History of UNGD

While conventional drilling has been used to extract natural gas since the 1940s, unconventional gas wells have only been widely used since 2004, with 18% of global gas production now coming from unconventional sources (International Energy Agency, 2016). Conventional gas wells are drilled vertically and only access gas that has escaped from a high permeability source rock into a reservoir. Unconventional gas wells, on the other hand, use horizontal drilling and hydraulic fracturing to access natural gas still trapped in low permeability source rock, resulting in generally higher yields from these wells. Although hydraulic fracturing has existed in different forms since an “exploding torpedo” was first patented in 1865, the current process is much different from its early practice commercialized by Halliburton and Stanolind in the late 1940s. The current process has been termed “high-volume slick-water hydraulic fracturing” because the total volume of injected water and the many chemical additives (hence “slick”) have dramatically changed along with the conversion from vertical to horizontal drilling over the past several decades (Arthur, Bohm, & Layne, 2008; Montgomery & Smith, 2010). The large injected volumes and the range of chemicals used, initially not disclosed in areas of use, contributed to early concerns about the potential environmental and health impacts. This article will focus on operations to extract natural gas via horizontal hydraulic fracturing, which will henceforth be referred to collectively here as unconventional natural gas development (UNGD); this does not include the related but distinct practices of conventional drilling, extraction of shale oil, mining of tar sands, or “dry fracking.”

Benefits of UNGD

Natural gas, consisting of methane, is the cleanest burning of the fossil fuels, with lower emissions of carbon dioxide per unit of derived energy and virtually no release of combustion toxicants. The rise of unconventional shale gas has resulted in power plants switching from coal to shale gas for electricity generation resulting in lower emissions from the electricity sector. This rise in unconventional shale gas production in the United States has provided the country with a strong domestic energy industry and dramatically lowered the costs of fuel while creating local and national economic benefits. In addition to being used for electricity and residential heating and cooking, natural gas also has industrial uses and can be used as a fuel in transportation. Natural gas currently provides 29% of the total U.S. energy supply and accounts for approximately 33% of the country’s electricity generation (U.S. Energy Information Administration, 2017).

Growth of UNGD and Resultant Public Concerns

While the largest increases in natural gas production from UNGD have occurred to date in the United States, and particularly in the states of Pennsylvania and Texas, shale gas reserves are widely distributed throughout the world, with an estimated total global availability of around 7,500 trillion cubic feet (Figure 1) (U.S. Energy Information Administration, 2015).

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Figure 1. Global shale gas reserves.

Source: U.S. Energy Information Administration (2015).

UNGD has received a great deal of media attention in the United States, especially since the release of the documentary Gasland in 2010. Filmmaker Josh Fox interviewed residents of towns where UNGD was occurring in Pennsylvania, infamously showing these residents lighting their tap water on fire. Gasland was widely viewed in the United States over the following year, concomitant with a spike in public concern over UNGD. The New York Times and National Geographic also did extensive reporting on UNGD and environmental, community, and public health impacts. Informally referred to as “fracking,” UNGD has been highly politicized and information on the benefits and consequences highly questioned (Brantley & Meyendorff, 2013). The core of the argument in the political arena has often come down to “claimed” environmental and health consequences versus “claimed” economic benefits. Proponents of UNGD have sponsored several studies that have estimated high economic value and employment opportunities (Hoy, Kelsey, & Shields, 2017).

The UNGD Process

To understand what people who live near UNGD operations may be exposed to, it is important to understand the UNGD process (Figure 2). UNGD begins with the construction of roads and then the clearing of three to five acres of land for the well pad. Additional infrastructure built on or near well pads includes water holding ponds (formally termed impoundments) to hold water for hydraulic fracturing. This pre-drilling activity typically requires 400 to 2,000 truck trips to and from the well pad. To create the well, a rig drills down vertically to 7,000 to 10,000 feet before turning horizontally to drill for another 2,000 to 10,000 feet. Flexible steel is put down the hole to keep the well open and cemented to seal it, typically including four layers of steel pipe: conduction casing down to around 40 feet; surface casing down to around 50 feet below groundwater; intermediate casing down to around 1,000 feet; and production casing all the way to the end. A perforation gun is then sent down into the horizontal part of the well to perforate the steel to allow access to the shale.

