2
Baseline Information on Indoor Radon and Radon in Water in the United States

Several databases provide a national picture of indoor radon and radon in water for the United States. We provide these data here as context for the discussions in later chapters on ambient radon, transfer factors, uncertainty, mitigation, and a multimedia approach to risk reduction. Figure 2.1 is a geologic-physiographic map of the United States that will serve as a general reference for areas of the country that are important as sources of radon (Schumann and others 1994); it is derived from standard geologic and physiographic maps.

Indoor Radon

The concept of radon potential can be used as a basis for estimating indoor radon concentrations. Although it is not possible to accurately predict radon concentrations in individual houses because of the highly variable nature of factors that control radon entry and concentrations in a specific house, one can estimate the distribution of indoor radon concentrations on a regional basis. Several approaches have been taken to develop indoor-radon potential maps of the United States, and succeeding studies have built on previous ones; the most recent maps of predicted indoor radon encompass a statistical analysis of variables that account for the greatest variation in indoor radon: geology, climate, and house structure.

Figure 2.2 shows the geologic-radon potential map of the United States developed by the US Geological Survey (Gundersen and others 1992) on the basis of geology, indoor radon measurements, the aerial radiometric data collected by the National Uranium Resource Evaluation (summarized in Duval and



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--> 2 Baseline Information on Indoor Radon and Radon in Water in the United States Several databases provide a national picture of indoor radon and radon in water for the United States. We provide these data here as context for the discussions in later chapters on ambient radon, transfer factors, uncertainty, mitigation, and a multimedia approach to risk reduction. Figure 2.1 is a geologic-physiographic map of the United States that will serve as a general reference for areas of the country that are important as sources of radon (Schumann and others 1994); it is derived from standard geologic and physiographic maps. Indoor Radon The concept of radon potential can be used as a basis for estimating indoor radon concentrations. Although it is not possible to accurately predict radon concentrations in individual houses because of the highly variable nature of factors that control radon entry and concentrations in a specific house, one can estimate the distribution of indoor radon concentrations on a regional basis. Several approaches have been taken to develop indoor-radon potential maps of the United States, and succeeding studies have built on previous ones; the most recent maps of predicted indoor radon encompass a statistical analysis of variables that account for the greatest variation in indoor radon: geology, climate, and house structure. Figure 2.2 shows the geologic-radon potential map of the United States developed by the US Geological Survey (Gundersen and others 1992) on the basis of geology, indoor radon measurements, the aerial radiometric data collected by the National Uranium Resource Evaluation (summarized in Duval and

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--> Figure 2.1 Geologic-physiographic map of the United States (courtesy of USGS). others 1989), soil permeability, and foundation housing characteristics. It is a map of the land potential, not a map of exposure or risk. It was compiled from individual state geologic-radon potential maps (Gundersen and others 1993) that served as the basis of the Environmental Protection Agency (EPA) map of radon zones that has been incorporated into one of the national building codes (EPA 1993). Figure 2.3 shows the most recent and most comprehensive map of indoor-radon potential and represents a prediction of the geometric mean of annual exposure to indoor radon. The elements used in the map include the radium content of the surficial soil derived from the aerial radiometric data collected by the National Uranium Resource Evaluation (summarized in Duval and others 1989), information on the geologic province that comprises most of the county (from the US Geological Survey), soil characteristics, the fraction of homes with basements and with living-area basements, and radon-concentration surveys conducted nationally and in each state from EPA and other sources. Those elements are used in a Bayesian mixed-effects regression model to provide predictions of the geometric mean indoor radon concentration by county. Additional details of the model are given in Price (1997). The predicted county means have standard errors of 15–30% for typical counties; the uncertainty in a given county depends on the number of radon measurements in the county and the level of detail in the geologic information.

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--> Figure 2.2 Geologic-radon potential map of the United States (Courtesy of USGS).

