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Vapor Intrusion

Vapors from past chemical spills can impact indoor air quality

When a chemical spill enters the environment, some of this contamination ends up in the ground and can stay there for a many years. Certain types of chemicals, such as volatile organic compounds (VOCs), can turn into vapor. VOCs include petroleum products like gasoline, and dry cleaning and industrial solvents. Once in vapor form, these chemicals can move through soil and along underground utility lines to the foundation of a nearby building. When the vapor enters the building through cracks in the foundation or along utility lines, this is called vapor intrusion. Vapor intrusion can present health risks to building occupants who breathe in the chemical vapors.

Vapor Intrusion 101

Radon vs. Chemical Vapors

Vapor intrusion is similar to the concept of radon gas entering homes. However, while radon gas occurs naturally in soil, vapor intrusion results from chemical contamination.


Vapor intrusion can make people sick

While exposure to vapor intrusion can affect everyone, newer evidence shows that exposure to a VOC known as trichloroethylene, or TCE, during pregnancy can have serious effects on the developing fetus, including an increased risk of heart defects. This can happen at low levels of TCE and very early in pregnancy, before someone may know that they are pregnant. Therefore, individuals who are or may become pregnant should avoid exposure to TCE above health-based guidelines.

Pregnant adult with hands around belly

TCE is especially harmful for the developing fetus—even when indoor air levels of TCE are low.

Health effects from exposure to vapor intrusion depend on many factors, including the specific chemical, how much of the chemical someone was exposed to and at what levels, and for how long someone was exposed. In addition, humans can have different reactions depending on their individual health status, previous exposure to other chemicals, and genetic factors.

Short-term exposures to higher levels of VOC vapor in indoor air can cause eye and respiratory irritation, headache, drowsiness, slowed reflexes, and nausea. These symptoms are usually only temporary and should go away when the person is moved to fresh air. If you experience strong VOC odors indoors, then you are being exposed and you should consider increasing ventilation and avoiding that location until the odors dissipate. Health officials are also concerned about low-level VOC vapor exposures that happen over many years without someone realizing it, as this may raise that person’s lifetime risk for developing cancer (for example, exposures to VOCs that have no odor).

Expect contamination to be inspected

The most common vapor intrusion cases involve solvents from commercial and industrial properties and are usually not accompanied by an odor. In many cases, these chemical releases are not immediately discovered. By the time they are discovered, the contamination has had time to spread away from the area where it was spilled, through the soil and groundwater.

If you live or work near a site with reported VOC contamination, you should expect that the potential for vapor intrusion is also being investigated. You may be contacted by the people responsible for investigating and cleaning up the contamination or others, such as environmental consultants, that are working for them on the cleanup and have information about the project. Your cooperation and consent would be requested before any testing/sampling could be done on your property. You may ask the person contacting you any questions about the work being done. It is important to understand that the people responsible for the contamination are required to investigate how far the contamination spreads, including at neighboring properties.

For an online database of sites with environmental contamination in Wisconsin, click on the “Launch” button for "BRRTS on the Web" on the Department of Natural Resources' (DNR) Remediation and Redevelopment page.

Air samples are taken from around the home

In some cases, the potential for vapor intrusion can be ruled out by collecting soil gas or groundwater samples near the contamination site. In other cases, sampling closer to your home or business may be necessary.

Samples may also be taken from beneath a building’s foundation, called sub-slab sampling, to see if chemical vapors have reached the foundation. Indoor air samples are often collected at the same time to determine if the chemical vapors are getting into the occupied areas through cracks in the floor, piping, or other entryways.

Install a mitigation system to reduce exposure

Radon mitigation system outside of a house.
If vapor intrusion is having an effect on the air in your home or business, the most common solution is to install a vapor mitigation system that looks and operates very similar to a radon mitigation system. These mitigation systems prevent chemical vapors from entering the home or business. Systems designed to prevent chemical vapors from entering a building also prevent radon gas from entering, an added health benefit.

