Levels of radon, a known carcinogen, rising since 2004, around the time that drilling for a new type of natural gas well began 
 
 Johns Hopkins Bloomberg School of Public Health researchers say that levels of radon in Pennsylvania homes - where 42 percent of readings surpass what the U.S. government considers safe - have been on the rise since 2004, around the time that the fracking industry began drilling natural gas wells in the state. 
 
 The researchers, publishing online April 9 in Environmental Health Perspectives, also found that buildings located in the counties where natural gas is most actively being extracted out of Marcellus shale have in the past decade seen significantly higher readings of radon compared with buildings in low-activity areas. There were no such county differences prior to 2004. Radon, an odorless radioactive gas, is considered the second-leading cause of lung cancer in the world after smoking. 
 
 "One plausible explanation for elevated radon levels in people's homes is the development of thousands of unconventional natural gas wells in Pennsylvania over the past 10 years," says study leader Brian S. Schwartz, MD, a professor in the Department of Environmental Health Sciences at the Bloomberg School. "These findings worry us." 
 
 The study, conducted with Pennsylvania's Geisinger Health System, analyzed more than 860,000 indoor radon measurements included in a Pennsylvania Department of Environment Protection database from 1989 to 2013. Radon levels are often assessed when property is being bought or sold; much of the study data came from such measurements. The researchers evaluated associations of radon concentrations with geology, water source, season, weather, community type and other factors. 
 
 Between 2005 and 2013, 7,469 unconventional natural gas wells were drilled in Pennsylvania using hydraulic fracturing ("fracking") to liberate natural gas from shale. Up until recently, most natural gas wells were created by drilling vertically into porous zones of rock formations like sandstone to release the gas. These are known as conventional natural gas wells. In recent years, there has been a huge uptick in the drilling of unconventional natural gas wells in 18 states throughout the country. In contrast to the conventional wells, gas is not sitting in shale waiting to be pumped out. Instead, the gas is contained in the shale, which needs to be broken apart to release large volumes of natural gas. This is done by first drilling deeper into the ground vertically and again horizontally. Then, in the fracking process, millions of gallons of water containing proprietary chemicals are pumped in to help extract the gas. 
 
 The disruptive process that brings gas to the surface can also bring heavy metals and organic and radioactive materials such as radium-226, which decays into radon. Most indoor radon exposure has been linked to the diffusion of gas from soil. It is also found in well water, natural gas and ambient air. 
 
 Averaged over the whole study period, houses and other buildings using well water had a 21 percent higher concentration of radon than those using municipal water. Houses and buildings located in rural and suburban townships, where most of the gas wells are, had a 39 percent higher concentration of radon than those in cities. 
 
 Since radon is naturally occurring, in areas without adequate ventilation -- like many basements -- radon can accumulate to levels that substantially increase the risk of lung cancer. 
 
 The study's first author is Joan A. Casey, a Robert Wood Johnson Foundation Health & Society Scholar at the University of California-Berkeley and San Francisco, who earned her PhD at the Bloomberg School in 2014. She says it is unclear whether the excess radon in people's homes is coming from radium getting into well water through the fracking process, being released into the air near the gas wells or whether natural gas from shale contains more radon than conventional gas and it enters homes through cooking stoves and furnaces. Another possibility, she says, is that in the past decade buildings have been more tightly sealed, potentially trapping radon that gets inside and leading to increased indoor radon levels. In the past, most radon has entered homes through foundation cracks and other openings into buildings. 
 
 "By drilling 7,000 holes in the ground, the fracking industry may have changed the geology and created new pathways for radon to rise to the surface," Casey says. "Now there are a lot of potential ways that fracking may be distributing and spreading radon." 
 
 Natural gas typically travels via pipeline at 10 miles per hour, meaning radon can go statewide in one day. Radon has a half-life of about four days, meaning it has lost 95 percent of its radioactivity after 20 days. 
 
 The state of Pennsylvania recently took a comprehensive set of measurements near 34 gas wells, including air samples for radon near four wells, which did not show high levels of the radioactive gas. But the researchers say their study, which looks at levels in hundreds of thousands of homes and other buildings, is a better way to assess the potential cumulative impacts of all the wells. 
 
 "I don't think we can ignore these findings," Schwartz says. "Our study can be improved by including information that was not available for our analysis, such as whether natural gas is used for heating and cooking, whether there is any radon remediation in the building, and general condition of the building foundation. But these next studies should be done because the number of drilled wells is continuing to increase and the possible problem identified by our study is not going away." 
 
 ### 
 
 "Predictors of Indoor Radon Concentrations in Pennsylvania 1989-2013" was written by Joan A. Casey, Elizabeth L. Ogburn, Sara G. Rasmussen, Jennifer K. Irving, Jonathan Pollak, Paul A. Locke and Brian S. Schwartz. On April 9, this article will be available to download for free at http://ehp. niehs. nih. gov/ 1409014 . 
 
 The research was supported in part by the National Institutes of Health's National Institute of Environmental Health Sciences (R21 ES023675). 

 In this June 25, 2012 file photo, a crew works on a gas drilling rig at a well site for shale based natural gas in Zelienople, Pa. (Photo: Keith Srakocic, AP) 
 
 Levels of cancer-causing radon gas in Pennsylvania homes have increased as the fracking industry has expanded, a new study shows. 
 