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--> Figure 2.3 Indoor-radon potential [predicted geometric mean air concentration in living area, Bq/m3]

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--> From figures 2.2 and 2.3, it is obvious that the Appalachian Mountains, Rocky Mountains, Colorado Plateau, and northern glaciated states (states north of the limit of glaciation) tend to have the highest radon potential and indoor radon. The principal geologic sources of radon in the United States are: Uranium-bearing metamorphosed rocks, volcanics, and granite intrusive rocks that can be highly deformed or sheared (shear zones in these rocks cause the largest indoor-radon problems in the United States), found predominantly in the Appalachian Mountains, Rocky Mountains, and Basin and Range; Glacial deposits derived from uranium-bearing rocks and sediments found in the northern tier of states above the limit of glaciation; Marine black shales found in the Appalachian Plateau and Great Plains and to a smaller extent in the Coastal Plain, Colorado Plateau, and Basin and Range; High-iron soils derived from carbonate, especially in karstic terrain found in the Appalachian Plateau, Appalachian Mountains, and Coastal Plain; and Uranium-bearing fluvial, deltaic, marine, and lacustrine deposits and phosphatic deposits found in the Colorado Plateau, Rocky Mountains, Great Plains, Coastal Plain, Basin and Range, and Appalachian Plateau. Radon in Groundwater and Public Water Supplies In the 1980s, a number of national studies of radon and other radionuclides in public water supplies and groundwater in the United States were published (see (Longtin 1988; Michel and Jordana 1987; Hess and others 1985; Horton 1983). These studies examined geographic distribution, the controls of hydrogeology, and differences among private well, small public, and large public water supplies. The most common conclusions of the studies suggest that the highest radon concentrations in groundwater and public water supplies generally occur in portions of the Appalachian Mountains, Rocky Mountains, and Basin and Range. Private well sources and small public water supplies tend to be higher in radon than large public water supplies. Private well sources and small water supplies tend to be in aquifers with low capacity. When these types of aquifers are uranium bearing granite, metamorphic rocks, or fault zones (as found in the mountain states), the radon concentration in the water tends to be high. Large public water supplies tend to use high-capacity sand and gravel aquifers, which generally comprise low-uranium rocks and sediments and tend to be lower in radon. The study of Hess and others (1985) examined 9,000 measurements of radon in water from national and state surveys. Data were compiled for all but 10 states. Public water supplies originating in surface water tended to have radon concentrations less than 4,000 Bq m-3. Private water supplies were higher in radon than public water supplies by factors of 3 to 20. States with the highest radon in private well water were Rhode Island, Florida, Maine, South Dakota, Montana, and

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--> Georgia. The New England states overall had the highest radon concentrations in water from all sources; state geometric means ranged from 18,500 Bq m-3 in Massachusetts to 88,800 Bq m-3 in Rhode Island. A population-weighted geometric mean for the United States of 6,900 Bq m-3 was reported. Two major national databases collected by EPA exist for radioactivity in public water supplies. Beginning in November 1980, EPA systematically sampled the 48 contiguous states, focusing on water supplies that served more than 1,000 people (Horton 1983). Radon samples were analyzed with liquid scintillation-counting methods, and samples were targeted to be from as close to the groundwater source as possible and to exclude surface waters. The more than 2,500 public water supplies that were sampled represented 45% of the water consumed by US groundwater consumers. High radon concentrations were found in the waters of the New England states, North Carolina and South Carolina, Georgia, Virginia, Arizona, Colorado, Nevada, Montana, and Wyoming. Individual sample measurements ranged from 0 to over 500,000 Bq m-3, the average was 12,600 Bq m-3 and the geometric mean was 3,700 Bq m-3. From 1984 to 1986, EPA conducted the National Inorganics and Radionuclides Survey on the basis of 990 randomly distributed samples from the inventory of public water systems in the Federal Reporting Data System (Longtin 1988). The random sample was stratified into four general categories that represented the population served by the system and represented finished water in the distribution system, generally sampled at the tap. Radon was measured with liquid scintillation-counting methods. Longtin (1988) calculated a population-weighted average radon concentration of 9,200 Bq m-3 but did not calculate unweighted statistics. Our committee examined the unweighted data; of the 990 records, 275 had censored observations of less than 3,700 Bq m-3. Values ranged from below the detection level to 949,000 Bq m-3. The distribution of the concentrations was assumed to be log normal and statistics were estimated with the method of maximal likelihood, using SAS and LIFEREG, which accounts for censored data. The geometric mean radon concentration was estimated at 7,500 Bq m-3, the average 20,000 Bq m-3 and the geometric standard deviation 4.06. A comparison of the two data sets with the data of Hess and colleagues (1985) is shown in figure 2.4. The distributions appear similar in most respects. The 9,000 measurements of Hess and others included the 2,700 measurements of Horton (1983) and some state studies but did not include the Longtin (1988) data. The Hess data have higher percentages of readings in the highest concentration categories than either of the other two data sets. The Horton data have the highest percentage of radon measurements less than 18,500 Bq m-3. Ambient Radon Ambient radon concentration is the concentration of radon in the atmosphere. The outdoor concentration of radon varies with distance and height from