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Figure 2. UNGD well pad timeline.

Source: United States Environmental Protection Agency (2016).

In the next phase, termed “hydraulic fracturing” or “stimulation,” millions of gallons of water are pumped into the well under high pressure mixed with a proppant (usually sand) and a number of chemical additives that vary in composition by drilling company and geology. Chemical additives include many classes of chemicals including friction reducers, gelling and foaming agents, antibacterial agents, surfactants, cross-linkers, breakers, pH buffers, iron control chemicals, clay protection chemicals, and scale inhibitors (Board on Chemical Sciences and Technology, 2015). Fracking with this mixture at high pressure opens and extends the fractures so that the natural gas can leave the formation and enter the drilling pipe. The injected frac fluid transports the proppant along the fracture length to promote patency of the fracture and, in some cases, transports radioactive tracers through the fractures to determine the injection profile and track the locations of fractures (Harper, 2008). Five to 30% of the fracking fluid injected returns to the surface as “flowback water” (Kondash, Albright, & Vengosh, 2017; United States Environmental Protection Agency, 2016), followed by “produced water” (i.e., water that is extracted along with gas from the shale at depth during production), which contains organic molecules as well as several inorganic ions (e.g., sodium, barium, strontium) eroded out of the geologic formations at depth (Vengosh, Jackson, Warner, Darrah, & Kondash, 2014). The drilling process and fracking fluids also acquire and bring to the surface a number of naturally occurring radioactive materials (NORMs) (Brown, 2014).

The well pad surface is prepared to receive natural gas and process (i.e., separate it from other organics and water vapor), compress (with diesel-powered compressor engines), store (in tanks with relief valves for off-gassing), and transport it via pipeline. Waste liquids from the water that returns to the surface, including the aforementioned flowback waters and also water that returns more slowly over many months, termed “produced waters,” are primarily contaminated by salts, organics, and TENORMs; this aqueous mixture is stored in the impoundments until it can be treated, reused, or disposed. This UNGD process from well pad to the start of natural gas production usually requires only several months. Production drops rapidly from unconventional wells, with generally around an 80% to 90% decline in gas production from the initial peak within the first three years (Hughes, 2018). This has important implications including that for the industry to increase gas production over time, many wells must be continually drilled. Wells also may be re-stimulated to boost production, so some of the early development concerns can happen later at the same well. Residents in UNGD areas typically experience development over many years, because of the large number of drilled wells at each well pad, which are closer to homes compared to wells for conventional gas production (Adgate, Goldstein, & McKenzie, 2014). After development, gas production is expected to last decades, with much less visible activity but many potential ongoing environmental and community impacts (Shonkoff, Hays, & Finkel, 2014), which will be discussed in the “Environmental and Community Impacts from UNGD” section. While Pennsylvania had approximately 8,000 wells in production, some have estimated that at full development, the state could have as many as 50,000 wells (Johnson, 2010).

Regulatory and Legal Issues

In the Energy Policy Act deliberations led by then Vice President Dick Cheney in 2005, several long-standing exemptions of the oil and gas industry from environmental regulations were expanded and extended (U. S. Congress, 2005). Termed “The Halliburton Loophole” because of Cheney’s prior position as CEO of Halliburton, the U.S. oil and gas industry was declared in this legislation to no longer be subjected to many or all provisions of a range of important environmental legislation, including the Clean Water Act, Safe Drinking Water Act, Superfund Act (CERCLA), and the Resource Conservation and Recovery Act (RCRA) (Patterson et al., 2017). This has forced the states to shoulder the bulk of responsibility for UNGD governance risks, with some local officials disputing control over land use, and most states are not fully equipped or motivated to adequately govern UNGD’s risks (Davis, 2017; Wiseman, 2014). Even among the air and water standards that apply to UNGD (mostly at the state level), there have been many violations of these regulations, and there is no legislation that governs inter-state waste disposal issues (Alawattegama et al., 2015; Rabe, 2014).