 The study is a preliminary "first look" into a possible connection between fracking and radon, a naturally occurring radioactive gas, says co-author Joan Casey. While the study doesn't conclusively prove that fracking releases radon from the ground, the findings are concerning, says Casey, a researcher at the University of California-Berkeley and University of California-San Francisco. 
 
 Fracking, or hydraulic fracturing, has spurred a boom in oil and natural-gas production. The fracking process blasts millions of gallons of water -- mixed with sand and chemicals -- deep underground to break apart shale deposits and release natural gas. 
 
 While supporters of fracking says it's a safe source of energy, opponents are concerned that the process could contaminate local water supplies and even contribute to earth quakes. 
 
 Authors of the new study, published today in the journal Environmental Health Perspectives, say they focused on Pennsylvania because it has one of highest residential radon levels in the country, and because the state has a huge, detailed database of home radon measures. 
 
 Pennsylvania's high radon levels stem from the type of bedrock that runs through much of the state, which contains radioactive materials such as uranium and radium, which degrade into radon, an invisible gas, Casey says. 
 
 Radon can seep into basements through cracks in a home's foundation and become trapped in homes that aren't well ventilated. 
 
 Doctors are concerned about radon because it's the second-leading cause of lung cancer, behind only tobacco, says Janice Nolen, assistant vice president of national policy at the American Lung Association. Radon causes 21,000 lung cancer deaths a year, according to the Environmental Protection Agency. 
 
 Authors analyzed more than 860,000 indoor radon measurements from Pennsylvania's database, taken from Jan. 1, 1989 to Dec. 31, 2013. 
 
 Researchers found that radon levels fluctuated from 1989 to 2004. But radon levels in the state began to rise around 2004, when fracking really took off, the study says. 
 
 Authors also noticed that radon concentrations were 21% higher in buildings with well water than in those using municipal water. Radon can dissolve in water. So it's possible that radon enters homes through showers and faucets, then spreads into the air, says study coauthor Brian Schwartz, a professor of environmental health sciences at the Johns Hopkins Bloomberg School of Public Health in Baltimore. 
 
 Schwartz notes that it's possible that something other than fracking caused home radon levels to rise. For example, homes may have become more energy efficient since 2004. Although well-insulated homes save energy, they can also trap radon inside, he says. 
 
 A top industry group was unimpressed with the study. The Marcellus Shale Coalition, Pennsylvania's leading natural gas organization, provided USA TODAY with this statement: 
 
 "It's unfortunate, yet not unexpected, that some anti-shale activists continue to peddle profoundly flawed and unsubstantiated claims, such as this, based purely on hypothetic and perhaps pre-determined narrative-driven 'cause and effect' conclusions with the goal of generating fear," the statement read. "Thankfully, however, these suggestive scare tactics veiled as 'research' are easily refuted with readily available unbiased, fact-based data and independent scientific findings." 
 
 Authors of today's study acknowledge that their findings conflict with those of a January study from Pennsylvania's Department of Environmental Protection, which reported that "there is little potential for additional radon exposure to the public due to the use of natural gas extracted from geologic formations located in Pennsylvania." 
 
 Pennsylvania officials, however, say it's difficult to compare the two studies, because they measured radon in very different ways. 
 
 While Casey and Schwartz's paper included radon measurements from homes, the Pennsylvania state report measured radon at fracking wells, gas processing facilities, disposal sites and waste water processing facilities and other places, says the Pennsylvania Department of Environmental Protection's Ken Reisinger. The state report measured radon levels in the natural gas coming out of the ground, as well as in air near the fracking facilities. Radon levels weren't higher than expected, Reisinger says. 
 
 Reisinger questioned Casey and Schwartz' conclusion that fracking may be causing radon levels to rise. That's because their report also found rising radon levels in parts of the state with no fracking. 
 
 Casey and Schwartz say researchers should conduct more detailed studies to see if their findings can be confirmed. 
 
 Some health experts say the link between radon and fracking is worrisome. 
 
 "There are a tremendous number of poorly understood and potentially serious health risks associated with fracking, one of which is exposure to radioactivity," says Aaron Bernstein, a pediatrician and associate director of the Center for Health and the Global Environment at the Harvard H.T. Chan School of Public Health. "We simply do not have anything close to adequate safeguards for people's health." 
 
 Fracking has been linked to a wide spectrum of health problems for Americans across the country, according to a December report by the Natural Resources Defense Council, an environmental group. 
 
 That report said Americans who live near oil and gas drilling wells are exposed to fracking-related air pollution in the form of chemicals such as benzene and formaldehyde. 
 
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 Environ Health Perspect; DOI:10.1289/ehp.1409014 
 
 Predictors of Indoor Radon Concentrations in Pennsylvania, 1989–2013 
 
 Joan A. Casey,1 Elizabeth L. Ogburn,2 Sara G. Rasmussen,1 Jennifer K. Irving,3 Jonathan Pollak,1 Paul A. Locke,1,4 and Brian S. Schwartz1,3,5 
 
 Author Affiliations open 1Department of Environmental Health Sciences, and 2Department of Biostatistics, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA; 3Center for Health Research, Geisinger Health System, Danville, Pennsylvania, USA; 4Department of Health Policy and Management, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA; 5Department of Medicine, Johns Hopkins School of Medicine, Baltimore, Maryland, USA 
 
 PDF Version (1.2 MB) 
 