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--> Figure 2.4 Distributions of radon in water measurements in several studies across the United States. its principal source in the ground (rocks and soil) and distance from other sources that can locally or regionally affect ambient radon, such as bodies of water, mine or mill tailings, vegetation, and fossil-fuel combustion. The decrease in radon with height from the source is not simply tied to ground exhalation, nor is the variance a simple mathematical function. A number of studies have documented the decrease in ambient radon with increasing height above the ground and concluded that it is due predominantly to dilution by atmospheric mixing and turbulence (Gogolak and Beck 1980; Druilhet and others 1980; Bakulin and others 1970; Pearson and Jones 1966; Servant 1966; Moses and Pearson 1965; Pearson and Jones 1965). The ambient radon concentration can decrease by more than half in the first 10 m, but many studies show decreases of only one-tenth to one-third in the first 10 m. Concentrations of outdoor radon also change daily and seasonally in response to temperature, changes in atmospheric pressure, and precipitation. Gesell (1983), Blanchard (1989), and Harley (1990) reviewed available studies of outdoor radon from around the world and observed consistent diurnal and seasonal trends. Generally, the diurnal pattern of outdoor radon concentration includes early morning and evening maxima related to cooling and air stability. Minimum concentrations typically occur in the afternoon because of

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--> warming, evaporation and transpiration from soil, and mixing of air. Maximum concentrations occur after midnight and in the early morning hours because of inversion, cooling, and the increased stability of air masses. The marked diurnal pattern is illustrated in figure 2.5, which shows 7 y of hourly data from a site in suburban northern New Jersey (N. Harley, personal communication). As in diurnal patterns around the world, the radon concentration overnight was much greater than that during the day. The ratio of maximums to minimums generally ranges from 1.5 to 4. Seasonally in the United States, maximum outdoor radon concentrations often occur in the summer to early winter and minimum concentrations in the late winter to spring in reaction to meteorologic changes and moisture conditions in the ground. The seasonal pattern varies somewhat in different parts of the world because of variations in seasonal wet and dry periods. Some moisture greatly increases radon emanation (Tanner 1980) whereas too much moisture or saturation of the soil greatly decreases radon transport to the atmosphere. Large barometric-pressure changes and precipitation events yield short-lived but large variations in ambient radon and soil radon on a particular day or seasonally (Schumann and others 1992; Clements and Wilkening 1974). The influence of barometric pressure is illustrated in figure 2.6, which shows outdoor-radon data from Fort Collins, Colorado (Borak and Baynes 1999). A change in barometric pressure changes the pressure gradient between the atmosphere and soil. The soil response depends on the magnitude and duration of the change; a dramatic increase in barometric pressure suppresses radon transport to the atmosphere, and a decrease Figure 2.5 Diurnal variation of ambient radon at a site in northern New Jersey averaged over 7 y.

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--> Figure 2.6 Seasonal variation in ambient radon and barometric pressure during 1994 from a site at Fort Collins, Colorado. (Radon concentration is lower curve.) in pressure enhances radon transport from the soil. The radon response often lags slightly behind the barometric-pressure change and is diminished if the pressure change is relatively small or gradual, which allows equilibration. The ratio of maximums to minimums for seasonal variation of outdoor radon generally ranges from 2 to 5 and is larger in summer than in winter. Ambient radon concentrations differ geographically because of differences in ground concentrations of radon related to geology, soil texture, moisture, atmospheric dilution by adjacent water bodies, and climatic and meteorologic sources. Overviews of ambient radon concentrations around the world include those by Gesell (1983), NCRP (1988), UNSCEAR (1988), and Harley (1990). Those studies report averages above continental land masses generally in the range of 4–75 Bq m-3 and averages above water bodies or islands generally less than 2 Bq m-3. Wilkening and Clements (1975) estimated that the ocean contributes only 2% of atmospheric radon. Studies of Ambient Radon For the purposes of this report, we have compiled and examined most of the outdoor-radon studies conducted in the United States during the last 15 y. There has been only one national study in the United States, but several ambient-radon studies have been carried out on the state or regional scale or at a single site over

