Basics of Vapor Intrusion

Feb 15, 2019

Many assume that indoor air quality problems originate from within a property, such as mold growth from a burst pipe or asbestos-containing materials getting disturbed. However, some problems can originate from outside a property. One such problem that arises from the exterior is vapor intrusion. In this blog post, I will cover what is vapor intrusion and some of the common sources.

Basics of Vapor Intrusion

Vapor intrusion refers to the migration of volatile chemicals which travel from the groundwater or soil into the building above it. Vapor intrusion can be caused by a variety of sources. A common source is volatile organic compounds (VOCs) in the soil. VOCs can be introduced into the soil and groundwater via a chemical spill or leaking underground storage tanks to name a few.  

Chemical Spills

Volatile organic compounds are a common culprit in vapor intrusion problems due to chemical-based spills. VOCs which leak into the soil form a plume of soil gas. The volatiles can migrate through porous layers of soil and eventually can contaminate the groundwater. As VOCs are volatile in nature they can migrate from the soil or groundwater upward and infiltrate through a property through cracks in the foundation or via utility lines.

One common example of VOC related vapor intrusion is perchloroethylene from dry cleaners. Perchloroethylene, also known as tetrachloroethene, tetrachloroethylene, or simply “perc” is a VOC and halogenated solvent. Halogens are a group of elements (the 2nd column from the right side of the periodic table) which are renowned for their reactivity and are often used in cleaners and solvents. Due to its volatile and non-flammable nature, it was used in dry cleaning. Using perchloroethylene in dry cleaners has caused soil and groundwater contamination in some cases. This is due to various factors such as equipment leaks or spilling the containers into the ground. The International Agency for Research on Cancer (IARC) has designated perc as a “probable human carcinogen.”

Another example of chemical spill related vapor intrusion related to trichloroethylene also known as trike. This compound is a halogenated solvent similarly to percholorethylene. For decades IBM used tricholoethylene as an industrial degrasser to clean metal parts in Endicott, New York. The compound eventually sinked beneath factory and contaminated the soil and groundwater. After the vapor intrusion was uncovered in several of the homes in town, IBM installed ventaliation systems into 400 of the homes.

Underground Storage Tanks

According to the EPA, an underground storage tank (UST) is any tank with associated piping that has at least 10% of its volume underground. Underground storage tanks are commonly used at gas stations to hold petroleum and also may be used as septic tanks and for industrial purposes. USTs are an environmental concern because in the past they were constructed of bare steel which can corrode over time. If a gas station based tank corrodes and leaks, it can release petroleum-derived VOCs such as benzene, toluene, ethylbenzene and xylenes (commonly referred to collectively as BTEX) into the groundwater and the soil. Benzene is a carcinogen, while toluene and xylenes do have negative health effects but they are currently not classified as carcinogens.

Another underground storage tank leak issue is associated with septic tanks. In septic tanks, bacteria cause the production of hydrogen sulfide. Hydrogen sulfide is a common component of sewer gas and is toxic and flammable. Hydrogen sulfide is recognizable due to its pungent rotten egg-like odor.


While I have focused on volatile organic compounds, there are a wide variety of other culprits in the soil and groundwater to be concerned with.  Other issues include radon, semi-volatile organic compounds (SVOCs), elemental mercury, hexavalent chromium, and a host of other compounds. Vapor intrusion can be mitigated through either drawing the gas out from under the foundation such as a radon mitigation system, creating a negative pressure under the home, and/or remediating the soil and groundwater from chemical-based contamination. While we do not offer soil and groundwater testing or mitigation systems, at Indoor Science we can test for vapor intrusion. Vapor intrusion shows us that radon is not the only concern in the soil.