Is It Soil Vapor? Soil Gas?

Apr 5, 2021

Environmental Enlightenment #321

Here is an illustration taken from a video by California EPA. The name is “Supplemental Guidance: Screening and Evaluating Vapor Intrusion” and the illustration shows contaminated “gas” intruding into a building.

Credit Cal EPA

Another illustration there asks, “What Is Vapor Intrusion?” and there again, “gas” is the intruder.

And so it goes throughout the environmental protection world, the terms soil vapor and soil gas are used interchangeably.


We open our book of physics and find that vapor is low energy gas that comes from liquids or solids; and we find that under normal conditions of pressure and temperature, vaporized substances move back and forth between being solid, liquid or gas; that with a bit of pressure or cooling, vaporized substance will return to its original state. And once you take gaseous substance above certain critical temperature, no pressure in the world will take it down to being liquid or solid.


Volatile Compounds are those that at regular temperatures in atmospheric pressure will evaporate readily. Spill gasoline on the pavement and in a few minutes it’s gone. Same with kerosine, dry-cleaning solvents, and a slew of other commercial and industrial substances.

Thus, technically, we can say soil vapor or soil gas at random.

Transactions in commercial/industrial real estate are usually concerned about harmful impacts to the land, impacts that originate with spills of volatile, hazardous liquids. In these scenarios, the term soil vapor communicates better, and you will find that this term is gaining the upper hand in our reports.




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