Radon is believed to be second only to smoking as the major cause of lung cancer in the United States, causing between 7,000 and 30,000 deaths annually according to EPA (U.S. Environmental Protection Agency). The link between radon and lung cancer in uranium miners was documented in the early 1950s, but it did not become known that harmful concentrations of radon could occur in buildings until 1984 when a home in eastern Pennsylvania was found to have radon concentrations as high as in some mines. Upon further study several homes in this area and in adjacent New Jersey were found with very high concentrations of radon. Pilot tests in other parts of the United States revealed many other areas in which radon concentrations were very high.
Radon-222 is a colorless, odorless gas that is one of the intermediate products of the radioactive decay of uranium-238, which occurs in varying amounts in all rocks and soils. Radon concentration is usually expressed in picocuries per liter (pCi/L) in the United States. One picocurie is a radioactive decay rate of about 2.2 nuclear disintegrations per minute. The average concentration of radon in outside air is about 0.4 pCi/L, which is equivalent to the concentration of radon required to produce about one nuclear disintegration per minute per liter of air. Analyses of lung cancer death rates of miners exposed to high radon concentrations indicate that radon concentrations of 4 pCi/L or more may be cause for concern. It is estimated that radon will cause between one and five lung cancer deaths per 100 people living for 70 years in houses with 4 pCi/L radon. EPA recommends remedial action for homes and other inhabited buildings in which radon concentrations are 4 pCi/L or more.
In February 1989 NBMG began a pilot study to determine the importance of radon as a health hazard in Nevada communities and to determine the relationships between indoor radon concentration and local rock and soil types. Preliminary results of this study, which were presented in the Summer 1989 issue of Nevada Geology, indicated that radon did pose a potential health hazard to Nevadans and more detailed study was required. NBMG continued its studies of radon on a broader scale in 1990 with the aid of EPA grants administered by NDOH (Nevada Division of Health); these studies are still in progress.
Preliminary results of the expanded 1990 studies were reported in the Winter 1990 issue of Nevada Geology and, in 1994, a detailed report of all NBMG radon studies up through 1993 was published in NBMG Bulletin 108, Radon in Nevada. Portions of this bulletin are summarized in the following sections.
In addition to the screening survey of 1989, NBMG conducted three surveys of radon in indoor air: a statewide random survey in 1990-91, a more intensive survey of eight selected communities in 1992, and a survey of state office buildings in Reno/Sparks and Carson City in 1992-93. The numbers of measurements and summarized results of these surveys are presented in table 1.
Survey Number Mean Highest Percent pCi/L pCi/L >4 pCi/L Statewide screening survey, 1989 307 2.9 31.6 20 Statewide random survey, 1990-91 2-day charcoal canisters 2,065 2.9 46.7 19 1-year alpha-track detectors 243 1.4 9.4 5 Targeted communities survey, 1992 Austin 92 5.2 40.7 43 Genoa 52 2.3 19.7 9 Pioche 80 4.0 80.9 16 Lake Tahoe communities 38 3.2 14.4 24 State office survey Reno/Sparks, 1992 125 2.1 11.8 15 Carson City, 1993 134 2.2 12.8 11
All measurements by 7-day charcoal canisters except as noted in the statewide random survey. Lake Tahoe communities involved were: Crystal Bay, Glenbrook, Incline Village, Stateline, and Zephyr Cove .
The statewide survey involved the measurement of radon in the indoor air of about 2,000 homes throughout Nevada. Using a list of telephone numbers randomly selected by EPA, UNR telephone interviewers called homeowners throughout the state to determine if their homes met the criteria for inclusion in the study and if they were willing to participate. Only owner-occupied, single-family, detached dwellings were included in the project; mobile homes were included if they were skirted to restrict airflow beneath them. Homeowners agreeing to take part in the survey were sent a short-term (charcoal) radon detector to expose in the lowest habitable floor in their homes for two days and then mail to a laboratory for analysis. Ten percent of the participants were sent a short-term detector every three months for a year to determine seasonal variation in radon concentration in their homes; these participants were also sent one or more long-term (alpha-track) detectors to expose in their homes for one year. The tests were performed at no cost to the homeowners and, except for informing the homeowner, individual results are confidential.
