The Nevada Indoor Radon Survey is a statewide investigation of radon gas in single-family homes in Nevada. It is being conducted by NBMG and the Nevada Division of Health (NDOH), with guidance and partial funding provided by the U.S. Environmental Protection Agency (EPA). This survey is part of an ongoing program by EPA to measure levels of radon throughout the United States.
Radon, specifically radon isotope-222, is a colorless, odorless, tasteless radioactive gas that is produced as a natural decay product of uranium. Uranium and radon occur in varying amounts in all rocks and soils, and radon gradually seeps from the Earth into the atmosphere and may find its way into buildings. Radon can build up indoors, especially in lower levels of the home. EPA recommends that the lowest living area (basement or ground floor) of all homes and other buildings with frequent human occupation should be tested for radon. Radon is present in outdoor air as well, but the concentrations outdoors are usually substantially less than those found indoors.
According to EPA, radon-222 is responsible for up to 20,000 lung cancer deaths per year in the United States. EPA recommends that remedial action be taken if radon concentration exceeds 4 picocuries of radon per liter of air (4 pCi/L). Studies indicate that radon will cause between I and 5 lung cancer deaths per 100 people living for 70 years in homes with this concentration. The radon concentration of outside air is generally less than 0.5 pCi/L.
The first phase of the Nevada project was the measurement of the concentrations of radon in the indoor air of about 2,000 homes across the state. This was done to determine the distribution of radon in Nevada and to determine whether radon poses a health hazard to Nevadans. Using a list of randomly selected telephone numbers supplied by EPA, telephone interviewers at NBMG began calling homeowners across the state in January 1990. Homeowners agreeing to take part in the project were sent short-term radon detectors to be placed in their homes for two days and then mailed to an EPA laboratory for analysis. Ten percent of the homeowners were also sent a longer-term radon detector to place in their homes for one year, and are being sent a short-term detector every three months.
Approximately one month after the short-term detectors were mailed to the laboratory, the homeowners were sent the results of their homes' individual radon measurements, along with information explaining how to interpret the results and, if needed, what remedial action to take. Homeowners participating in this study incurred no costs for the radon detectors or their participation, and all radon information pertaining to each individual home is kept confidential.
To date, NBMG has received results of analyses of 1,724 of the 2,063 detectors mailed out during the winter and spring of 1990. The individual indoor measurements average 2.7 pCi/L and range from 0.0 to 46.7 pCi/L. EPA's recommended guideline for remedial action of 4 pCi/L is exceeded by 17% of the analyzed detectors. However, on the basis of numbers of households, population centers were sampled less intensively than rural areas. For example, only 12% of the homes sampled in 1990 were in Clark County where more than half of the state's population lives. When weighted to compensate for variability in sampling intensity (giving more weight to urban data), the data indicate that about 10% of Nevadans live in houses that have radon levels greater than 4 pCi/L. The decrease from the 17% based on raw data to the 10% based on weighted data is due largely to the fact that only 3% of the sampled homes in Clark County exceed 4 pCi/L. The weighted average for the state is 2.0 pCi/L.
Comparable results were obtained in a preliminary statewide radon survey conducted by NBMG the previous winter and spring (reported in Nevada Geology, no. 3, Summer 1989). In that survey, the 307 short-term radon detectors averaged 2.9 pCi/L and 20% of them exceeded 4 pCi/L. The combined results of the 1989 and 1990 winter and spring surveys are shown in the graph and summarized by county in the table.
Some relationships between the occurrence of higher levels of indoor radon and local geology are indicated by the data currently collected in this survey. Granite and related plutonic rocks can contain above-average amounts of uranium; some granitic rocks have been shown to be sources of high indoor radon concentrations in some parts of the country. There is a cluster of cities and towns in west-central Nevada with a relatively high proportion of elevated radon measurements, which may be due to the proximity of granitic rocks in the local mountain ranges. In central Nevada, the town of Austin is surrounded by granitic rocks, and the mountain ranges surrounding the town of Lovelock contain some granitic rocks.