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--> time. In general, the studies examined in this section reported readings taken at about 1–2 m above the ground with various methods, most commonly the use of alpha-track detectors, continuous or semicontinuous radon monitors, and electret ion chambers. The accuracy and precision of the individual methods has been examined in numerous studies and recently reviewed by Fortmann (1994), Lucas (1957) and Busigin and others (1979). Studies on the quality of data in the range of 1–40 Bq m-3 are rare, although measurement detection limits for the different devices range from 1 to 18 Bq m-3 (Blanchard 1989). Measurement errors reported in the studies that the committee compiled generally range from 8 to 20% but can be substantially higher when very low concentrations were measured. Quality control and duplicate measurements were used in all the studies. Only a few studies measured radon progeny and calculated equilibrium factors and outdoor dose rates or dose (Wasiolek and others 1996; Wasiolek and Schery 1993). National Studies In the late 1980s, EPA conducted a national survey of ambient radon across the United States (Hopper and others 1991) to confirm previously reported concentrations and in response to section 302 of the Indoor Radon Abatement Act, which stated that ''the national long-term goal of the United States with respect to radon levels in buildings is that the air within buildings in the United States should be as free of radon as the ambient air outside of buildings.'' Section 303 also required EPA to include information regarding outdoor ambient radon concentrations around the country in the updated Citizens Guide to Radon. From 1989 to 1991, measurements were made quarterly in 50 cities, one in each state, across the country. The sites chosen coincide with 50 EPA's Environmental Radiation Ambient Monitoring System stations that were established in 1973 and most coincide with the capital cities of the states. The ambient radon concentrations measured at the sites are shown in table 2.1. Summary statistics and the frequency distribution of the seasonal averages are shown in figure 2.7. Measurements were made at each station with three electret ion chambers placed in ventilated shelters 1 m above ground. In each shelter, there were also three thermoluminescent dosimeters to measure gamma radiation to provide the needed gamma correction of the electret measurements. Every 90 d, the devices were exchanged for new ones, and the old ones were measured at EPA's Las Vegas facility. Three devices were used to assess precision and allow for backup in case a device failed, and readings were taken quarterly to examine seasonal variation. (EPA 1992d; Hopper and others 1991). Measurements were reported in pCi/L, but for this report we have converted the measurements to Bq m-3 [note: 1 m3 = 1,000 L]. The limit of detection of the devices was determined to be 2 Bq m-3. During the first quarter, several stations were started several weeks late and problems with the setting up of the stations and the measuring protocol were found in several states (these data were not included). Corrections of the proce-