 Abstract 
 
 About This Article 
 
 Supplemental Material Background: Radon is the second-leading cause of lung cancer worldwide. Most indoor exposure occurs by diffusion of soil gas. Radon is also found in well water, natural gas, and ambient air. Pennsylvania has high indoor radon concentrations; buildings are often tested during real estate transactions, with results reported to the Department of Environmental Protection (PADEP). Objectives: We evaluated predictors of indoor radon concentrations. Methods: Using first-floor and basement indoor radon results reported to the PADEP between 1987 and 2013, we evaluated associations of radon concentrations (natural log transformed) with geology, water source, building characteristics, season, weather, community socioeconomic status, community type, and unconventional natural gas development measures based on drilled and producing wells. Results: Primary analysis included 866,735 first measurements by building, with the large majority from homes. The geologic rock layer on which the building sat was strongly associated with radon concentration (e.g., Axemann Formation, median = 365 Bq/m 3 , IQR = 167–679 vs. Stockton Formation, median = 93 Bq/m 3 , IQR = 52–178). In adjusted analysis, buildings using well water had 21% higher concentrations (β = 0.191, 95% CI: 0.184, 0.198). Buildings in cities (vs. townships) had lower concentrations (β = –0.323, 95% CI: –0.333, –0.314). When we included multiple tests per building, concentrations declined with repeated measurements over time. Between 2005 and 2013, 7,469 unconventional wells were drilled in Pennsylvania. Basement radon concentrations fluctuated between 1987 and 2003, but began an upward trend from 2004 to 2012 in all county categories (p < 0.001), with higher levels in counties having ≥ 100 drilled wells versus counties with none, and with highest levels in the Reading Prong. Conclusions: Geologic unit, well water, community, weather, and unconventional natural gas development were associated with indoor radon concentrations. Future studies should include direct environmental measurement of radon, as well as building features unavailable for this analysis. 
 
 Citation: Casey JA, Ogburn EL, Rasmussen SG, Irving JK, Pollak J, Locke PA, Schwartz BS. 2015. Predictors of indoor radon concentrations in Pennsylvania, 1989–2013. Environ Health Perspect 123:1130–1137; http://dx.doi.org/10.1289/ehp.1409014 Address correspondence to B.S. Schwartz, Johns Hopkins Bloomberg School of Public Health, 615 N. Wolfe St., Room W7041, Baltimore, MD 21205 USA. Telephone: (410) 955-4158. E-mail: bschwar1@jhu.edu We thank R.K. Lewis at the Pennsylvania Department of Environmental Protection, who facilitated the acquisition of the radon data. We also thank J.M. Crisp, who assisted with the assembling and cleaning of the Marcellus well data, and R.D. Peng, who provided statistical support. This research was supported in part by the National Institutes of Health (grant R21 ES023675). The authors declare they have no actual or potential competing financial interests. Received: 30 July 2014 
 
 Accepted: 31 March 2015 
 
 Advance Publication: 9 April 2015 
 
 Final Publication: 1 November 2015 Note to readers with disabilities: EHP strives to ensure that all journal content is accessible to all readers. However, some figures and Supplemental Material published in EHP articles may not conform to 508 standards due to the complexity of the information being presented. If you need assistance accessing journal content, please contact ehponline@niehs.nih.gov. Our staff will work with you to assess and meet your accessibility needs within 3 working days. 
 
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 Introduction 
 
 Exposure to radon-222—an inert, odorless, and carcinogenic gas—is the second leading cause of lung cancer worldwide (Darby et al. 2005; Pawel and Puskin 2004). The U.S. Environmental Protection Agency (EPA) estimates that indoor radon exposure causes or contributes to about 21,000 lung cancer deaths in the United States annually (Pawel and Puskin 2004). In 1986, the U.S. EPA set an action level of 148 Bq/m3 (4 pCi/L; there are 37 Bq/m3 per pCi/L) based on the current state of radon testing and mitigation technologies [National Research Council (NRC) 1999a; U.S. EPA 1992]. 
 
 Uranium-238 occurs naturally in soil and bedrock and decays to radium-226, which decays to radon. Both uranium-238 and radium-226 persist in the environment (half-lives of 4.5 billion years and 1,600 years, respectively). Radon-222 has a half-life of 3.8 days, and its radioactive decay products are responsible for its carcinogenicity. Pressure differentials between soil gas and indoor air cause the migration of radon through cracks and other openings into buildings, the primary source of indoor radon. Radium and radon are soluble in water, with concentrations increasing as salinity increases (Warner et al. 2012). 
 
 Several counties in eastern Pennsylvania overlie the Reading Prong, a physiographic section known to have high bedrock uranium concentrations (Gundersen 1991) and elevated indoor radon levels. The entire state has had some of the highest indoor radon levels in the United States. The Pennsylvania Department of Environmental Protection (PADEP) established a Radon Division that administers a program of radon monitoring and remediation (http://www.portal.state.pa.us/portal/ser​ver.pt/community/radon_division/21923). 
 
 U.S. Geological Survey (USGS) analysis of 548,547 indoor and short-term radon test results compiled by the PADEP during 1990–2007 reported that 39% of radon tests exceeded the U.S. EPA action level and that concentrations varied dramatically by geologic unit, a rock layer of a given lithology and geologic period (e.g., Annville Formation, high-calcium limestone from the Ordovician period) (Gross 2013). Geologists have identified 195 geologic units in Pennsylvania. Other factors that have been associated with higher indoor radon levels include the use of radon-rich well water [Folger et al. 1994; United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2009], colder months, less precipitation, more expensive housing, rural area, and higher individual socioeconomic status (SES) (Cohen and Gromicko 1988; Folger et al. 1994; UNSCEAR 2009). Radon is present in natural gas used for cooking and heating; calculations performed in the 1970s suggested that it would not be expected to result in an increase in indoor radon levels (Johnson et al. 1973). Radon can also enter buildings from ambient air; however, outdoor radon concentrations are generally low, around 10 Bq/m3, but can range from 1 to 100 Bq/m3 (UNSCEAR 2009). 
 
