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.

Low and high VPs

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.

HazCom to RiskCom

Per OSHA, about 32 million workers work with and are potentially exposed to one or more chemical hazards. There are an estimated 650,000 existing chemical products, and hundreds of new ones are introduced annually. Most chemicals in commerce are ingredients in mixtures. OSHA HazCom 29 CFR 1910.1200 requires employers train workers on information on safety data sheets, labels, and placards. VP information is found in SDS Section 9: Physical and chemical properties.

OSHA does not specify how much a worker should know about VP. An employer merely saying “vapor pressure” may suffice for worker understanding of the term during required HazCom and SDS training.  Required HazCom, however, is the foundation for voluntary risk communication. A good understanding of VP is necessary for good awareness of RiskCom.

The following is an example of HazCom to RiskCom: An open container of paint contains a mixture of 65% solids and fillers that include silica and lead oxide, all with VPs below 0.1 mmHg, along with 35% solvents with equal proportions that include propanol VP 18.0 mmHg, methyl isobutyl ketone VP 19.95 mmHg, and ethanol VP 43.7 mmHg. As required, the SDS includes health hazard information on the chemical contents of the paint, with silica and lead being the most toxic components. The container of paint evaporates. How much silica and lead did workers working near the open container of paint inhale?

RiskCom means that workers understand that silica and lead are not harmful under conditions of use at the workplace. Paint must be dried then sanded, sawed, or other physical means to liberate silica and lead into the air. All factors being equal, which solvent evaporates faster, propanol or ethanol? You should know enough now to answer that question. An explanation of a chemical’s VP, such as provided above, may help workers and others differentiate between hazard and risk.  This concept should apply to all chemicals used or found in the workplace.

Expert witness

I recently served as an expert witness for the defense in a toxic tort claim. Details of the claim have been changed to preserve confidentiality.  The basics of the case are similar to the open container of paint scenario described above. My knowledge and experience was sought by an attorney to help determine if a plaintiff’s health problems were caused, as proposed by the plaintiff’s experts, by proximity to a chemical product.

The worker stated that the odor from the product caused him to become nauseous, lightheaded, with labored breathing. The worker was diagnosed by his physician with a host of ailments that may be attributed to the toxic effects from chemicals in the product. Other specialists, such as a toxicologist, support the diagnosis. The worker alleges that he is permanently disabled and no longer can hold a job. The worker hires a lawyer who files a toxic tort claim against the manufacturer of the chemical product, supplier of the product, and others including his employer, that may be liable for the worker’s poor health and disabling injuries. Did the worker’s proximity to the chemical product cause his aliments? Expert witnesses, both for the plaintiff and defense, earn high dollars when they can answer that question with a reasonable degree of scientific certainty.

Association is not causation

Chemicals with a bad reputation, such as silica and lead, are often attacked by lay people and even experts as causing health problems simply because of their presence. An SDS often reads like a smoking gun when an illness happens. But dose i.e., how much of the chemical enters the body, must always enter the argument of causation. Association is never causation without confirmation of dose.

In my opinion, the major problem with the lawsuit I was involved with is that the plaintiff’s expert witnesses, all with impressive experience and credentials, focused on the chemical in the product with the worst reputation. The plaintiff’s toxicologist, in particular, referenced dozens of toxicological studies to demonstrate the health dangers of the chemical. But like my silica and lead example, the chemical being attacked by the plaintiff’s experts had a very low VP. Under conditions of use the chemical could not readily evaporate, which is necessary for it to become airborne, inhalable, and toxic.

In follow-up arguments, I acquired samples of the chemical product that I used under near worst-case conditions while conducting air sampling followed by laboratory analysis. Air samples for the chemical were below analytical detection. I looked forward to my deposition by the plaintiff’s lawyers, so I could more fully explain the concept of VP and airborne potential. Perhaps based on my expert report, the plaintiff’s lawyers canceled the scheduled deposition at the last moment and released the defense’s client from the lawsuit.

Learn more about VP?

There is a lot to know about how chemical behavior is impacted by VP. I routinely use its concepts in RiskCom. If you would like to learn more about VP, search 40 CFR 796.1950. That reference aptly describes vapor pressure concepts as used in Chemical Fate Testing Guidelines under Protecting Environment. The EPA’s Vapor Intrusion Screening Level Calculator (VISL) at is also a good learning source. Among other things, the free online VISL can help calculate risks from indoor air concentrations, mostly based on understanding of chemical VPs.