Among the properties of chemicals such as molecular weight, solubility, reactivity, pH and such, vapor pressure (VP) is the most important property for worker health considerations. In basic terms, VP is the tendency of molecules at the surface of the chemical to break free from the whole and become airborne. VP is associated with the term evaporation. The ability of a chemical to evaporate into the air, makes the understanding of vapor pressure very important.
Every chemical has a VP, commonly expressed in millimeters of mercury (mmHg). Temperature greatly effects each chemical’s VP. Unless otherwise stated, a chemical’s VP is reported at 25 degrees C. Chemicals with very low VPs are solids while chemicals with high VPs are gases. In between are liquids and vapors. The air you are breathing now has a VP at 760 mmHg. Water has a VP at 17.5 mmHg. Place a drop of water and a drop of oil on a table at the same time. Which of these chemicals evaporates, or totally dries, quicker? Water, therefore, has a higher VP than oil. Further, if there were an emulsion of water and oil, not considering toxicity or other factors, which chemical would present the greatest initial inhalation exposure? The one with the higher VP. If there were solids within the oil water emulsion, the solids may never evaporate. This concept may be greatly expanded to help explain worker exposure risk to mixture of chemicals.