 The development of unconventional natural gas in the Marcellus shale in Pennsylvania has the potential to exacerbate several pathways for entry of radon into buildings. The USGS reported 91,020 Bq/m3 (n = 14) as the median radium concentration in produced water from Marcellus wells (Rowan et al. 2011), a value nearly 500 times the federal drinking water limit (185 Bq/m3) and one that far exceeds the industrial discharge limit of 2,220 Bq/m3. Underground, radon collects in porous geological formations and thus in natural gas production wells (Gogolak 1980). Shales also tend to contain both higher concentrations of uranium (3.7–40 ppm) than other geologic formations and higher concentrations of radon in their natural gas (Gogolak 1980). The USGS reported preliminary data from 11 wellheads in Pennsylvania with corrected concentrations of radon (devices were calibrated for air measurement, but used in natural gas with correction factor = gas measurement × 1.47) ranging from 37 to 2,923 Bq/m3 (median = 1,369) (Rowan and Kraemer 2012), suggesting that shale gas may have higher radon levels than other natural gas sources. 
 
 To our knowledge, no prior studies have evaluated predictors of radon concentrations in Pennsylvania. Our main objective was to identify the independent contribution to indoor radon concentrations of geologic unit, water source, building characteristics, season, weather, community SES, community type, and Marcellus shale development. 
 
 Methods 
 
 Study design. We obtained radon data on 1,983,705 indoor radon tests conducted in 806,469 buildings between 1987 and 2013 from all 67 counties in Pennsylvania; these tests were submitted by certified testers, laboratories, or homeowners to the PADEP Bureau of Radiation Protection, Radon Division. Buildings are most often tested during real estate transactions (World Health Organization 2009), and the PADEP requires reporting of all test results to their GreenPort website (http://www.depgreenport.state.pa.us/). We used the subset of radon measurements taken between 1 January 1989 and 31 December 2013 in our analysis because few samples were available from earlier years (n = 4,294) (Figure 1). The Institutional Review Board at the Johns Hopkins Bloomberg School of Public Health reviewed the study protocol and did not consider it to be human subjects research. 
 
 Figure 1 – Flow diagram of radon tests included in six primary models. View larger image (TIF File) 
 
 Outcome: indoor radon concentration. Data included the address of the tested building, building type (12 types; Table 1), test location (i.e., basement, first floor, second floor), test type (i.e., activated charcoal, alpha-track detectors, charcoal liquid scintillation, continuous radon monitors, electret ion chamber), test dates, and radon concentration (Bq/m3). Results were available for both short-term (2–7 days) and long-term (up to 1 year) testing periods. We used ArcGIS (version 10.0; Esri) and 10 street maps [e.g., TeleAtlas (TomTom), TIGER files (https://www.census.gov/geo/maps-data/dat​a/tiger.html), and StreetMap Premium (Esri) from 2000–2012] to obtain latitude and longitude of buildings. 
 
 We excluded tests from buildings that could not be geocoded to an address, that were out of state, that were not taken on the first floor or basement, or that appeared in the database more than once (n = 394,008 buildings). Many buildings (n = 307,245) had multiple radon measurements (range, 2–56) taken on the same floor and day. For example, in buildings with two measurements per floor (n = 291,098), the correlation of floor-specific measurements was very high (ρ = 0.91). Because we had no information on building remediation, our primary analysis included only measurements taken during the first test day at each building (n = 866,735, including 224,666 averaged concentrations from the same floor and day). In a sensitivity analysis, we included up to four tests over time from each building. 
 
 Data sources. We obtained data on the public water service areas compiled by the PADEP from the Pennsylvania State University’s Spatial Data Access website (Pennsylvania Spatial Data Access; http://www.pasda.psu.edu). Any home outside the public water supplier’s service area was assumed to use well water. Statewide bedrock geology and physiographic sections were downloaded as shapefiles from the Pennsylvania Department of Conservation and Natural Resources (PADCNR; http://www.dcnr.state.pa.us). On average, each geologic unit covers 749 noncontiguous square kilometers. One important geologic unit is the felsic gneiss, which is found throughout the state. The Reading Prong section primarily contains felsic gneiss; however, the section is present in only three counties, identified as Reading Prong counties in Figure 2. 
 
 Figure 2 – County category groupings, the Reading Prong section, and location of spudded Marcellus wells (through 2013). View larger image (TIF File) 
 
 We downloaded monthly average temperature and rainfall in 10 regions from the Pennsylvania State Climatologist (http://climate.psu.edu). Based on 2000 U.S. Census boundary files, buildings were assigned to a minor civil division: cities, moderate- to high-density boroughs, and suburban and rural townships. Community socioeconomic deprivation, an indicator of community SES, was derived from six z-transformed U.S. Census 2000 variables (Schwartz et al. 2011). Marcellus shale development data covering 1 January 2005 through 31 December 2013, came from PADEP and PADCNR, with the latitude and longitude of each well, the date of well drilling, natural gas produced, and number of producing days. 
 
