Hazardous Sites & Substances

Environmental Health Division

Hazardous Sites & Substances
Vapor Intrusion

December 2007

On this page:
What is vapor intrusion?
Why is vapor intrusion a concern?
What happens if vapor intrusion is suspected?
What can be done to reduce vapor intrusion and improve indoor air quality?
Related topics
For more information
Printable factsheet: Vapor Intrusion

What is vapor intrusion?
Chemicals that have been spilled or dumped on the ground can pollute soil and groundwater. One group of chemicals, volatile organic compounds (VOCs), easily evaporate. Petroleum products, dry cleaning solvents, and many industrial chemicals contain VOCs. VOCs evaporating from the polluted soil and groundwater rise towards the ground surface. If these vapors come to a basement as they travel to the surface, they may enter through cracks in the foundation, around pipes, or through a sump or drain system. In this way, the VOC vapors enter buildings and contaminate indoor air. This process, when pollution moves from air spaces in soil to indoor air, is called vapor intrusion.

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Why is vapor intrusion a concern?
People may be able to smell chemical vapors, especially those from petroleum, if there are high concentrations in indoor air. At these concentrations, some people experience headaches, eye and throat irritation, nausea, or other symptoms. Some people may be more sensitive to these effects than others. The effects are usually temporary, and will go away when the person leaves the building or breathes fresh air. Fortunately, this situation is rare.

More often, health officials are concerned about the possible health effects of long-term exposure to low levels of contaminants. At low levels, there is usually no odor to warn people that contaminants are in the air. Long-term exposure to some of the VOCs that have been found at vapor intrusion sites may be associated with increased risk of damage to the liver, nervous system, and to the developing fetus. Exposure may also lead to an increased risk of developing certain types of cancer. Even though these risks are usually very low, they are avoidable, and health and environmental officials want to identify and take steps to reduce or eliminate vapor intrusion where possible.

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What happens if vapor intrusion is suspected?
Anytime VOCs are in soil or groundwater, there could be vapor intrusion into nearby buildings. If there is contamination from a spill or leak at a gas station, dry cleaner or other industrial plant, the possibility that vapor intrusion is occurring should be investigated. Officials begin by collecting soil, groundwater and soil gas samples to look for the presence of chemicals and the levels. If chemicals are present near buildings, it may be necessary to collect samples of soil gas beneath the slab or indoor air. Indoor air samples are typically collected over 24 hours using specialized canisters. After the air sample is collected, it is brought to the laboratory for analysis.

Many factors affect vapor intrusion, such as weather, time of year, type of building construction, and ventilation. Indoor air sample data can be difficult to interpret. Indoor air quality varies greatly from day to day, and from building to building. Many of the chemicals that may be present in soil vapor can also be found in common household products, cigarette smoke, and vehicle emissions. If the chemicals found in soil and groundwater are not detected in indoor air, it lessens the possibility that vapor intrusion is occurring, but does not eliminate it. On the other hand, if the same chemicals found in soil and groundwater are found in indoor air (especially in the basement), it suggests vapor intrusion may be occurring but does not eliminate other sources within the building. Several indoor air samples taken at different times may be necessary to determine if vapor intrusion is occurring, and if so, the actual concentrations that are due to vapor intrusion.

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What can be done to reduce vapor intrusion and improve indoor air quality?
If soil and groundwater contamination is found at a site, some sort of cleanup action may be required. This could involve digging up and removing contaminated soil, installing a soil vapor extraction (SVE) system, or pumping out and treating the contaminated groundwater. Any of these actions should help reduce vapor intrusion. If chemical vapors are found in indoor air at levels that might affect people’s health, it may be necessary to install a system to direct vapors away from indoor air. These systems are the same systems installed to keep radon, a naturally occurring radioactive gas, from entering homes. They direct the vapors that collect underneath the foundation to the outside air. They are relatively inexpensive to operate, simple to design and install, and are a proven solution to radon and vapor intrusion problems. Usually, when the contamination is the result of a spill or leak, the responsible party or a government agency pays for the installation.

Indoor air quality can be affected by many factors, not just vapor intrusion. To keep the indoor air in your home healthy, we recommend the following steps:

If you smoke, do it outdoors. Or, even better, take this opportunity to quit.
Store chemicals in their original containers, and keep them in a well-ventilated area. Do not buy more than you can use at one time, and follow the directions carefully. Check with your local household hazardous waste center for proper disposal.
Make sure your home is properly ventilated. A home that is too airtight can have problems with moisture and mold. Contaminants in the air can build up.
Make sure your furnace and air conditioner are in good working condition, and have all major appliances regularly inspected by a professional.
If there are strong odors in your home and they smell like petroleum or natural gas, leave the building immediately and then call the fire department.
Test your home for radon. For information on obtaining a low cost radon test kit, contact the MDH Indoor Air unit.

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For more information
For questions about chemical leaks or spills, please contact us.
For indoor air questions, including testing for radon, contact the Indoor Air Unit

Printable information sheet: Vapor Intrusion (PDF: 61KB/2 pages)

Prepared in cooperation with the US Agency for Toxic Substances and Disease Registry

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