The 2,065 charcoal canisters that were returned and analyzed in 1990-91 averaged 2.9 pCi/L and about 19% of them exceeded 4 pCi/L (table 1). However, this average is not truly representative of the whole state. To obtain adequate samples in the smaller communities, they were sampled more intensively than the population centers. For example, only 12% of the sampled homes were in Clark County where more than half of the state's population lives and where the radon measurements were among the lowest in the state.
Indoor-air radon concentrations vary throughout the year, being lowest in summer primarily because homes are better ventilated then. In the 1990-91 survey, most (1,601) of the charcoal canister measurements were made during the winter months (December, January, and February), 260 were made in the spring (March, April, and May), 130 in summer (June, July, and August), and 74 in fall (September, October, and November). Average radon concentrations by season are shown in figure 1. If each season is given equal weight, the yearly average for the 1990-91 survey would be 2.2 pCi/L, lower than the unweighted average of 2.9 pCi/L for the charcoal canisters but higher than the 1.4 pCi/L average obtained by the 243 alpha- track detectors exposed for a full year (table 1).
In 1992, the radon study was focused on several small communities: Austin, Genoa, Pioche, and the Lake Tahoe communities of Crystal Bay, Glenbrook, Incline Village, Stateline, and Zephyr Cove. A total of 262 charcoal canister measurements were made in these communities (table 1). In addition, a study of indoor-air radon in state office buildings (including the University of Nevada, Reno) was begun in 1992 in Reno/Sparks and completed in 1993 in Carson City (table 1).
The combined results of the 1989, 1990-91, and 1992 residential surveys are summarized by county in table 2. At least 20% of the tested homes exceeded 4 pCi/L in nine of the state's 17 counties. EPA has designated these counties (Pershing, Mineral, Lander, Lincoln, White Pine, Carson City, Douglas, Eureka, and Elko) as having high potential health hazard from indoor radon.
County Number Mean Highest Percent pCi/L pCi/L pCi/L >4 pCi/L Carson City 98 3.7 31.6 2 Churchill 141 2.2 20.1 9 Clark 261 1.0 11.0 3 Douglas 154 3.8 21.9 29 Elko 258 2.9 18.0 22 Esmeralda 20 1.1 3.0 0 Eureka 37 3.9 35.4 24 Humboldt 259 2.3 43.4 13 Lander 162 4.3 46.7 34 Lincoln 234 4.4 80.9 29 Lyon 75 2.4 11.0 19 Mineral 75 4.2 23.6 35 Nye 170 1.6 17.5 6 Pershing 40 6.4 40.7 45 Storey 16 2.5 14.7 13 Washoe 495 2.9 40.6 18 White Pine 245 3.4 23.7 27 TOTAL 2,740 3.0 80.9 20
All measurements by charcoal canisters in residences (alpha-track detectors and offices excluded).
Nevada communities where ten or more residential indoor radon measurements were obtained in the 1989-92 surveys are shown in figure 2 along with the percentage of measurements greater than 4 pCi/L. A high proportion of the measurements exceed 4 pCi/L in several communities, notably Zephyr Cove, Orovada, Lovelock, Panaca, Wells, Ely, Hawthorne, Gardnerville, Yerington, Caliente, Austin, and Eureka. The high indoor radon concentrations in these communities has been found to be related in many cases to the local geology; these relations are discussed in detail in NBMG Bulletin 108.
The primary source of radon in homes is from the radioactive decay of uranium in the soil and rock beneath them. The radon so produced diffuses into the lower level of homes at rates dependent on the concentrations of radon and the barriers to gas diffusion; radon diffuses unimpeded into unsealed crawlspaces. Aside from the type of building construction and ventilation, the amount of uranium present in underlying soil and rock is usually the most important factor affecting indoor-air radon concentrations. The map of potential radon hazard (figure 3) is based primarily on an aerial radiometric survey of uranium content of near-surface rock and soil throughout Nevada made by the U.S. Department of Energy in the 1970s. The hazard ratings are also based on 1989-93 indoor-air radon survey data in those areas where measurements were made. The criteria by which the three hazard levels were determined are explained in NBMG Bulletin 108.
The radon studies summarized here are presented in greater detail in NBMG Bulletin 108, which also includes:
NBMG Bulletin 108 is available for $10.00 at the NBMG sales office or for $11.00 by mail.
---Dick Meeuwig, Editor