 Marcellus shale development metrics. The Marcellus Formation is 1,500–2,500 m underground and underlies a large section of Pennsylvania from the southwest curling northeast. Only unconventional wells (horizontal wells, hydraulic fracturing) were included (Figures 2 and 3). 
 
 Figure 3 – (A) Number of new unconventional wells drilled annually during 2005–2013 (gray) and cumulative number of wells. (B) Unconventional natural gas produced (billions of cubic meters) during 2005–2013. View larger image (TIF File) 
 
 The spud date was the day well drilling began, and the production start date was the day the well first produced natural gas. We estimated a start date of production for each well: 
 
 Production start date at well i = (l p – k p )I p , [1] 
 
 where l p is the last day of production in period p, k p is the number of days in production in period p, and I p equals 1 when period p is the first period of production for well i, and 0 otherwise. We estimated daily natural gas production for each well in its first production period as the volume of gas produced in its first period divided by the number of days of reported production. In subsequent periods we estimated daily gas production as the volume of gas reported in each period, divided by the number of days in that production period. When wells were missing one or more production volumes by period, we imputed missing volumes for periods in which there were data before and after (n = 102 wells), assuming a linear decline over time. We imputed missing spud dates (n = 149 wells) using conditional mean imputation based on production start date, stimulation (hydraulic fracturing) date, year, and geographic region. 
 
 Two primary Marcellus development metrics were created based on all wells in the state, one on drilled wells and the other on wells in production. Wells drilled prior to the start of an indoor radon test were included in that building’s exposure assignment. Once a well was drilled, it was assumed to contribute until the end of the study period, 31 December 2013. We calculated drilled-well exposure assignment: 
 
 Building j metric = Σn 
 
 i = 1Σl 
 
 k = 1(I A (k,m)/d ij 2)/m, [2] 
 
 where n is the number of drilled wells, m is the duration of the indoor radon test in days, k is the day with 1 equal to 1 January 2005, and l is equal to 3,287 (to 31 December 2013), I A (k,m) is 1 when well i has been drilled before day k and the indoor radon test at building j is conducted from day k to day k + m, and 0 otherwise, and d2 ij is the squared-distance between the coordinates of the wellhead of well i and building j. We calculated the producing-well exposure assignment: 
 
 Building j metric = Σn 
 
 i = 1Σl 
 
 k = 1(I A (k,m)g p /d ij 2)/m, [3] 
 
 where n is the number of producing wells, m is the duration of the indoor radon test in days, k is the day with 1 equal to 1 January 2005 and l is equal to 3,287 (to 31 December 2013), I A (k,m) is 1 when well i is producing on day k and the indoor radon test at building j is conducted from day k to day k + m, and 0 otherwise, g p is the estimated amount of natural gas produced (in thousands of cubic meters) by well i on day k, and d2 ij is the squared distance between the coordinates of the wellhead of well i and building j. 
 
 Statistical analysis. The goal of the analysis was to evaluate associations of year, county category, geologic unit, community type, community SES, well water use, and metrics of unconventional natural gas development with indoor radon concentrations. Building was the unit of analysis. The distribution of radon concentrations was skewed, so we used natural log-transformed radon concentration (ln-radon) as our outcome variable to improve compliance with assumptions of linear regression (i.e., homoscedasticity and normality of residuals). We used one-way analysis of variance (ANOVA) to assess unadjusted differences in indoor radon concentrations by other covariates. To evaluate associations with indoor ln-radon concentration, we used multivariable linear regression and generalized estimating equations to account for within-building correlation when models included more than one measurement per building. When beta coefficients are < 0.1, 100 × β can be interpreted as approximating the percent change in radon concentration associated with a 1-unit change in the independent variable. In models used to assess the spatial distribution of radon levels, we wanted to remove the contribution of building-related factors. Models used to assess associations of unconventional natural gas development with radon levels did not contain county, minor civil division, or community SES because of concern about overadjustment. Covariates were included in models 1–4 because of a priori hypotheses that they could confound the relationship between our primary variables of interest and ln-radon concentration or based on the quasi-likelihood information criterion (Hardin and Hilbe 2013). 
 
 Model 1A included only measurements taken on the first test date at each building (n = 762,725 buildings and n = 866,735 radon values), which included some averaged values when multiple tests were performed on the same floor on the same day. Model 1A was adjusted for test year (1989–2013), test location (basement or first floor), well water use (yes or no), 13 building types (including “unknown”), test type (listed above), test duration, season, weather (average temperature and rainfall for 10 regions during the month radon measurement began with linear, quadratic, and cubic terms to account for nonlinearity), minor civil division, county (n = 67), and 179 mutually exclusive geologic units [reference group = Stockton Formation (n = 62,026) plus 12 geologic units with < 20 tests]. We used model 1B to evaluate changes over time in within-building basement radon levels by estimating model 1A, restricted to basement measurements, from up to four testing dates per building (n = 714,097 buildings and n = 1,015,764 radon values). We also assessed changes in radon levels over time for buildings with high initial concentrations by restricting model 1B to buildings with initial radon concentrations ≥ 740 Bq/m3 (n = 55,161 buildings and n = 99,293 radon values). 
 
 In model 2, we assessed differential changes in basement radon concentration by place and time by removing county from model 1A and restricting to basement radon values (n = 705,798 buildings and radon values). We ran five separate regressions by county category [Philadelphia, Reading Prong (which have no Marcellus activity), low Marcellus activity (< 100 wells drilled by 2014), high Marcellus activity (≥ 100 wells drilled by 2014), and no Marcellus activity] (see Supplemental Material, Table S1). We then plotted the predicted values of the geometric mean radon concentration by county category and year; 95% confidence intervals (CIs) were estimated using the delta method (Cox 1998). 
 
