To most of the public, radon appears to have gone the way of the Hula Hoop--still around, but no longer a rage. Beginning about 1984, and for a few years after, radon was a hot news story. Homeowners were urged to test their residences with inexpensive charcoal test kits, which were readily available in grocery and hardware stores. Radon testing and mitigation companies appeared almost overnight and a multiplicity of legislative actions were pondered by lawmakers.
Although indoor radon emerged as a public health issue, geologists soon became an integral part of the story because radon is a potential health problem with a geologic origin. This colorless, odorless gas is a daughter element in the radioactive decay series of uranium, which is widespread in small quantities in rocks and sediments. Geologists have long been aware of radon and have used it in the exploration for oil and gas and even in the prediction of earthquakes. Public health officials have known about the harmful effects of high concentrations of radon from studies of uranium miners, but neither geologists nor health officials suspected that a number of homes in the country harbored potentially harmful concentrations of this inert gas. Even more surprising to geologists was the fact that some of these high-radon homes were built on rocks or sediments that had comparatively low concentrations of uranium. It quickly became apparent that there were complex geological factors involved in the occurrence of indoor radon.
The radon issue generated an unusual meld of science and politics in which the U.S. Environmental Protection Agency (USEPA) issued radon alerts to the public suggesting the indoor radon levels of 4 picocuries per liter (pCi/l) or greater posed a risk of lung cancer, although epidemiological data to support this claim, at such low concentrations, were lacking. The EPA estimates of 7,000 to 30,000 annual lung cancer deaths due to indoor radon exposure were statistical calculations based on extrapolations from studies of uranium miners, and others, who were working in poorly ventilated environments in which radon exposures were as high as several thousand picocuries. The USEPA, caught between the proverbial "rock and a hard place," was faced with either choosing a conservative approach and recommending a low "action level," or choosing a higher standard such as 10 to 20 pCi/l, which is prevalent in many European countries. Similar to the issues of asbestos and second-hand smoke, they chose the former, more conservative, approach. However, some scientists are not convinced that casual exposure to low levels of any of these carcinogens is a significant health risk and argue that many billions of dollars are spent in remediation and mitigation efforts that produce little, if any, reductions in health risks.
At the time of the initial radon campaign by the U.S. Environmental Protection Agency, which was triggered by the discovery of extremely high radon levels in a home in eastern Pennsylvania, little was known about how widespread the problem might be and what geologic parameters might be involved. Initial analyses of the distribution of radon across the United States relied on aerial radiometric data collected in the late 1970's and early 1980's by the U.S. Department of Energy as part of the National Uranium Resource Evaluation (NURE) project. The U.S. Geological Survey used NURE data to construct aerial radiometric maps of the country and individual states.
The Ohio map depicts some surprising patterns of comparatively high surface concentrations of uranium. Most geologists expected the higher levels to coincide with the outcrop belt of the Upper Devonian Ohio Shale, a dark, organic shale that has long been known to have elevated concentrations (10 to 40 ppm) of uranium. Although the aerial radiometric map shows some coincidence of elevated levels of surface uranium with Ohio Shale outcrops, there are areas, particularly in the central and western portions of the state, that are not associated with such outcrops and, surprisingly, resemble the distribution of glacial moraines.
Health authorities, in conjunction with television stations and sponsoring businesses, mounted large programs to encourage homeowners to purchase radon test kits. Dayton and Columbus, in particular, initiated extensive campaigns. Partly as a result of this publicity, more than 50,000 homes across Ohio were tested.
Comparatively large public interest in radon in the late 1980's and a lack of information on geologic controls on radon distribution spurred a number of research efforts by faculty and students at several universities in Ohio. Investigations included the occurrence of radon in ground water, glacial deposits, and the Ohio Shale, and development of a database for indoor radon measurements and their correlation with geologic deposits.
The results of the latter study by James A. Harrell, John P. McKenna, and Ashok Kumar, were published by the Ohio Division of Geological Survey in 1993. Report of Investigations No. 144, Geological controls on indoor radon in Ohio, compares radon levels with geologic features across the state.
Geometric mean indoor radon concentrations for Ohio counties (from Harrell, McKenna, and Kumar, 1993). |
More than 50,000 radon measurements in 1,270 zip code areas were available; however, only 698 of these zip code areas had five or more measurements--considered a minimum by the authors for statistical validity. These data indicate that 64.9 percent of the zip code areas in the state have average indoor radon levels below the U.S. EPA "action level" of 4 pCi/l. The national average is 93 percent. In Ohio, 4.7 percent of the zip code areas had radon levels above 8 pCi/l. Nationally, about 1.6 percent of homes exceed 8 pCi/l. On a county basis, 38 percent (33) of Ohio's 88 counties were above 4.0 pCi/l, but only Licking County in east-central Ohio was above 8.0 pCi/l (geometric mean of 11.5 pCi/l for the county). Seven counties--Carroll, Fairfield, Franklin, Harrison, Knox, Pickaway, and Ross--had geometric mean indoor radon concentrations between 6 and 8 pCi/l.