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--> Table 2.1 Seasonal Ambient Radon, Bq m-3 for the United States (Arithmetic Average of Three Detectors at Each Site) Site Quarter 1 Sum. 1989 Quarter 2 Fall 1989 Quarter 3 Winter 1989 Quarter 4 Spring 1990 Quarter 5 Sum. 1990 AL 13.3 16.4 12.5 12.0 26.5 AK 12.7 6.8 10.6 8.5 11.1 AR 11.7 21.6 19.5 11.8 15.2 AZ 8.8 16.2 28.5 20.0 13.6 CA 11.8 16.8 20.5 11.3 11.7 CO 12.3 27.5 17.3 8.0 10.5 CT 13.4 25.9 13.0 ND 15.4 DE 17.5 13.9 13.6 10.5 13.9 FL 25.9 14.7 14.1 9.6 11.0 GA 14.8 28.7 19.0 13.9 13.0 HI 9.5 6.8 7.0 5.9 8.9 IA 24.8 14.6 25.9 14.1 17.3 ID 13.3 16.3 27.4 9.1 9.7 IL 22.7 21.8 24.8 16.5 20.5 IN 19.4 15.7 15.0 19.6 15.7 KY 19.5 16.4 20.0 11.6 18.6 KS 27.3 19.2 23.4 15.8 19.6 LA 17.4 13.8 8.3 5.7 9.1 MA 22.9 17.0 15.2 8.0 14.4 MD ND 19.7 22.6 12.7 16.7 ME 19.2 18.1 20.1 13.2 15.4 MI 16.3 12.1 15.3 10.5 14.9 MN 18.5 14.1 17.6 8.8 10.6 MO 17.4 28.0 25.3 17.0 16.7 MS 15.5 16.0 11.0 11.7 15.7 MT 12.8 18.9 17.0 15.3 19.4 NC 18.7 14.6 15.9 5.3 12.2 ND 19.4 27.5 19.1 15.5 14.7 NE 20.0 22.0 21.2 14.6 18.7 NH 24.4 15.4 14.1 10.1 15.0 NJ 15.3 18.5 17.4 12.5 15.7 NM 7.8 6.2 10.2 2.5 4.4 NV 5.1 10.0 12.6 5.7 6.3 NY 12.7 11.8 15.0 9.3 10.4 OH 16.9 14.1 19.7 10.1 12.2 OK 10.4 13.4 13.6 10.4 23.8 OR ND 9.0 17.5 10.2 11.6 PA 20.9 21.5 24.2 10.0 16.5 RI ND 5.1 16.3 10.6 9.7 SC 15.4 35.9 17.5 12.2 16.7 SD 17.9 21.2 21.7 16.0 18.5 TN 17.3 18.0 19.8 11.1 20.2 TX 17.9 17.0 21.1 7.0 20.7 UT 7.8 15.0 15.2 5.9 9.3 VA 14.6 20.0 16.2 13.9 18.6 VT 18.9 14.1 14.3 13.8 15.0 WA 15.0 24.9 16.0 14.3 20.8 WI 13.4 22.4 15.0 11.2 13.0 WV 22.0 11.6 ND 15.0 20.5 WY 7.4 16.7 ND 8.8 10.5 Avg. 16.1 17.3 17.5 11.4 14.8

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--> Table 2.1 Seasonal Ambient Radon, Bq m-3 for the United States (Arithmetic Average of Three Detectors at Each Site) Site Quarter 6 Fall 1990 Quarter 7 Winter 1990 Quarter 8 Spring 1991 Quarter 9 Sum. 1991 Avg. All Quarters AL 13.7 13.4 7.8 13.3 14.3 AK 11.6 ND ND ND 10.2 AR 20.5 15.5 16.0 15.7 16.4 AZ 18.3 18.0 14.6 12.5 16.7 CA 18.5 13.4 10.5 12.5 14.1 CO 16.9 13.2 5.8 3.6 12.8 CT 15.7 13.4 7.5 12.6 14.6 DE 13.7 15.0 7.9 15.9 13.6 FL 9.1 9.0 11.2 18.5 13.7 GA 11.3 14.8 10.1 15.4 15.7 HI 8.5 7.3 6.3 10.2 7.8 IA 20.4 18.3 13.1 20.8 18.8 ID 17.4 23.3 5.9 13.9 15.1 IL 17.5 10.1 11.8 20.5 18.5 IN 13.8 11.5 10.2 15.9 15.2 KY 18.9 14.4 12.7 24.5 17.4 KS 24.7 22.0 15.3 19.2 20.7 LA 6.8 9.7 8.0 9.7 9.8 MA 15.7 13.7 10.7 14.8 14.7 MD 15.2 17.0 11.1 16.8 16.5 ME 17.3 10.5 16.2 17.4 16.4 MI 16.2 11.8 11.5 14.8 13.7 MN 15.0 14.7 10.5 11.7 13.5 MO 20.6 16.2 11.0 14.7 18.5 MS 17.6 9.9 7.5 14.8 13.3 MT 20.2 17.6 12.5 20.1 17.1 NC 11.8 9.3 8.3 ND 12.0 ND 25.2 22.4 18.0 14.1 19.5 NE 22.8 21.7 15.8 24.4 20.1 NH 16.5 11.5 11.8 14.6 14.8 NJ 19.6 11.3 17.4 11.0 15.4 NM 6.7 1.7 4.4 3.9 5.3 NV 12.4 8.5 4.2 ND 8.1 NY 11.5 8.9 12.3 ND 11.5 OH 14.2 9.6 11.6 16.8 13.9 OK 13.3 10.9 25.2 10.6 14.6 OR 13.1 11.2 9.6 11.2 11.7 PA 16.4 11.6 15.5 19.9 17.4 RI 9.0 8.3 15.2 11.5 10.7 SC 13.7 11.8 10.0 15.2 16.5 SD 24.7 23.8 30.7 21.0 21.7 TN 21.2 11.2 10.5 21.1 16.7 TX 13.2 11.7 7.0 8.9 13.8 UT 11.5 14.9 6.4 5.6 10.2 VA 29.5 12.5 13.7 16.2 17.2 VT 14.4 11.0 5.3 14.1 13.4 WA 18.9 14.8 12.8 17.9 17.3 WI 14.6 10.4 15.0 18.6 14.8 WV 19.2 18.4 13.3 27.3 18.4 WY ND 10.4 10.4 5.2 9.9 Avg. 16.1 13.3 11.6 15.0 14.8