The party responsible for investigating and cleaning up the contamination is also responsible for paying for the installation of this system. Because certain contamination persists for long periods of time in the environment, it is likely that these mitigation systems will be needed for many years. According to state rules, these mitigation systems can be turned over to the property owners to operate once the investigation and cleanup are deemed complete by the DNR, even if some contamination remains in the environment. The systems need to continue to be operated until no longer required by the DNR. In homes with high radon levels, DHS recommends that these mitigation systems remain in place permanently.

Fact sheets on this topic and other resources for property owners are available on the DNR's vapor intrusion website.

Household products can be a source of VOCs

Paints, paint strippers and thinners, cigarette smoke, aerosol sprays, moth balls, air fresheners, new carpeting or furniture, hobby supplies (glues and solvents), stored fuels, and dry-cleaned clothing all contain VOCs. For this reason, household products are more likely to be a source of air quality problems in your home than vapor intrusion from a contamination site.

Tips to improve indoor air quality in your home:

  • Do not buy more chemicals than you need at a time.
  • Store unused chemicals in appropriate containers in a well-ventilated location.
  • If you smell a chemical odor that does not seem to be from an indoor source, contact your local health department.
    • For very strong odors, your local fire department can determine if there is a fire hazard.
  • Air out your home - fresh air will help prevent both build up of chemicals in the air and mold growth.
  • Fix all leaks promptly and control moisture that encourages mold growth.
  • Have all major appliances and fireplaces checked annually by a professional.
  • Test your home for radon!
  • Visit our Air Quality Issues webpage and Breathe Easily fact sheet, P-02166, for more ideas on how to protect yourself from other common indoor air pollutants.


Resources for Environmental Professionals

Featuring "Through the Cracks" Toolkit and Inventory

Phone books from the mid-century are the main data sources for DHS' new toolkit, "Through the Cracks: Preventing Vapor Intrusion Exposure in Wisconsin and Keeping Children and Families Safe.”

The purpose of this toolkit is to introduce targeted audiences, including local health department officials and city developers, to the public health risks associated with contaminated properties from years of occupancy by former dry cleaner businesses, and to guide these audiences through the process of documenting locations of former dry cleaner operations in an effort to prevent harmful exposures to current or future building occupants. Old phone books are essential to this process.

Throughout much of the 20th century, dry cleaners often used cleaning solvents containing chemicals known as chlorinated volatile organic compounds, or CVOCs. Inadequate disposal practices of the past were often responsible for spills of these chemicals into the environment. CVOCs take a long time to break down, and contamination often remains for decades in the ground below and adjacent to the former dry cleaning site. Over time, CVOCs can turn into vapor and present vapor intrusion-related health risks to building occupants who breathe in the chemical vapors.

While we can find just about everything on the internet these days, we would likely have trouble finding locations of dry cleaning businesses that operated between 1930 and 2000. This is where old phone books come in handy: these books typically contain not only the addresses of former businesses, but also helpful hints about the services they offered (for example, whether dry cleaning happened on-site) that are found in advertisements scattered throughout the musty yellow pages.

Anyone looking to complete this project will utilize old phone books to create an inventory of past dry cleaning businesses, and then prioritize these sites based on special criteria to determine the likelihood that contamination, and thus a public health risk, remains on-site. After these locations are identified and ranked, a number of actions outlined in the toolkit can be taken to prevent potential vapor intrusion exposure.

Who would have thought phone books could be so essential to preventing an emerging environmental public health hazard?

For questions or assistance with this project, please email


Preventing vapor infusion exposure in Wisconsin, P03070

Through the Cracks: Preventing Vapor Intrusion Exposure in Wisconsin and Keeping Children and Families Safe Toolkit, P-03070 (PDF)

Template Inventory

The Template Inventory (Excel) is a free resource available to anyone completing this project. This file contains detailed instructions and examples for building an inventory of locations presenting potential vapor intrusion concerns.


Historical phone books are the preferred data sources for this project because they are comprehensive, use relatively consistent categorization, and contain advertisements, like this one, that can provide clues about business activities to help guide your search. For example, dry cleaners that advertised service in four hours or less are more likely to have used CVOC chemicals on-site.


Questions? Can't find what you're looking for? Contact us!

Last revised December 23, 2021