Radon Test Results Summary and Interpretation

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This dataset includes radon test kit data supplied to the CDPH Indoor Radon Program from the certified radon laboratory and testing community. The radon test data includes radon levels found in homes and apartments across the state. However, radon levels may be different from house to house, even on the same street, so do not rely on the dataset to determine whether your home may have radon. CDPH recommends that all homeowners test for radon, regardless of your local area results.

The dataset includes about 57,000 test measurements of indoor radon concentrations reported in units of picocuries per liter (pCi/liter) of air. The measurements were made between 1992 and 2024 using a variety of test kit types and detectors. Most of the tests were made with charcoal test kits. Most of the test kits were placed by homeowners, using instructions provided with the kit for proper placement of the kits in living areas. This dataset includes test kits placed on various floors of houses depending on house design and use.

An EPA 2018 report estimated that the typical outdoor radon concentration is about 0.7 pCi per liter. EPA also estimated that the typical indoor radon concentration is about 1.7 pCi per liter. The first grouping interval that we’ve chosen for reporting (<=2pCi/liter) would cover the typical indoor radon level and at those levels there is typically no recommendation for further action. EPA also estimated that about 6% or 7% of homes have radon above 4 pCi/liter. Elevated levels of indoor radon have been found in every State, and EPA recommends that all homes be tested for indoor radon.

The reports calculate the percentage of measurements in each of three ranges of results:

1. Radon measurements levels less than or equal two picocuries per liter of radon, which include normal levels of radon and radon levels less than the detection limit of kits. Some of the data in this range are non-detect values. Therefore, calculation of summary statistics such as arithmetic mean or standard deviation values is inappropriate.

2. Radon measurements between 2 pCi/liter to 4 pCi per liter, which include levels of moderate radon results above the normal range but less than the typical action level of 4 pCi/liter. Note that the World Health Organization action level for radon is 2.7 pCi/liter, which falls in this range.

3. Radon measurements above 4 pCi/liter indicate elevated radon levels that recommend remediation or at least follow-up testing.

The California Geological Survey created reports for CDPH focusing on areas where potential radon levels may be elevated due to the underlying geology. The latest of these reports in 2024 covers Western El Dorado County. In this report CGS estimated that areas where the geology indicated likelihood of elevated radon levels had the percentage of radon measurements above 4pCi/liter were above 20%. In areas with moderate levels of radon predicted, the measurement levels were estimated to be between 5% and 20% above 4 picocuries per liter. The basis for determining areas of elevated radon potential was the underlying geological formations and the history of other measurements in the area from a previous dataset collected in 2011-2012.

For the California Indoor Radon Program, the purpose and the use of the data is to find areas that have apparently elevated radon concentrations, where more testing might be warranted and to find areas where there is insufficient testing to determine the estimated levels of radon present. The data from our database cannot be used to determine whether a particular residence in an area should be remediated or tested. In fact, all housing units need to be tested to determine an actual level if determining radon level is of an interest.

We have created reports based on grouping the radon measurement data by different geographical criteria such as city, county and zip code and grouped the readings into the three intervals we discussed above and calculated the percentage of the measurements for that geographic unit that fall in the in each range and those reports are available. It should also be noted that this data set contains measurements using different types of measurement systems and there is a lot of variability introduced through homeowner replacement of the devices. One should not use this data to define an area as elevated or to decide about a specific house or unit of housing. Further analysis of the data might include geographical plotting of the data using a GPS system which might provide more detailed geographical information. The California indoor radon program is not pursuing that approach or mapping of the data at this point.

Multiple types of test kits were used in the in the measurements accumulated in the database. Also, the different types of test kits have different detection limits and uncertainties associated with them. Most of the measurements were made with short term activated charcoal test kits.