Vapor Intrusion: Just the Facts

What is Vapor Intrusion?

Vapor intrusion can occur in an area where various chemicals were not properly handled or disposed of and have seeped into the ground. Some culprits can be properties that were once gas stations, dry cleaners, or businesses that used industrial degreasers. Once in the ground, these chemicals can move through the soil or groundwater and create plumes of contamination. If these chemicals are exposed to air, they can become harmful vapors that pose a danger to human health. They are often invisible and have no smell, and thus may be very hard to detect. The real danger occurs when these vapors find their way into buildings through cracks in foundations or basement walls, around pipes, or other points of entry.


Why Does Vapor Intrusion Matter?

These airborne toxins can make the air unsafe to breathe. They are harmful to health, and even short-term exposure can cause serious health issues. It’s only fairly recently that vapor intrusion has been understood. While groundwater contamination has been a concern for a long time, the ability for that contamination to become airborne was only identified in the 1980s when radon intrusion was being discovered. The general rule of thumb is that if a toxin can contaminate water, then it can also become airborne.


How Can I Know if I Have a Vapor Intrusion Issue?

Since harmful vapors elude the senses, a known contamination will have to be the tipoff that there may be an issue. When a contamination is identified, samples of the soil and groundwater around the area will be taken to determine the levels of chemicals. If the level of chemicals is high enough to warrant concern, vapor testing may be the next step. “Sub-slab vapor sampling” is a method in which small holes are drilled in the floor of the lowest level and an air valve is installed. This allows for an air sample from below the building to be collected. If the vapor testing proves that the contaminant vapor levels are high, then an indoor air quality test may be called for to determine if the vapors are penetrating the air. If other sources of chemicals are present indoors, such as gasoline or paints, these items must be removed before testing to avoid producing a false positive.


What Can I Do about Vapor Intrusion?

Vapor intrusion mitigation methods are divided into two categories: active or passive. While active methods may be less costly, passive methods are generally more effective.

One active method of controlling vapor intrusion is called “sub-slab depressurization”, and it’s the most common method. This is a system of pipes and fans that pull up the offending vapors and vent them outside. “Building over-pressurization” is another active method typically used with larger buildings. This involves adjusting the indoor air pressure relative to the sub-slab area so that the vapors stay below the surface.

Passive methods include sealing openings, installing vapor barriers, and passive venting. When sealing openings, it is important to make sure that all cracks in the floor slab and gaps around pipes and utility lines are filled. If the floor is unfinished dirt, concrete will often be used to create a seal. A vapor barrier is a sheet of “geomembrane” or strong plastic that is placed beneath the building. This is generally done during construction, but it can also be installed in a crawl space. Passive venting is often used in conjunction with a vapor barrier. This involves creating a layer beneath the building that accumulates vapors. Either the buildup of vapors or wind will cause these vapors to move through the venting layer to be expelled outside. This system can be either installed when a building is built or in existing buildings.

These vapor mitigation systems should be checked regularly to ensure that they are functioning properly and that no new cracks have formed. The geomembrane should be checked for any damage from time to time. Electric fans must be inspected to make sure that they are working. Any equipment should be allowed to keep running until the appropriate agency approves for it to be shut off.

Publication Details

November 28, 2022


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