Water Quality Impacts

Early concerns surrounding UNGD focused on impacts on water resources, as evidenced by an extensive review of the subject by the U.S. Environmental Protection Agency, which showed that despite data gaps and uncertainties, there is sufficient scientific evidence that hydraulic fracturing activities are impacting drinking water in several ways (Figure 3). These were summarized as: (1) from water withdrawals being made in times or areas of low water availability; (2) large volumes or high concentrations of chemicals from produced water and other fluids onsite spilling; (3) hydraulic fracturing fluids being injected into wells with inadequate mechanical integrity as well as directly into groundwater; (4) hydraulic fracturing wastewater being inadequately treated and discharged into surface water; and (5) hydraulic fracturing wastewater being disposed of or stored in unlined pits and leaching into groundwater (United States Environmental Protection Agency, 2016). Another comprehensive review found that risks to water resources from UNGD activity also include the contamination of shallow aquifers with fugitive hydrocarbon gases and the accumulation of toxic and radioactive elements in soil or stream sediments near disposal or spill sites (Vengosh et al., 2014).

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Figure 3. UNGD well pad water activities.

Source: United States Environmental Protection Agency (2016).

Specific agents of concern that contaminate water include BTEX (butane, benzene, toluene, ethylbenzene, and xylene) and other volatile organic compounds (VOCs) (Drollette et al., 2015), and salts and metals (including mercury, bromide, iodide, and ammonium) (Grant et al., 2015; Harkness et al., 2015). Prior to UNGD, many areas with underlying shale gas reservoirs already had natural background levels of methane in groundwater due to upward migration of natural gas from the shale formation (Brantley et al., 2014). A series of studies has reported that methane and ethane levels in groundwater were higher from water wells that were closer to, particularly within 1 km, of UNGD wells; while the methane could be from preexisting biogenic sources, ethane is only from thermogenic sources (Jackson et al., 2013; Osborn, Vengosh, Warner, & Jackson, 2011; Warner et al., 2012) and thus had to arise from fossil fuel deposits. Further evaluation of seven candidate scenarios that could account for hydrocarbon gases in shallow aquifers identified faulty production casing and gas traveling through the void between the well casing and piping as the likely culprits (Darrah, Vengosh, Jackson, Warner, & Poreda, 2014).

Toxicity Concerns of Chemicals Used and Produced

Several reviews of the toxicity of chemicals used for UNGD have found Category 2 oral toxicants (second highest category of toxicity out of 4) (Stringfellow, Domen, Camarillo, Sandelin, & Borglin, 2014; Yost, Stanek, DeWoskin, & Burgoon, 2016a, 2016b) as well as reproductive and developmental toxicity (Elliott, Ettinger, Leaderer, Bracken, & Deziel, 2017) and carcinogenicity (Yost, Stanek, & Burgoon, 2017). A series of water quality as well as in vitro and in vivo studies have also generated concern for the potential reproductive health impacts (Kassotis, Bromfield, et al., 2016; Kassotis, Iwanowicz, et al., 2016; Kassotis et al., 2015; Kassotis, Tillitt, Davis, Hormann, & Nagel, 2014). While many of these chemicals used in and produced from hydraulic fracturing are known to be toxic, many of them are lacking well-characterized exposure pathways to impacting human health, and others may be present in doses that are likely too low to cause a health response.

Physical Hazards

Physical hazards from UNGD activities include high noise from drilling, hydraulic fracturing, and increased truck traffic (Goodman et al., 2016); light pollution from flaring of natural gas and also from large lights illuminating the well pad and far beyond at night (Kiviat, 2013); radioactive elements from drill tailings and flowback fluids (Casey et al., 2015; Field et al., 2014; Konkel, 2015); and induced seismicity from deep underground injection (Carpenter, 2016). Exposure to excess noise, light, and vibration can potentially impact health by disturbing sleep, exposure to increased earthquakes can potentially impact health through injuries as well as through stress pathways and anxiety, and exposure to radioactive materials can potentially impact health through various outcomes, greatly depending on the intensity, frequency, and duration of the exposure (Hays, McCawley, & Shonkoff, 2017; Zhang, Hammack, & Vidic, 2015).