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--> Figure 2.7 Average ambient radon concentrations at 50 sites in the United States. dure were made by the second quarter. The authors (Hopper and others 1991) emphasize that the study does not statistically represent the distribution of ambient radon concentrations in the United States but indicates that estimates of annual average ambient radon concentrations and the associated error estimates can be derived for each site. The authors used only quarters 2–5 for their original report for the sake of timeliness, but provided our committee with the entire data set for this report (R. Hopper, private communication). The updated Citizens Guide To Radon (EPA 1992c) reported an average outdoor concentration of 14.8 Bq m-3 on the basis of the survey. As can be seen from table 2.1, ambient radon concentrations above the average (all quarters, all sites, 14.8 Bq m-3) tend to occur in the Appalachian Mountains, the northern Midwest, and the northern western states. Sites in the southern and western coastal states, the Great Lakes states, and several of the central and southwestern states tend to be at or below the average. These trends probably reflect the geology or other sources at the sites and the proximity to large water bodies. The bar graph in figure 2.7 illustrates the average of all data from each site by season, showing the spring minima and fall maxima. State Studies Statewide or regional studies have been conducted in California (Liu and others 1991), Nevada (Price and others 1994), Minnesota and Iowa (Steck and

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--> others 1999), and Maine (Hess and others 1982). In the Nevada study, Price and others (1994) used the same method as Hopper and others (1991) and measured 50 sites across the state during a 30-d period in the summer of 1992. Sites were chosen to reflect different rock types and represent the principal population centers in Nevada. About half the sites were in residential areas, and the rest were in remote areas near rock outcrops. Results indicate that radon in soil gas corresponds well with the geology, outdoor radon concentration and indoor radon. Measurements across the state ranged from 2.6 to 52 Bq m-3; the geometric mean was 13.1 Bq m-3. The range and values of concentrations were generally very similar to what Hopper and others found for the United States. As part of a statewide radon study, indoor radon was measured at 300 sites throughout California (Liu and others 1991). At 68 of those sites, outdoor radon was also measured by using alpha-track detectors in cups suspended 1–2 m above the ground and exposed for a year starting in April 1988. Indoor radon was found to correlate well with broad geologically defined areas of the state. The geometric mean outdoor radon concentration was 15.54 Bq m-3, and the range was 0.3 to 55.5 Bq m-3. Steck and others (1999) measured annual average atmospheric radon concentrations at 111 locations across Iowa beginning in 1993 and ending in the spring of 1997. They also measured ambient radon at 64 selected sites in western and northern Minnesota during 1995–1996. Comparisons were made with indoor radon; at some sites seasonal variations and variation with height were tested. Large-volume alpha-track detectors were enclosed in protective housings and placed 1.5–2 m above the ground for a year at each site. In Minnesota, concurrent annual average indoor radon measurements were also made with the same type of device. In Iowa, the researchers found that elevated outdoor radon concentrations twice the annual average reported by Hopper and others persisted over long periods and covered wide areas of the state. In both states, some outdoor concentrations were the same as or higher than the national average indoor radon concentration (46 Bq m-3). In general, outdoor radon concentrations were distributed in a geographically similar pattern to indoor measurements. Etched-track detectors were used to measure outdoor radon and multi-room indoor radon at 100 sites in Maine from October 1980 to May 1981 to determine integrated average radon concentrations during the heating season (Hess and others 1982). The outdoor cups were in open sheltered areas on porches, garages, and sides of homes approximately one meter above the ground. Over half the houses were in geologic regions where high concentrations of radon in water were previously found. The remaining measurements were made in regions of low or intermediate concentrations of radon in water. Outdoor radon corresponded well with geology and in some instances was comparable with indoor radon concentrations. The average ambient radon concentration of 26 Bq m-3 reported by Hess and others (1982) is nearly twice the US average reported by Hopper and others (1991).

