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Report on the Environment

U.S. Homes Above EPA's Radon Action Level

Report on the Environment Workgroup
by email at: roe@epa.gov

The metadata includes data set information and responses to technical questions.

1. Indicator Title
U.S. Homes Above EPA's Radon Action Level
2. ROE Question(s) This Indicator Helps to Answer
This indicator is used to help answer one ROE question: "What are the trends in indoor air quality and their effects on human health?"
3. Indicator Abstract

This indicator presents the estimated number of homes with indoor radon levels over EPA's radon action level—the recommended maximum concentration for indoor radon—and the number of homes with operating radon mitigation systems. Radon is a human carcinogen that seeps into basements and dwellings from the surrounding soil, and is an important indoor air quality issue in homes nationwide.

4. Revision History
May 2008 - Original indicator posted
December 2008 - Indicator updated
March 2010 - Metadata updated
June 2010 - Indicator updated
Data Sources
5. Data Sources

This indicator is based on data extracted from the National Residential Radon Survey and radon vent fan sales data provided by three manufacturers that account for approximately 99 percent of the radon vent fan market.

6. Data Availability

U.S. Census data on housing stock are publicly available, as are data on the spatial distribution of radon levels in homes. The vent fan sales data are unpublished, but have been provided to EPA by three major manufacturers.

7. Data Collection

Study Design

This indicator presents (1) the number of U.S. homes estimated to be at or above the EPA recommended radon action level of 4 picocuries per liter (pCi/L) and (2) the number of homes with an operating radon mitigation system. The data for this indicator were developed from multiple sources (U.S. EPA, 1992a, 2009) and U.S. Census data, which are all viewed as being scientifically valid.

This indicator is based on two types of data. First, to determine the number of homes above the action level, data were extracted from the National Residential Radon Survey (U.S. EPA, 1992a), which estimated radon levels in the U.S. housing stock in the early 1990s. The number of homes at or above 4 pCi/L in subsequent years was estimated by applying the results of the Radon Survey (U.S. EPA, 1992a) to U.S. Census data on the number of households nationwide. The 1992 National Residential Radon Survey was based on the housing stock that would be covered by EPA’s radon testing policy (i.e., homes that should test). This included only homes intended for regular (year-round) use and covers single-family homes, mobile homes, and multi-unit and group quarters (U.S. EPA, 1992b). The 1992 residential survey estimated that about one in 15 homes in EPA's "should test" category would have a radon level of 4 pCi/L or more.

Second, the measure of the number of homes with operating mitigation systems was derived from radon vent fan sales data. The primary sources for these data are three radon vent fan manufacturers that are responsible for 99 percent of the market. EPA-solicited annual updates from these manufacturers on the number of radon vent fans sold per year, their potential non-radon application or use, and estimates of their useful life (the latter is also checked against similar information from practicing mitigation practitioners in the field). Individual company information is treated as confidential business information (CBI). Only homes that have had a radon measurement are mitigated. Following mitigation, a test confirming reduction of the radon level to less than 4 pCi/L is routinely made.

This indicator, as a measure of public health and risk reduction, is the most appropriate and current available for indoor radon levels. The approach used to portray the data is viewed as being appropriate because it tracks two related trends: the number of homes affected by radon and the number of affected homes for which some mitigation has occurred.

EPA believes that the physical measurements of radon in air—taken by the established radon measurement industry in the U.S.—are consistent with EPA, state, and industry protocols, and in line with existing quality assurance (QA) guidance. Measuring radon (radiation) in homes is well understood and has been widely practiced since the mid-1980s (U.S. EPA, 1993).

The indicator is not designed with special emphasis on sensitive populations or ecosystems.


Supporting documentation for this indicator is in an interim update report titled, "National Radon Results: 1985 to 2003" (Gregory and Jalbert, 2005). Specifically, documentation of the sampling approach used to characterize radon in homes is in the chapter titled "Radon Testing," and more information on the approach used to evaluate radon vent fan sales is in the chapter titled "Mitigation of Existing Homes." The Census data and data collection methodologies are publicly available from the U.S. Census Bureau (http://www.census.gov).