Other rocks that have been shown to contribute to elevated indoor radon levels in some parts of the country include some shales, some metamorphic rocks, and phosphatic rocks. Most of Nevada's shale is located in the eastern half of the state and may be contributing to some of the elevated radon content found in homes in east-central and northeastern Nevada. Metamorphic rocks occur in various mountain ranges of Nevada. Their role in the state's radon picture is unclear but they may be a source of indoor radon in some communities such as Carson City and Hawthorne. Phosphatic rocks occur in few areas in Nevada, and are restricted mostly to northeastern Elko County. It is unlikely that phosphatic rocks are a source of radon in any of the communities involved in this survey.
Because of their relatively high uranium contents silicic volcanic rocks have the potential to generate significant soil radon. However, studies in other areas of the country have not yet demonstrated a close connection between silicic volcanic rocks and high levels of radon in buildings. However, based on some of the radon data collected in Nevada over the past two years and on some geochemical analyses, it appears that elevated indoor radon levels found in some Nevada communities may be due to certain kinds of volcanic rocks. Many of the elevated radon values in Lincoln County are likely due to nearby silicic volcanic rocks or sediments derived from them. Some of the higher indoor radon levels in homes in Elko County and in the towns of Lovelock, Eureka, and Yerington, among others, may also be derived from silicic volcanic rocks.
Thirty-four states, including Nevada, have participated in EPA's radon survey. Of the 47,598 homes measured for indoor radon in those states, 21% exceeded 4 pCi/L, and the weighted average radon value for all the states was 3.3 pCi/L. Some homes in other states had readings as high as a few thousand pCi/L, and in Iowa, radon in 71% of the homes surveyed by EPA exceeded 4 pCi/L.
Compared to some parts of the country then, the data from Nevada may not seem very high or indicate that there is any need for alarm regarding radon in Nevada. However, some areas in Nevada contain radon in amounts substantially higher than the average radon value for the United States, and some areas contain high percentages of homes that exceed 4 pCi/L. Anyone living within these so-called hot spots (generally any area where more than 15% of the radon measurements exceed 4 pCi/L) should be especially concerned about radon.
Results of this survey should be considered preliminary. Although 1,950 homes in Nevada have now been measured for radon content under this program, this is far less than a 1% sample. Furthermore, radon measurements made in your neighbor's house cannot be used to predict the radon content of your house. This is because radon concentration in buildings depends on many factors besides the type of underlying rock or soil. These other factors include the floor on which the measurement was taken, the specific types of construction and building materials used in the design and construction of the building, the barometric pressure and air temperature, and whether windows and doors in the building were opened or closed during the testing period. For these reasons, the only way to determine the amount of radon present in a particular building is to test that building for radon using a radon detector.
Individuals wishing to determine radon concentrations in their homes can conduct their own testing using relatively inexpensive (about $12-30) radon detectors which are available by mail from several private companies. For a list of companies selling radon detectors, contact the Radiological Health Section of the Nevada Division of Health (505 E. King Street, Carson City, Nevada 89710, phone 702-687-5394). Detectors are also available from the American Lung Association of Nevada, P.O. Box 7056, Reno, Nevada 89510, phone 702-825-5864. Radon detectors can also be found locally in some hardware stores. For general information on radon, and to obtain an EPA booklet on radon, call 1-800-SOS-RADON.
During the coming year we will use a portable radon detector to obtain more data on apparent hot spots; some of these hot spots are based on too few measurements to be statistically significant. Work will also be conducted to try to correlate radon production, particularly in the hot spots, with specific types of rocks and soils found in the state. Work will also be getting underway to determine if lung cancer incidence in Nevada can be correlated with the distribution of radon. In addition, after all quarterly short-term detectors are analyzed, we will be able to determine the influence of season on indoor radon accumulation by comparing their results to the winter and spring results reported here. The NDOH and NBMG plan to produce several brochures and reports on their findings on radon in Nevada over the next two years to better alert and advise Nevadans concerning the measurement, distribution, and mitigation of radon.
---Jim Rigby, Environmental Geologist