilopascals (kPa), pounds per square inch (psi) or millibars. One standard atmosphere equals 760 mmHg, 101.3 kPa or 14.7 psi. OSHA's flammable liquids standard, 29 CFR 1910.106, defines vapor pressure for that standard as the pressure in pounds per square inch absolute measured by the Reid method (ASTM D323).
Vapor pressure sets the highest concentration a chemical's vapor can reach in still air at a given temperature. This is called the saturation concentration. It can be estimated by dividing the vapor pressure by atmospheric pressure. For toluene, 21 mmHg divided by 760 mmHg is about 2.8 percent, or roughly 27,600 ppm. That is far above toluene's OSHA permissible exposure limit of 200 ppm and its IDLH value of 500 ppm. Normal ventilation keeps most workplace air well below saturation, but inside tanks, drums, pits and other enclosed spaces, concentrations can approach it.
Liquids with high vapor pressure release more vapor at lower temperatures, so they tend to have lower flash points. The examples above follow this pattern: acetone has a flash point of 0°F, benzene 12°F and toluene 40°F. Even small spills of a high vapor pressure liquid can quickly create a vapor cloud above the lower explosive limit (LEL). OSHA classifies flammable liquids in 29 CFR 1910.106 by flash point and boiling point rather than vapor pressure, but all three properties are linked.
Because vapor pressure rises with temperature, closed drums and containers of volatile liquids can build internal pressure when they sit in the sun or near heat sources. This can cause bulging, leaks or a spray of liquid when a bung is opened.
Vapor pressure is also used to define volatile organic compounds (VOCs) in some regulations. The EU's Industrial Emissions Directive treats an organic compound as a VOC if its vapor pressure is 0.01 kPa or more at 20°C.
A maintenance team planned to clean parts with a solvent in a small, poorly ventilated pump room during summer. The supervisor checked Section 9 of two candidate products' SDSs. The first had a vapor pressure of over 100 mmHg and a flash point below room temperature. The second, a higher-boiling solvent, had a vapor pressure of less than 1 mmHg and a flash point above 140°F.
The team chose the lower vapor pressure product, set up a portable fan to exhaust air from the room and kept the parts washer lid closed when not in use. Air readings during the job stayed well below the solvent's exposure limit, and no flammable vapor was detected.
Vapor pressure describes how readily a liquid evaporates. Relative vapor density compares the weight of the vapor to air, showing whether it rises or sinks.
No. Toxicity depends on the chemical itself. A high vapor pressure means more of it can get into the air, which increases the potential for exposure.
Vapor pressure rises as temperature rises. A value measured at 68°F will understate the vapor released on a hot day or from a heated process.
Check Section 9 of the SDS. The NIOSH Pocket Guide to Chemical Hazards also lists vapor pressure for many common workplace chemicals.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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