Community/Social Impacts

UNGD in a community has many potential impacts on its built and social environments, including changing roads and inhabitants, decreasing green space, and increasing traffic that lead to changes in social capital, social support, crime, civic engagement, employment opportunities, and land and home values (Adgate, Goldstein, & McKenzie, 2014). The increased truck traffic is often a major change and has the potential to lead to more motor vehicle accidents (Goodman et al., 2016). Home value changes can also be major, with one study showing that the price of a house using groundwater and within 2 km of a spudded well lost, on average, $16,059 of value, but the price of a house with public water and within 2 km of a well gained $5,070 on average (Muehlenbachs, Spiller, & Timmins, 2015). Focus groups conducted with residents of UNGD-impacted communities of West Virginia found that UNGD contributed to a disruption in these residents’ sense of place and social identity, generating widespread social stress (Sangaramoorthy et al., 2016). Another study analyzed 215 letters to the editor in the newspaper from the most heavily drilled county in Pennsylvania and found that these letters showed that citizens were facing discord and stress in four major areas: socioeconomic impacts, perceived threats to water, population growth and implications, and changes to the rural landscape (Powers et al., 2015).

Air Quality Impacts

A comprehensive literature review conducted by the Natural Resources Defense Council found that fifteen different UNGD processes and sources—including the drilling process, wastewater, and condensate tanks—release air contaminants including

These air quality impacts are of concern because, in studies from related industries, exposure to these elevated levels of diesel PM, H₂S, and VOCs has been linked to eye, nose, and throat irritation, respiratory illnesses, cardiovascular problems, central nervous system damage, birth defects, cancer, or premature death (Macey et al., 2014). While some of these air contaminants mainly have negative impacts in communities close to well pads, significant amounts are emitted from natural gas infrastructure located offsite (Litovitz, Curtright, Abramzon, Burger, & Samaras, 2013), as well as from several thousand truck trips per well pad. These truck trips create diesel PM, VOC, CO₂, and NO_x emissions over wide areas away from the pad causing regional air quality impacts including ground-level ozone (McCawley, 2015). Ozone exposure is associated with a variety of respiratory and cardiovascular effects, and its formation is highest from UNGD activity in the winter (Edwards et al., 2014). Regulations on air quality continue to evolve including the USEPA issuing performance standards for VOCs emitted from new wells in 2015 (United States Environmental Protection Agency, 2012). However, air quality is still a concern for health, especially since EPA monitoring relies heavily on self-reported emissions data, the rules only apply to new gas wells, and protocols used for assessing compliance with air standards generally do not adequately determine the intensity, frequency, or durations of actual human exposures to the mixtures of toxic materials released regularly at UNGD sites (Brown, Weinberger, Lewis, & Bonaparte, 2014).

Climate Change Implications

Natural gas is a fossil fuel that can contribute to climate change in several ways including from (1) clearing of land (i.e., the loss of carbon dioxide sinks from forests); (2) fugitive emissions (i.e., emissions of methane from pressurized UNGD equipment due to leaks during drilling, storage, and transport operations); and (3) combustion during use. Direct measurements of methane emissions at 190 onshore natural gas sites in the United States found 957 Gg (gigagrams) in emissions for completion flowbacks, pneumatics, and equipment leaks; coupled with EPA national inventory estimates for other categories, the authors estimated natural gas production contributed 2,300 Gg of methane emissions per year (Allen et al., 2013). Subsequent investigation expressed concern about the substantial amount of methane (which has ~20x the global warming potential as carbon dioxide over 100 years [Howarth, 2015]) that leaks from UNGD operations and how there is a clear need to help obtain better fugitive emissions data and to increase efforts on reducing methane leakage in order to minimize the climate footprint of natural gas (Jiang et al., 2011). Estimates for fugitive emissions from UNGD (coal and petroleum do not have fugitive emissions) range from 1% to 6% and some estimate that if fugitive emissions exceed 3% or so then UNGD and subsequent natural gas use are worse for climate than is coal use (Howard, 2015).