 We produced two maps of statewide basement radon concentrations for 2006–2013. The first displayed median radon concentrations per geologic unit (with ≥ 10 measurements). In the second, we removed variability due to building-level factors (which could help target remediation efforts to certain locations). We did this with model 3 by regressing ln-radon on building-level factors (i.e., year, building type, test type, test duration, season, average temperature and rainfall). In model 4, we fit a linear regression of the residuals from model 3 on only geologic unit, county, and well water use (n = 304,278 buildings and radon values) and then used model 4 to output new predicted radon concentrations in a 500-m × 500-m grid statewide. Split samples suggested that model 4 predicted well, and residual semivariogram plots did not exhibit spatial autocorrelation. 
 
 We used models 5 and 6 to evaluate two a priori hypotheses of the possible contribution of unconventional natural gas development on indoor radon concentrations: a) Ambient air could contribute to indoor radon concentrations through the release of radon and radium in the drilling process, primarily in the summer when buildings are more likely to be open; and b) produced natural gas containing radon could enter building air through use of natural gas for cooking or unvented heating and, given a transit speed of about 16 km/hr in pipelines (Gogolak 1980), all buildings in the state could be affected. 
 
 In model 5, we evaluated the associations of the drilled well metric (Equation 2) with ln-radon concentration by restricting model 1A to the years 2005–2013 (primary years of Marcellus development); measurements taken only during July, August, and September; and buildings located within 20 km of a drilled well at the time of the radon test. Because summer months had little variability in temperature, we did not include temperature in model 5. We also fit model 5 separately for first floor (n = 1,044 buildings and radon values) and basement (n = 18,123 buildings and radon values) because of hypotheses about pathways of radon entry. Model 5 excluded 3 first-floor and 130 basement radon concentrations from buildings located within 800 m of a well because we did not have enough data to fit a curve for distances < 800 m and 9 first-floor radon values that were outliers (studentized residuals > 3). As a counterfactual analysis, we re-ran model 5 including buildings from 1989 through 2005 that would be located within 20 km of a Marcellus well by December 2013. 
 
 To evaluate associations of the producing-well metric (Equation 3) and ln-radon concentrations, in model 6, we restricted model 1A to the years 2005–2013 and excluded buildings located within 800 m of a producing well (n = 315). Because year and the production well metric were highly correlated (ρ = 0.95), the regression models could not separate their independent influence; therefore, we presented model 6 production associations as unadjusted and adjusted for year, as well as year associations unadjusted for production. Regression analysis was performed using Stata 13 (StataCorp). We tested for linear trend by year by including year as a continuous variable. Alpha was set at 95%, and statistical significance was p < 0.05. Exposure metric creation and radon predictions were performed using R, version 3.0.0 (R Core Team 2013) and the sp package. 
 
 Results 
 
 Our primary analysis included 866,735 first indoor radon values from 762,725 buildings collected during 1989–2013. Every county reported results (see Supplemental Material, Table S2), with a median of 3,447 and ranging from 59 in Forest to 99,590 in Allegheny. Most (81.4%) of the values were from basements (n = 705,798), with a median concentration of 118.4 Bq/m3 [interquartile range (IQR) = 59.2–262.7]; 42.2% of these values in basements (n = 297,614) met or exceeded the U.S. EPA action level (Table 1). Radon concentrations varied within and between county categories across the study period, with Reading Prong counties having significantly higher and Philadelphia significantly lower radon concentrations. 
 
 In total, 7,469 unconventional natural gas wells were drilled in 39 Pennsylvania counties during 2005–2013 (Figure 3A). More than 5,000 of those wells entered production, producing 191 billion m3 of natural gas during 2009–2013 (Figure 3B). We identified 1,056 buildings with radon values from the first floor collected during the summer and located within 20 km of a drilled well at the time of the test. The median of the drilled well metric of these buildings was 0.6 wells/km2 (IQR = 0.2–1.3). The median of the producing-well metric of buildings statewide was 294 m3/day/km2 (IQR = 3–4,464). There were increasing median radon concentrations across quartiles of the production well metric for both the first floor and basement (Table 1). 
 
 In unadjusted analysis, several variables were associated with indoor radon concentrations: well water, building type, duration of test, season, weather during the test, community SES, community type, and county; geologic unit associations were strong, with large variation by unit [e.g., Axemann Formation, median = 365 Bq/m3 (IQR = 167–679), vs. Stockton Formation, median = 93 Bq/m3 (IQR = 52–178); Table 1]. Communities with lower SES had lower radon levels, but this variable was not included in subsequent models because of concerns regarding mediation (i.e., drilling improves individual SES and community SES, but richer individuals have more tightly sealed homes and higher radon concentrations). 
 
