Scientists evaluate the radon potential of an area and create a radon potential map by using a variety of data. These data include the uranium or radium content of the soils and underlying rocks and the permeability and moisture content of the soils. Usually maps of these factors are not available, and other indirect sources of information about these factors, such as geologic maps, maps of surface radioactivity, and soil maps, are used.
Another type of information that scientists use in determining the radon potential of an area is radon measurements of local soil air. Existing indoor radon data for homes also are useful. These data are the most direct information available about indoor radon potential, even though the houses that have been sampled may not be typical for the area and exact location information for measured houses is seldom available.
A geologic map shows the type of rocks and geologic structures in a specific area. Because different types of rocks have different amounts of uranium, a geologic map can indicate to a geologist the general level of uranium or radium to be expected in the rocks and soils of the area. Such maps are especially important in showing where rocks with high levels of uranium occur.
Because radon that enters buildings usually comes from the upper several feet of the earth's surface, knowing the radon levels of the near-surface (surficial) materials is important. Surficial geologic and engineering maps show and describe these surface materials for many parts of the United States. These maps are useful for understanding the physical properties of the materials at the surface, like permeability, but are generally not as useful for determining what the uranium concentrations in the surface materials might be.
Radioactivity maps give an indication of the uranium levels of surface materials. The most common type of radioactivity map is an aeroradioactivity map, which is based on radioactivity measurements made from an aircraft flying at low altitude with instruments that measure the radioactive energy radiating from the ground.
A large amount of aeroradioactivity data was collected as part of a U.S. Department of Energy program to evaluate the uranium resources of the United States. Most of the energy detected during these flights was from rocks and soils within 800 feet of flight lines that were spaced 1 to 6 miles apart. Many major metropolitan areas were not covered by the survey because of flight restrictions. Therefore, only a small part of the surface of the United States was measured. The data from this survey, however, give a good indication of the background uranium concentration of soils and rocks underlying most of the United States.
The digital data from the survey were processed by the U.S. Geological Survey to produce a map showing the uranium content of surface materials in the conterminous United States (the lower 48 States). The smallest data point on the map covers about 1.6 by 1.6 miles, limiting the amount of detail that can be seen. (You can tell how parts of a region, a State, or possibly a county vary in surface uranium concentration, but you can't tell how uranium varies from neighborhood to neighborhood or from house to house.
Soil-air radon data
Scientists also measure radon in soil air. These data give direct evidence about soil radon, but extensive sets of these data are not commonly available. The two basic methods for measuring the radon concentration of soil air are the same as those used to measure radon in buildings. Both methods measure the alpha particles produced by the decay of the radon in the air.
One method involves burying a passive device, such as a charcoal cannister or an alpha-track detector, in the soil and leaving it open to the soil air. This method allows long-term measurements, but the devices can be affected strongly by soil moisture. In the other method, a sample of soil air is collected from a probe driven into the ground, and the radon in the sample is measured by using electronic equipment. This method provides data quickly, but these short-term measurements may vary greatly due to daily, weekly, and seasonal changes in soil and atmospheric conditions that are averaged out during long-term measurements.
The best data on the physical properties of soils are in soil surveys published by the Soil Conservation Service of the U.S. Department of Agriculture in cooperation with state and county officials. Soil surveys provide descriptions and maps of the soils that underlie the areas described.
Modern soil surveys include permeability data for the mapped soils at varying depths. In older soil reports, no permeability data are given, and soil names and statements regarding internal drainage must be used to estimate permeability.
Indoor radon data
Indoor radon has been measured in many houses, schools, and commercial buildings across the United States. For the most part, these measurements have been made by private homeowners using passive detection devices purchased at a nearby store, ordered by phone, or ordered through the mail. Radon concentrations in some homes and businesses are being measured by private companies as part of real estate transactions. Many local, State, and Federal agencies are measuring radon in buildings for which they are responsible.
Most indoor radon measurements are confidential transactions between homeowners and measurement vendors. The data from these private measurements are not generally available to the public. When they are available, the data are usually given as summaries by state, county, or zip code. Nonetheless, these summaries are useful in determining which regions of the counties, states, or United States seem likely to have elevated indoor radon levels.
By careful examination and correlation, scientists can evaluate the effects of varying geology and soils on actual readings of indoor radon. The indoor radon information can be used as an additional aid to create a radon potential map or it can be used as a way of expressing the radon potential of areas mapped by the geologist. However, differences in house construction also can cause variations in the indoor radon levels.
Scientists create radon potential maps by combining a variety of data, such as the locations of rocks containing high levels of uranium, locations of fractures, aeroradioactivity data, soil data on permeability and radon content, and indoor radon data. Not all of these types of data are available for every area, and radon potential maps for different areas may vary if they are based on different types of data. For instance, radon potential maps and data sets prepared by the U.S. Geological Survey (USGS) of Montgomery and Prince Georges Counties, Maryland, and Fairfax County, Virginia, are based on different data. The radon potential of Montgomery County was estimated by USGS geologists using measurements of soil radioactivity, measurements of soil-air radon, general geologic and soil maps, and indoor radon measurements reported by homeowners.
Three levels of radon potential were identified in the counties. Low radon potential means that the majority of homes contain less than 4 pCi/L of indoor radon. Moderate radon potential indicates that one-third to one-half of the homes have more than 4 pCi/L. High Radon potential means that the majority of homes contain more than 4 pCi/L.