Ask TreeHugger: What Is Vapor Intrusion?

Question: Could you discuss the issues and health risks related to "vapor intrusion" of volatile organic chemicals into buildings from contaminated sites? What can a homeowner or purchaser do to find out if there is a vapor intrusion risk at home, schools, or day care centers?

Response: "Vapor intrusion" (sometimes called "soil gas vapor intrusion") is what happens when chemicals move from the ground or water into your home or other buildings. Chemicals that enter your home through the ground often belong to a class of chemicals called "volatile organic compounds" (VOCs), which as their name implies are volatile, and thus like to exist as a gas. This property is important, allowing VOCs to move easily through openings that exist between the soil grains and thus to move from areas of high pressure to areas of low pressure. Since basements tend to be at lower pressure than the ground below, this pressure-related movement can cause VOCs to enter your home from the ground through openings or cracks in your foundation. Once inside, the VOCs can spread through out your home with the help of natural air flow, room or house fans, or other home ventilation devices. When this occurs, vapor intrusion can be an important source of indoor pollution in your home.Whether vapor intrusion also presents additional health risks depends on many factors. For example, health risks will depend on the people that live or spend time in your home, as people have different responses to chemical exposures. Health risks will also depend on the type and amount of your chemical exposures, more precisely the specific VOCs that enter your home, their levels inside your home, and the length of time that the VOCs remain elevated.

Although other kinds of chemical spills, leaks or contamination can also lead to vapor intrusion in homes, the most common VOCs associated with vapor intrusion are gasoline-related, the result of spills or leaks from an underground storage tank at a gas station. For gasoline-related intrusions, VOCs of concern may include (but are certainly not limited to) benzene, toluene, xylenes, and styrenes. Exposures to these and other gasoline-relaed VOCs have been linked to a variety of adverse effects, including short term (and generally reversible) effects such as eye and respiratory irritation, headaches, and nausea, and long-term effects, such as cancer.

There are several ways to test if VOCs or other chemicals are entering your home through the ground. You can collect samples in the nearby ground (soil vapor samples), in the ground directly below your foundation (sub-slab vapor samples), in your indoor air, and in your outdoor air (for comparison). Samples in these different places should be collected at the same time, preferably during the heating season when the pressure difference between the ground and the basement should be highest. Similarly, indoor samples should be collected in the basement or first floor of the living space, since these are the areas in your home where the levels from vapor intrusions should be highest. Since there are many sources of VOCs inside your home and since indoor VOC concentrations can vary widely day-to-day, indoor sampling results may be more difficult to interpret than the soil and sub-slab sampling results, but interpretation of indoor sampling results will be easier with outdoor sampling data and information about your house, including its ventilation, characteristics, and other VOC sources.

Also, if you know that a nearby gas station, commercial or industrial facility or other site has had a spill or a leak of VOCs, you may want to call the owners, local government officials, or the people cleaning up the spill to see if any sampling is being performed to test for vapor intrusion and if so, to ask for testing to be performed at your home. The U.S. Environmental Protection Agency provides extensive technical guidance on vapor intrusion including "safe" levels of chemicals in groundwater and soil gas (See Draft Guidance at

If tests show that chemicals are getting into your home through the ground, there are many possible solutions, including sealing cracks and openings in the foundation or installing a radon remediation system. Often, the costs of these solutions will be paid for by the person or company who allowed the chemicals to get into the ground.

Previous Ask Treehugger columns can be found here.

Helen Suh MacIntosh is a professor in environmental health at Harvard University and studies how pollution behaves in the environment and how it affects people's health. Please keep in mind that her answers are just her interpretation of available information and should not be taken as the only viewpoint or solution to a problem. Use this column at your own risk. Having said this, please feel free to post any of your environmental health questions to (Please use a descriptive email subject line and mention if you want to remain anonymous or not).

Ask TreeHugger: What Is Vapor Intrusion?
Question: Could you discuss the issues and health risks related to "vapor intrusion" of volatile organic chemicals into buildings from contaminated sites? What can a homeowner or purchaser do to find out if there is a vapor intrusion risk at home,