Documentation of the radon vent fan sales data is contained in EPA program staff files and to a lesser (but sufficient) degree in the draft or published reports cited here.

8. Indicator Derivation

The data collection approach and the underlying data have both been peer-reviewed numerous times. For instance, earlier versions of the Radon Progress Report were peer reviewed, and the entire indicator was subject to public review and external peer review prior to publication.

The approach by which underlying data on radon measurements, Census data, and radon vent fan sales data were translated into this indicator is described in "Data Collection." This indicator is as direct and unencumbered a measure of risk reduction as is currently available within the existing limitations (see "Data Limitations"). No statistical methods have been used to portray the data beyond the times or spatial locations in which measurements were made.

9. Quality Assurance and Quality Control

The quality and integrity of the Census data are presumed adequate. The quality and integrity of the manufacturer-provided data are presumed sufficiently accurate given that they are primarily (unaltered) sales data. The quality and integrity of the National Residential Radon Survey data are also presumed to be accurate, given the extensive peer review conducted of this document.

10. Reference Points

The most relevant reference points, thresholds, and ranges of values include the following: (1) EPA's recommended radon action level for the public of 4 pCi/L; (2) the nationwide average indoor radon level of 1.25 (1.3) pCi/L; (3) the effectiveness of radon mitigation (i.e., radon reductions to less than 2 pCi/L in 70 percent of residential mitigations); and (4) Census data as the basis for estimating the number of homes above the action level. All of these and some other values were established through valid survey, assessment, and study designs.

11. Comparability Over Time and Space

The data are comparable over space and time. Radon tests are conducted according to the same standards regardless of location. Data are based on a single 1992 study.

12. Sources of Uncertainty

Content under review.

13. Sources of Variability

The uncertainty and variability are not expected to impact the conclusions. However, EPA's Indoor Environments Division currently prefers to use the central estimate in the range of a radon vent fan’s useful life (i.e., 10 years).

14. Statistical/Trend Analysis

The indicator presents a time series of estimated number of homes with elevated radon concentrations and the number of homes with operating radon vent fans. No special statistical techniques or analyses were used to characterize the long-term trends and their statistical significance.

15. Data Limitations

Limitations to this indicator include the following:

  1. The indicator presumes that radon vent fans are used for their intended purpose; the available information supports this premise. Even if fans are used for managing vapor intrusion, a radon risk reduction benefit still occurs.
  2. A home with an operating mitigation system is presumed to have a vent fan with an average useful life of 10 years. Each year the total of homes with operating mitigation systems is adjusted to reflect new additions and subtractions (i.e., vent fans installed 11 years earlier).
  3. The number of homes with radon levels at or above 4 pCi/L is an estimate based on one year of measurement data extrapolated for subsequent years based on population data, rather than on continuing measurements.
  4. This indicator does not track the number of homes designed and built with radon-reducing features, and without a vent fan. These features can help diminish radon entry in homes. Thus, more people are likely to have a reduced risk from exposure to radon in indoor air than suggested by the trends in operating radon mitigation systems alone. However, homes with radon-reducing features should be tested to determine if they are at or above EPA’s radon action level.
16. Data Reference

Gregory, B., and P. Jalbert. 2005. National radon results: 1985 to 2003. http://www.epa.gov/radon001/pdfs/natl_radon_results_update.pdf (7 pp, 184K, About PDF).

U.S. EPA (United States Environmental Protection Agency). 2009. Unpublished sales data provided by radon vent manufacturers.

U.S. EPA. 1993. Protocols for radon and radon decay product measurements in homes. EPA/402/R-92/003.

U.S. EPA. 1992a. National residential radon survey: Summary report. EPA/402/R-92/011.

U.S. EPA. 1992b. Technical support document for the 1992 Citizen's Guide to Radon. EPA/400/R-92/011.

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