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--> Site Studies Long-term studies of outdoor radon have been conducted at a number of sites around the country, including Socorro, New Mexico (Wasiolek and others 1996; Wasiolek and Schery 1993); Fort Collins, Colorado (Borak and Baynes 1999); Chester, New Jersey (Fisenne 1998); suburban New Jersey and Central Park, New York (N. Harley, personal communication). Researchers from the New Mexico Institute of Mining made semicontinuous measurements of 222Rn during the winter of 1991–1992 at a site 1 m above a golf course in Socorro, using a two-filter continuous monitor (Wasiolek and Schery 1993). Grab samples were also measured with a two-filter manual system, and meteorologic measurements were made 5 m above the ground. Both attached and unattached radon progeny were measured and an effective dose rate between 0.2 and 0.7 mSv y-1 was calculated. Radon concentrations varied over the period but generally were 5–10 Bq m-3 and had a geometric mean of 10.2 Bq m-3 . In a follow-on study, 220Rn was measured at the same site from February 1994 to February 1995 (Wasiolek and others 1996); an average effective dose of 220Rn decay products of 0.025 mSv was calculated. From 1990 to 1997, continuous and passive monitoring of ambient radon at a site in northern New Jersey (N. Harley, personal communication) yielded an average of 8 Bq m-3. Passive alpha-track detectors were measured seasonally and compared favorably with continuous (hourly) monitoring with a flowthrough scintillation counter. The Chittaporn and others (1981) continuous monitor used in the study is one of the few monitors that detects only radon and removes the decay products at formation with an electret. Decay products can introduce error into radon measurements that use flow-through scintillation counters. Monthly average outdoor radon concentrations were higher during the colder months of the year, however it is the daily variations of the outdoor radon that account for the variation by a factor of 2 in radon concentration. A site in Central Park, New York was also monitored for outdoor radon over a 3-y period using a passive alpha-track detector; they found an average of 7 Bq m-3 and trends similar to those in their northern New Jersey study (N. Harley, personal communication). In a 3-y study (1993–1995) at a single site in Fort Collins, Colorado measurements (Borak and Baynes 1999) were made 1 m above the ground at 15-min intervals with a 1 liter flowthrough scintillation flask. Researchers found that outdoor radon responded to changes in temperature, wind speed, barometric pressure, and precipitation and varied seasonally and diurnally. Daily averages varied from 2.5 to 64.5 Bq m-3, and monthly averages varied from 12 to 18 Bq m-3; the magnitude of the variation in concentrations seems to be due to diurnal changes and specific meteorologic events and less to long-term seasonal response. Fisenne (1988) measured outdoor radon in Chester, New Jersey with a continuous two-filter monitor for 9-y beginning in 1977. The annual average radon varied from 7.00 to 9.25 Bq m-3 and hourly measurements ranged from 0.4 to