Researchers compared the cumulative radiative forcing (defined as the difference between sunlight energy absorbed by the Earth and energy radiated back to space, measured in watts per square meter over the planet’s surface) created by alternative technologies fueled by UNGD compared to other fossil fuels using the best available estimates of greenhouse gas emissions from production, transportation, and use of each. They found that while using natural gas instead of coal for electric power plants can reduce radiative forcing immediately, a shift to compressed natural gas vehicles from gasoline or diesel vehicles leads to greater radiative forcing of the climate for 80 or 280 years, respectively, before beginning to produce benefits but that compressed natural gas vehicles could produce climate benefits sooner if fugitive emissions were prevented (Alvarez, Pacala, Winebrake, Chameides, & Hamburg, 2012). The impacts of climate change on health have been the subject of public health research for decades and include heat-related illness and death, increased respiratory diseases, increases in vector-borne diseases, increased mental health impacts from forced migration and civil conflict, and health impacts from severe weather events (Frumkin & McMichael, 2008; Haines, Kovats, Campbell-Lendrum, & Corvalán, 2006; Luber & Lemery, 2015; Patz et al., 2000).

Study Designs

The study designs employed are mostly retrospective cohorts (n = 6), followed by ecologic (n = 5), cross-sectional (n = 3), prospective longitudinal cohort (n = 1), and nested case-control (n = 2). Most were conducted with study populations in Pennsylvania (n = 13) plus one each in Colorado and Ohio, and two in Texas. Unfortunately, several of these studies had limitations, including three that only analyzed population-level data allowing inferences about predictors of population rates and not individual-level causes (Graham et al., 2015; Jemielita et al., 2015; Deziel et al., 2018); two that evaluated cancer that did not allow a sufficient latency period (time from first exposure to development of clinical disease) to pass before evaluating associations with cancer outcomes that can require many decades to appear after an exposure (Finkel, 2016; Fryzek et al., 2013); one that included primarily conventional wells in their well metric (McKenzie et al., 2014); and two which assessed both UNGD activity or exposure and the health outcome by self-report leading to same-source bias (Ferrar et al., 2013; Saberi, Propert, Powers, Emmett, & Green-McKenzie, 2014).

Measurement of the Outcome

The remaining nine studies categorized study participants by UNGD, identified a health outcome, and evaluated associations between UNGD and the health outcome. All of these studies necessarily focused on health outcomes that had a biologically plausible way in which UNGD could affect the outcome with a short latency period; the studies also focused on health outcomes that had a relatively high prevalence, since these are the ones that should first appear and also be amenable to study. The measurement of the health outcome varied: two used self-reported information from questionnaires and found positive associations for various symptoms including dermal and respiratory symptoms (Rabinowitz et al., 2015) as well as chronic rhinosinusitis, migraine, and fatigue symptoms (Tustin et al., 2017); five used birth records from governmental registries and found positive associations with various pregnancy and birth outcomes (Currie, Greenstone, & Meckel, 2017; Hill, 2012; Stacy et al., 2015; Whitworth, Marshall, & Symanski, 2017; Whitworth, Marshall, & Symanski, 2018); and three applied analytics to electronic health records (EHR) data to find these associations including with preterm birth and high-risk pregnancy (Casey et al., 2016) and asthma exacerbations (Rasmussen et al., 2016). One of the studies that used questionnaires had a limited sample size (n = 180), while the other was a large sample size (n = 7,785); for both studies, the assessment of the health outcome was not clinician verified. Using EHR data as well as governmental databases to identify cases allowed researchers to objectively obtain large sample sizes (n = 15,451 to 1,125,748), but using these strategies necessarily focused the studies on health conditions that were severe enough for patients to seek health care.

Measurement of UNGD Activity or Exposure

A key requirement of environmental epidemiology is the need to rank subjects along a gradient of exposure or dose. Exposure is defined as hazardous agents that are external to the human body and can be measured, for example, as concentrations of toxicants in air or sound levels for noise. Hazardous agents must have an exposure pathway to the human body and then are internalized through inhalation, ingestion, or dermal absorption. Exposure is considered in terms of the intensity (i.e., concentration), frequency, and duration of the exposure. Dose is defined after the exposure becomes internalized, and can be measured, for example, as blood or urine levels of toxicants or their metabolites. These measurements can be expensive, especially in large-scale studies, and current measurements are difficult to make relevant retrospectively for health outcomes that occurred many years in the past. Environmental epidemiology studies attempt to assign measures of recent or cumulative exposure or dose, depending on how the health outcome is hypothesized to occur, to individuals, and then perform analysis to evaluate whether risk of illness is higher or severity of illness is worse as exposure or dose is estimated to be higher.