RADON AND GEOLOGY IN OHIO
Although Ohio does not appear to have radon concentrations as high as those in some states, we are well above the national average. Geologists are intrigued by the origin and distribution of these elevated readings. As expected, there seems to be some correlation between elevated indoor radon and the outcrop area of the Ohio Shale, but there also is a coincidence between elevated indoor radon and glacial deposits. The highest radon levels are associated with Wisconsinan deposits in the Scioto and Miami Lobes, and particularly the Scioto Lobe.
Harrell, McKenna, and Kumar discount the presence of crystalline (igneous and metamorphic) erratics from the Canadian Shield as the primary source of uranium in these glacial deposits because the concentration of these materials is so volumetrically low. They conclude that much of the radon is derived from fragments of the uraniferous Ohio Shale that have been incorporated into the till and other glacial deposits by the glaciers.
Another probable uranium source in the glacial deposits is soil developed from the prolonged, deep weathering of carbonate rocks (limestone and dolomite) by solution. This process removes the soluble carbonate fraction, but leaves an insoluble residue of clay minerals, iron oxide, titanium dioxide, and other compounds, including uranium-bearing minerals. Although carbonate rocks generally have low (less than 3 ppm) uranium, the chemical weathering process concentrates this element in the residual soil. Commonly, under the right climatic conditions, reddish-brown soil known as terra rossa develops on the weathered limestone. It is possible that thick terra rossa soils developed on the limestone and dolomite bedrock of western Ohio prior to glaciation. Most of this material was removed by each of the three or more glaciations that covered up to two-thirds of the state. Possible remnants of terra rossa soil can be seen in some quarries in central Ohio where small sinkholes and fissures in the limestone contain red terra rossa. Each succeeding glacier incorporated till from preceding glaciers; most of the uranium of terra-rossa origin now resides in the uppermost, and most recent, Wisconsinan glacial deposits.
Numerous geological studies of radon occurrence have concluded that permeability of rock, sediment, or soil is a key factor in elevated levels of indoor radon. This gas has a short half life (3.8 days) and cannot travel long distances except in unusual circumstances. Consequently, dense, impermeable rocks or sediments with comparatively high uranium content may have a low potential to yield high indoor radon in overlying homes.
On the other hand, rocks or sediments (such as sand and gravel) that have low uranium concentrations but high permeability may yield relatively high radon values. Harrell, McKenna, and Kumar found a direct correlation between zip code areas with elevated radon levels and the distribution of sand and gravel deposits in the state. They speculate that the radon in these deposits is derived from uranium in the Ohio Shale or in other geologic source materials.
Studies of chert (flint) in neighboring Indiana have demonstrated that this rock has more than twice as much uranium as other rock types within the same formation. No studies of uranium concentrations in chert have been done in Ohio, but Harrell, McKenna, and Kumar point out that the zip code with the highest average radon level in Licking County (16 pCi/l) is located on Flint Ridge, an extensive flint/chert deposit of Pennsylvanian age.
In some areas of the country it is thought that ground water maybe a significant source of indoor radon. The gas is released when the water is aerated, such as in a shower. The USEPA estimates that 10,000 pCi/l of radon in ground water would release 1 pCi/l of radon into the air. Other studies suggest that 1,000 pCi/l would be required to elevate indoor-air radon by 1 pCi/l. Several studies of radon in Ohio ground water show no levels above 10,000 pCi/l, and most readings are below 1,000 pCi/l. These data suggest that radon in ground water in Ohio may not be a significant problem.
ADDITIONAL INFORMATION
The Division of Geological Survey can answer questions pertaining to the geology of radon. Please call 614-265-6580. The Bureau of Radiation Prevention of the Ohio Department of Health is Ohio's lead agency on radon and can answer questions and provide literature on radon mitigation. They can be contacted at Radon Program 246 N. High St., P.O. Box 118, Columbus, 0hio 43266-0118. Telephone: 800-523-4439 (toll free) or 614-644-2727.
ACKNOWLEDGMENT
We thank Dr. James A. Harrell of the Department of Geology, University of Toledo, for his assistance with this article.
FURTHER READING
Cole, L. A., 1993, Element of risk: The politics of radon: American Association for the Advancement of Science Press, 246 p .
Hansen, M. C., 1986, Radon: Ohio Division of Geological Survey, Ohio Geology, Fall issue, p.1-6.
Harrell, J. A., McKenna, J. P., and Kumar, Ashok, 1993, Geological controls on indoor radon in Ohio: Ohio Division of Geological Survey Report of Investigations 144, 36 p.
Otton, J. K., 1992, The geology of radon: U.S. Geological Survey, 28 p.