 In adjusted analysis (model 1A, n = 866,735 first basement and first-floor values), many variables were associated with radon concentrations. Strong associations were observed for specific geologies, for example Axemann, Bellefonte, and Nittany Formations were associated with 220–250% higher radon concentrations, compared with the Stockton Formation (see Supplemental Material, Table S2). Alpha track (generally long-term) and charcoal liquid scintillation tests were associated with 23% and 27% higher radon levels, respectively, compared with activated charcoal tests. Buildings using well water (vs. municipal water) also had 21% higher concentrations (β = 0.191; 95% CI: 0.184, 0.198). Buildings in cities versus townships were associated with lower radon levels (β = –0.323; 95% CI: –0.333, –0.314). There were nonlinear associations of rainfall and temperature; less rainfall and cooler temperatures were generally associated with higher radon concentrations. When up to four temporally ordered basement measurements per building were evaluated (model 1B, n = 1,015,764), we observed a significant decrease in radon concentration across tests, with a 37.1% (95% CI: 36.7, 37.3) decline from test 1 to test 2, 51.5% (95% CI: 51.1, 51.9) from test 1 to test 3, and 58.0% (95% CI: 57.4, 58.9) from test 1 to test 4 (see Supplemental Material, Table S3). Among buildings with an initial basement radon concentration ≥ 740 Bq/m3, we observed from the first test an 88.8% (95% CI: 88.6, 88.9) decline to the second test, and a 92.3% (95% CI: 92.1, 92.4) decline to the third test (see Supplemental Material, Table S4). 
 
 After controlling for confounding variables including geologic unit (model 2, basement values only), there was evidence of an upward trend from 2004 to 2012 (p < 0.001). Confidence intervals overlapped among the high, low, and no Marcellus activity counties, particularly between no activity and high activity counties before 2004, whereas there was little or no overlap after that time, with high activity counties having the highest estimated radon concentrations, followed by no activity and low activity counties, respectively. However, fewer measurements were taken in earlier years, resulting in less precise estimates with more variation from year to year (Figure 4). It should be noted that when both basement and first-floor values were included (model 1A; see Supplemental Material, Table S2) the upward trend began in 2006 (p < 0.001). There were large differences across the state in median radon concentrations by geologic unit (Figure 5A). Geologic unit and well water use did not appear to make large contributions to indoor radon concentrations in regions with many drilled Marcellus wells (Figure 5B, models 3 and 4). 
 
 Figure 4 – Geometric mean and 95% confidence intervals for indoor basement radon concentrations in five county categories, 1989–2013. High Marcellus shale counties had at least 100 unconventional wells drilled by 2013, and low Marcellus shale counties had 1–100. Predicted values were generated from five separate linear regression models (one for each county category) including only measurements taken on the first test date at each building (n = 705,798 values), adjusted for test year (1989–2013), well water use, 13 building types, five test types, test duration, season, weather (average temperature and rainfall with linear, quadratic, and cubic terms), minor civil division, and 179 mutually exclusive geologic units (model 2). View larger image (TIF File) 
 
 Figure 5 – (A) Unadjusted median basement radon concentrations (n = 304,278 tests) in Pennsylvania by geologic unit during 2006–2013. (B) Predicted contribution to basement radon concentration from geologic unit, county, and well water after accounting for variation due to year (2006–2013), building type, test type, test duration, season, average temperature, and average rainfall (based on models 3–4; n = 304,278 values). View larger image (TIF File) 
 
 The drilled well metric was significantly associated with first-floor summer radon concentrations in buildings located within 20 km of a drilled well; for each additional drilled well per square kilometer surrounding the building, first-floor radon levels were estimated to be 2.8% higher (drilled well β = 0.028; 95% CI: 0.001, 0.05) (model 5). We also found a positive, but attenuated, association with basement measurements (drilled well β = 0.010; 95% CI: 0.003, 0.020). In a sensitivity analysis, there was no association between the counterfactual drilled well metric for future wells and summer first-floor concentrations between 1989 and 2005 (drilled well β = 0.001; 95% CI: –0.022, 0.024). 
 
 The producing-well metric was not associated with indoor radon concentration when year was included in model 6 (production β = –0.001; 95% CI: –0.003, 0.002); when year was not associated, gas production was significantly associated with indoor radon concentration and radon concentrations were estimated to be 1.3% higher with each additional 100 m3 of natural gas produced per day per square kilometer (production β = 0.013; 95% CI: 0.005, 0.020). There was a positive association between year and radon concentrations between 2005 and 2013, when the production metric was removed from model 6 (year β = 0.012; 95% CI: 0.011, 0.014). 
 
 Discussion 
 
 We identified several predictors of indoor radon concentrations in Pennsylvania, a state with historically high radon levels (Alter and Oswald 1987). Water source, building type, test type, test duration, season, weather, county, and geologic unit were associated with indoor radon concentration. When data were aggregated to county categories, on average, Reading Prong counties had the highest indoor radon concentrations (Table 1, Figure 4). Nearly 300,000 homes had a first basement test result that exceeded the U.S. EPA action level. We observed fluctuating radon concentrations throughout the study period; low Marcellus activity counties consistently had lower radon concentrations than either high or no Marcellus activity, before and after drilling began. From 2005 through 2013 the high activity counties had higher basement radon levels than either low or no Marcellus activity counties, with confidence intervals that did not overlap, and there was evidence of a significant upward trend (Figure 4). In a model using first-floor and basement values and adjusting for each county (model 1A), radon concentrations only began increasing in 2006 (see Supplemental Material, Table S2). When we included multiple basement measurements per building, radon levels declined with repeated measurements within a building, which is good news for public health and also suggests that state remediation programs are effective. 
 
 Buildings located in cities had nearly 27% lower radon levels than those located in more rural townships (Table 2; see also Supplemental Material, Table S2). Previous work suggests that this difference is not due to weatherization of homes (Cohen and Gromicko 1988); it may occur because cities are sited in low-lying, alluvial sites, where radon levels are low (Briggs et al. 2008). However, the association persisted after adjustment for geologic unit and community SES. Buildings located in poorer communities also tended to have lower radon concentrations, consistent with past research (Cohen and Gromicko 1988). 
 