A complexity of research on the potential health impacts of UNGD is that UNGD could affect health through several different hazardous exposures and through behavioral or stress pathways. As discussed in the “Environmental and Community Impacts from UNGD” section, a variety of chemical and physical hazards are introduced to communities during development, and community impacts (e.g., industrialization, truck traffic, transient workforces) can influence behaviors or stress pathways. These in turn can affect health alone or especially in combination. Methods are available to estimate exposure or dose in individuals from each of these pathways, but studies that considered them all would be prohibitively expensive and could not be done retrospectively.

Most existing studies have thus used surrogates of exposure, in two primary categories. In the first, subjects were asked about what they saw and experienced around their residences and communities and this self-reported information was used to estimate proximity, intensity, frequency, and duration of exposure. Studies that used self-reported information about exposure and health outcomes are subject to same source bias, in that associations between exposure and health can be observed but are spurious due to biased reporting in persons concerned about what they are seeing in their communities and linking it to perceived health impacts. The second major approach was to use publicly available information on well locations, phase of development (e.g., well pad, drilling, stimulation, production), dates of development, well depth (a surrogate for volume of injected fluids and number of truck trips to site), and gas production (a surrogate for compressor engine activity and possibly VOC off-gassing), impoundment size and locations (a surrogate for VOC off-gassing from ponds), and compressor stations (locations, dates, number of compressor engines, estimated emissions from compressor engines). This information was available in space and time and geographic information systems (GIS) were used to link this UNGD activity to the residential address of study subjects. GIS-based approaches to UNGD activity characterization usually were based on proximity models, in which the inverse of distance or distance-squared is used to weight the information about the UNGD wells and infrastructure surrounding the residential address.

Health Impacts With the Most Evidence of Concern: Pregnancy and Birth Outcomes and Asthma Exacerbations

As of mid-2018, there have been six primary health studies investigating the potential impact of UNGD on pregnancy and birth outcomes (McKenzie et al., 2014 is not included here because most wells in Colorado at the time of the study were conventional and the information available at the time could not distinguish conventional from unconventional [McKenzie et al., 2014]). Hill used the gradual introduction of UNGD wells in Pennsylvania as a natural experiment to identify the potential impacts on infant health (Hill, 2012). The study examined the natality records of 2,437 singleton births to mothers residing within 2.5 km of a UNGD well in the state from 2003 to 2010. The authors compared birth outcomes (including low birth weight, premature birth, small for gestational age, and five-minute Apgar scores) before and after a gas well was completed for these mothers. Their results suggested that prenatal proximity to UNGD increased the overall prevalence of low birth weight by 25%, increased overall prevalence of small for gestational age by 17%, reduced five-minute Apgar scores, and was not associated with premature birth.

Stacy and colleagues investigated records for 15,451 live births in southwest Pennsylvania from 2007 to 2010 to examine the association of proximity to UNGD (mothers categorized into exposure quartiles based on inverse distance weighted well count) and perinatal outcomes (birth weight, small for gestational age, and prematurity, accounting for differences in maternal and child risk factors) (Stacy et al., 2015). The study found no significant association of UNGD activity with prematurity, but the authors found a positive association of UNGD activity with lower birth weight and a higher prevalence of small for gestational age (highest quartile had 34% increased odds over the lowest quartile).