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--> Table 2.2 Statistical Summary of Outdoor-Radon Surveys in the United States (Bq m-3). Location Time Avg. Geo. Mean Geo. Std. Dev. Range Method Reference California (68 sites) 1 y 18.1 15.5 1.8 0.3–56.0 Etched track Liu and others (1991) Iowa (111 sites) 4+y 30.0 29.0 1.4 7.0–55.0 Etched track Steck and others (1999) Maine (51 sites) 6 mo. 27.0 17.6   2.9–160.0 Etched track Hess and others (1982) Minnesota (64 sites) 1 y 22.0 19.0 1.8 4.0–55.0 Etched track Steck and others (1999) Nevada (50 sites) 30 d 15.1 13.1 — 2.6–52.0 Electret ion chamber Price and others (1994) Socorro, NM 4 mo. 12.5 10.2 2.0 1.3–50.3 Continuous 2-filter Wasiolek and Scherly (1993) Fort Collins, CO 3 y 18.0 15.0 1.7 2.5–64.5 (daily) Continuous monitor Borak and Baynes (1999) Suburban No. NJ 7 y 8.0 6.0 1.8 4.0–24.0 (daily) Continuous monitor Etched track N. Harley (personal communication) Chester, NJ 9 y 8.1 — — 0.4–63.0 (hourly) Continuous 2-filter Fisenne (1988) Central Park, NY 3 y 7.0 — —   Etched track N. Harley (personal communication) 63 Bq m-3. Again, diurnal changes in concentration and meteorologic events accounted for most of the variability in the hourly readings. Statistical Analysis and Summary of Data In all studies examined for this report, the spatial and temporal variation is striking and consistent and follows geologic, diurnal, meteorologic, and seasonal controls. However, comparison of the statistics from the studies (table 2.2) reveals that arithmetic and geometric means across time and geography were within the range of 6-30 Bq m-3. Individual readings and hourly or daily averages were

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--> in the range of 1–63 Bq m-3, with the exception of Maine. Both Iowa and Maine had higher average outdoor radon than the other areas, and the geometric mean for Iowa was significantly higher than all the others. Average Ambient Radon In the original charge to the committee, EPA requested "central estimates for a population-weighted average national ambient concentration for radon, with an uncertainty range. Comparisons of the contribution of radon in water to other sources of indoor radon will be made and comparisons will be made to outdoor levels." The charge was amended to include a discussion of alternatives to population-weighted averages and of spatial and temporal variation. The ambient-radon data of Hopper and others (1991) are the only data that provide some portion of national coverage over an extended period, but the committee has concerns about the appropriateness of using these data to develop a population-weighted average for the United States. Hopper and others contend that the data cannot be used to represent the ambient radon of the state that contains the sampling site. However, they do think that the ambient radon measured at each site is representative of that site. Using population data from the 1990 census, one can calculate a population weighted average (APW) by summing the products of each site's seasonal average (Ai) and the city population (Pi) and then dividing by the sum of the city populations. That assumes that the ambient radon measured at the site is representative of the ambient radon of the city. The population-weighted average radon concentration is given by: where N is the number of sites. The population-weighted average radon concentration is 14.0 Bq m-3. The total population of the cities for all the sites in the national outdoor radon survey was about 24 million, or slightly less than 10% of the US population. The calculation is dominated by New York City, which has a large population and a lower than average ambient radon measurement, and therefore the population weighted average is less than the unweighted average of 14.8 Bq m-3.Further, the sites were not chosen to be a statistical representation of the population across the United States, nor were the sites chosen to be representative of ambient radon within each state or sample the geology of the country. The overall distribution of the seasonal data for each site in the Hopper and others (1991) data is given in figure 2.8. The committee feels that it is more reasonable to recommend an (unweighted) arithmetic average radon concentration of

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--> Figure 2.8 Frequency distribution of average seasonal ambient radon for the United States. 15 Bq m-3 for the United States and that it lies within a confidence interval of 14–16 Bq m-3. It is evident that radon concentrations in air, in water, and indoors vary systematically across the United States and that this variation should be part of any regional consideration of multimedia assessment and mitigation. A comprehensive, geographically based ambient-radon study that incorporates the major population areas of the United States and their geologic variability would provide the basis for a valid population-weighted ambient radon concentration. Focused regional studies of ambient radon in high-radon areas such as the glaciated northern tier of states and states of the Appalachian Mountains, Rocky Mountains, and Basin and Range would yield better information on overall exposure and more-realistic baseline information for evaluating the contribution of the ambient concentration to what is observed in indoor air.