 We found that buildings using well water had 21% higher indoor radon concentrations than those using municipal water. The release of waterborne radon during showering or washing can contribute to concentrations in buildings. The NRC has estimated that 10,000 pCi/L (37,000 Bq/m3) of waterborne radon entering a building is needed to increase indoor air concentration by 1 pCi/L (37 Bq/m3) (NRC 1999b). Our 20% increase represented approximately 37 Bq/m3. An early study of Pennsylvania groundwater wells reported that only 10% exceeded 185,000 Bq/m3 (Swistock et al. 1993), putting our estimate at odds with the rule of thumb. 
 
 We found a statistically significant association between proximity to unconventional natural gas wells drilled in the Marcellus shale and first-floor radon concentration in the summer, with a positive—but attenuated—association for basement levels, which suggests a pathway through outdoor ambient air but does not rule out the possibility of radon moving from the basement to the first floor. Geographic location did not appear to account for the association because we did not find an association in buildings prior to 2006 that would be located near Marcellus wells in the future. Prior studies suggest that levels of radon in ambient air are low; our finding merits further study because the Marcellus shale is known to contain elevated levels of uranium (U.S. EPA 2008), and flowback water and reserve pit soil can contain elevated levels of radium, which could create an environmental exposure pathway (Rich and Crosby 2013; Rowan et al. 2011; Warner et al. 2012). It is also possible that radon could enter buildings through the use of natural gas containing radon. However, concentrations at the wellhead in Pennsylvania have a median of 1,369 Bq/m3 (Rowan and Kraemer 2012), much lower than the 37,000 Bq/m3 thought needed to increase radon concentrations by 12.2 Bq/m3 annually in homes that use gas appliances (Gogolak 1980). Our findings should be interpreted in the context of Pennsylvania’s recent Technologically Enhanced Naturally Occurring Radioactive Material (TENORM) study report from January 2015, which concluded that 
 
 There is little potential for additional radon exposure to the public due to the use of natural gas extracted from geologic formations in Pennsylvania. (Perma-Fix Environmental Services Inc. 2015) 
 
 However, the study did detect radon in several components of the unconventional natural gas development process and waste stream, such as natural gas, drill cuttings, and wastewater. 
 
 Our analysis had several limitations. We had no information on radon-resistant construction, construction year, types of remediation completed, type of heating and cooking systems, quantity of natural gas and water used in the building, degree of sealing of the building for energy efficiency, soil type near the building, wind speed and direction, or individual SES. These missing data make attributing increased radon levels to a particular source difficult. For instance, it is possible that the observed upward trend from 2004 to 2012 was simply the result of buildings being sealed more tightly during this time. 
 
 We did not know whether a radon professional or a homeowner performed each radon test. However, homes are usually tested during real estate transactions, and radon professionals generally perform these tests, ensuring impartial results. Tests are also performed when people are worried about their levels or want to retest after abatement. Worry about levels could introduce a form of selection bias sometimes observed in universal screening programs in which those with higher radon levels would be more likely to test first, which would account for the temporal trends up to 2005. We addressed the abatement concern by only including first measurements. In addition, our analysis should be considered exploratory because we did not perform any environmental radon measurements specifically directed at evaluating the Marcellus or well water hypotheses. 
 
 Conclusion 
 
 Radon continues to be a concern in Pennsylvania, and geology is an important contributor. Well water may contribute more to indoor radon than previously thought. There has also been a general rise in concentrations since 2006. The measurements of the Pennsylvania TENORM study should be periodically repeated given the projection of 60,000 wells in Pennsylvania by 2030 (Johnson 2010). Future studies of building radon levels should include more information about buildings, such as age, heating systems, remediation intervention, and radon-resistant construction. Radon exposure represents a major environmental health risk, and in addition to future studies to understand the impact of drilling on radon levels, there is continuing need for a radon program in Pennsylvania to track and evaluate radon concentrations and to encourage testing and remediation. 
 
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– "These findings worry us." That from the head of a new study out of Johns Hopkins Bloomberg School of Public Health suggests fracking could bring radon to the surface—raising levels of the odorless toxic gas in our homes. The researchers looked specifically at 860,000 readings taken in Pennsylvania homes, using a state database that covers 1987 to 2013. What they found per their study, published in Environmental Health Perspectives: "Basement radon concentrations fluctuated" during that period, "but began an upward trend from 2004-2012 in all county categories, higher levels in counties with ≥100 drilled wells vs. counties with none." That roughly coincides with when fracking began in the state: Between 2005 and 2013, 7,469 unconventional wells that use fracking to extract natural gas from shale were drilled there. A press release notes that homes and buildings in the rural and suburban areas where most unconventional natural gas wells are located had a 39% higher concentration of radon than their urban counterparts. The study doesn't pinpoint how the excess radiation got in; study author Joan A. Casey suggests one of three ways: via well water that's been affected by fracking, from air around the wells, or through the natural gas produced by these wells and used in homes. Still, USA Today notes that the study doesn't "conclusively prove" that fracking lets radon loose in the first place. As for why Pennsylvania, the authors studied it in part because it has one of America's highest residential radon levels; the state sits on bedrock that contains radium, which degrades into radon, the second-leading cause of lung cancer worldwide.