Casey and colleagues conducted a retrospective cohort study examining the association between exposure to UNGD activity (cumulative exposure estimated with an inverse-distance squared model incorporating distance to the mother’s home; dates and durations of well pad development, drilling, and hydraulic fracturing; and production volume during the pregnancy) and birth outcomes (term birth weight, preterm birth, low five-minute Apgar score, small size for gestational age birth, and physician-recorded high-risk pregnancy, controlling for potential confounding variables) (Casey et al., 2016). The researchers analyzed EHR data on 9,384 mothers and their 10,946 neonates in Pennsylvania from 2009 to 2013 and found an association between UNGD activity and preterm birth that increased across quartiles (highest quartile had 40% increased odds over the lowest), an association with physician-recorded high-risk pregnancy (highest quartile had 30% increased odds over the lowest), and no associations with Apgar score, small for gestational age birth, or term birth weight.

Whitworth, Marshall, and Symanski have conducted both a retrospective cohort study (2017) and a nested case control study (2018) in Texas, using the same exposure method as Casey et al. (2016). The research team analyzed the association between UNGD and birth outcomes using birth certificates from the Texas Department of State Health Services for over 150,000 singleton babies born between November 2010 and November 2012 and adjusting for confounders including maternal age, race/ethnicity, pre-pregnancy BMI, and adequacy of prenatal care. In their retrospective cohort study, they found 14% to 15% increased adjusted odds of preterm birth associated with UNGD activity in the highest tertiles of the half, two, and ten-mile metrics; 56% increased adjusted odds of fetal death in the second tertile of the two-mile metric and 34% increased adjusted odds in the highest tertile of the 10-mile metric. In their nested case-control study, they found increased adjusted odds of preterm birth in the third tertile of UNGD activity, with strongest associations between UNGD in the production phase and women in trimesters one and two of pregnancy.

Currie, Greenstone, and Meckel conducted a retrospective cohort study (2017) with a cohort of 1,125,748 infants from singleton births in Pennsylvania during the period from 2004 to 2013. Testing various distances of maternal residence from UNGD sites, analyzing data from Pennsylvania birth certificates, and adjusting for confounders including maternal marriage, maternal education, maternal age, and child sex in a difference-in-differences approach, they found that the introduction of UNGD reduces health among infants born to mothers living within three kilometers of a well site during pregnancy. They also found a 25% increase in the probability of a low-birth-weight birth for mothers living within one kilometer of a UNGD site and significant declines in average birth weight, as well as in an index of infant health. Overall, the three birth outcome studies have consistently revealed associations of UNGD activity with adverse birth outcomes. There were some methodologic differences between the studies that could explain the contrasting associations, but these health outcomes are the only ones for which multiple independent studies can be evaluated as a body of evidence. The findings, while not conclusive, are strong preliminary evidence that UNGD is associated with adverse birth outcomes from pregnancies in UNGD areas.

Asthma is a suitable health outcome to study in relation to UNGD because it is well known that air pollution and stress can both exacerbate asthma. This happens with short latency after the increase in air pollution or activities that promote stress, and exacerbations often end up bringing patients to see a health professional which provides a record in EHRs with dates, diagnoses, and treatments. To evaluate the association between UNGD and asthma exacerbations, Rasmussen and colleagues conducted a nested case-control study comparing 35,508 asthma patients treated at the Geisinger Clinic from 2005 to 2012 (Rasmussen et al., 2016). Patients were identified in EHRs and cases (asthma patients with exacerbations) were frequency matched with controls (asthma patients without exacerbations) on age, sex, and year of event. Cases were categorized into three groups based on mild exacerbation (new oral corticosteroid medication order), moderate (emergency department encounter), or severe (hospitalization). UNGD activity was assessed based on the distance from the patient’s home to each well and the well’s estimated activity metrics for four UNGD phases (well pad preparation, drilling, hydraulic fracturing, and production) on the day before each patient’s index date (date of exacerbation for cases, contact date for controls). The UNGD metrics also accounted for number of wells, for all wells in the state, and the size of the well. The analysis adjusted for several confounders and found associations between the highest quartile of the activity metric for each UNGD phase versus the lowest group for 11 of 12 UNGD-outcome pairs, with increased odds of 50% to 440% (the highest for the association of the production metric with mild exacerbations); several of the associations evidenced exposure-effect trends, with increasing odds along increasing quartiles of UNGD activity. They concluded that residential UNGD activity metrics were statistically associated with increased risk of mild, moderate, and severe asthma exacerbations.