Hydraulic fracturing and infant health: New evidence from Pennsylvania
Abstract
The development of hydraulic fracturing (“fracking”) is considered the biggest change to the global energy production system in the last half-century. However, several communities have banned fracking because of unresolved concerns about the impact of this process on human health. To evaluate the potential health impacts of fracking, we analyzed records of more than 1.1 million births in Pennsylvania from 2004 to 2013, comparing infants born to mothers living at different distances from active fracking sites and those born both before and after fracking was initiated at each site. We adjusted for fixed maternal determinants of infant health by comparing siblings who were and were not exposed to fracking sites in utero. We found evidence for negative health effects of in utero exposure to fracking sites within 3 km of a mother’s residence, with the largest health impacts seen for in utero exposure within 1 km of fracking sites. Negative health impacts include a greater incidence of low–birth weight babies as well as significant declines in average birth weight and in several other measures of infant health. There is little evidence for health effects at distances beyond 3 km, suggesting that health impacts of fracking are highly local. Informal estimates suggest that about 29,000 of the nearly 4 million annual U.S. births occur within 1 km of an active fracking site and that these births therefore may be at higher risk of poor birth outcomes.
Introduction
The growth in unconventional gas production involving hydraulic fracturing (“fracking”) has transformed the energy landscape, reducing energy prices, decreasing conventional air pollution by displacing coal in electricity generation, disrupting international energy trading arrangements, and increasing the prospects for energy self-sufficiency for the United States.
At the same time, continuing concerns about the possible local health effects of hydraulic fracturing have led some states and communities to ban the practice altogether. The absence of a systematic evaluation of fracking’s health effects has complicated the decision process for those governments around the world who are debating whether to allow hydraulic fracturing.
Hydraulic fracturing could affect human health through several channels, including water and air pollution. In the fracking process, water and other chemicals are forced into shale rock to fracture it and allow the gas or petroleum trapped in the shale to be tapped. Whereas much of the previous research has focused on water pollution (1–3), several recent studies address the possible effects of chemicals that have been found in both “fracturing fluid” (the fluid that is forced into the shale in order to fracture it) and in air emissions near fractured gas wells (4–6). One study measured various air pollutants weekly for a year surrounding the development of a newly fractured gas well and detected nonmethane hydrocarbons, methylene chloride (a toxic solvent), and polycyclic aromatic hydrocarbons, which have been shown to affect fetal outcomes (7).
There are at least two reasons to focus particularly on infant health in probing the health effects of exposure to hydraulic fracturing. First, there is increasing evidence that the fetus is vulnerable to a range of maternal pollution exposures (8–13). Second, because the fetus is in utero for at most 9 months, it is possible to pinpoint the timing of potential exposure. This is not the case with other possible health effects, such as cancer, that develop over long periods of time. Moreover, birth data are available with precise information on mothers’ residential locations, permitting researchers to examine the effects of proximity to fracturing sites on the health of newborns.
This paper provides evidence for impacts of hydraulic fracturing on human health, based on a large-scale analysis of vital statistics records from more than 1.1 million births in Pennsylvania during the period 2004–2013. Our empirical approach compares infants born to mothers living at different distances from hydraulically fractured well sites, both before and after hydraulic fracturing was initiated at the well site. In addition, we probe the robustness of the results by adjusting the estimates for maternal fixed effects to include comparisons of siblings who were exposed to fracking with those who were not. Further, we explore the relationship between infant health outcomes and residential distance from fracturing sites, comparing birth data from mothers residing at increasing 1-km intervals from the fracturing sites to investigate whether there is a gradient in the effects of exposure.
The results of our analysis suggest that the introduction of fracking reduces health among infants born to mothers living within 3 km of a well site during pregnancy. For mothers living within 1 km, we find a 25% increase in the probability of low birth weight (birth weight < 2500 g) and significant declines in average birth weight and in an index of infant health. There are also reductions in infant health for mothers living within 1 to 3 km of a fracking site, but the estimates are about one-third to one-half of the size of those within the 0- to 1-km band. There is little evidence of health effects at further distances, suggesting that health impacts are highly local.
This paper addresses four problems that have plagued the previous literature (14–16). First, the sample size of this analysis is much larger than that used in previously published work. Second, in addition to examining low–birth weight status, which is the most commonly used measure of infant health in the literature, we use an index of infant health outcomes informed by the literature on multiple hypothesis testing (17, 18) to incorporate the many other measures of infant health that are available in the vital statistics data. Third, we test for effects at various distances of maternal residence from fracking sites, rather than imposing one arbitrary assumption about the distance where health impacts may become apparent, or about the functional form of the distance gradient.
An additional innovation is that our models control for mother fixed effects. Estimates of fracking-independent aspects of maternal health in these models are controlled by comparing the health of fracking-exposed and unexposed siblings born to the same mother. In principle, this comparative technique controls for all the unobserved time invariant characteristics of the mother such as race that could confound conventional difference-in-differences estimates (that is, before and after comparisons of places with and without fracking). However, in practice, the mother fixed effects estimates are imprecise because there are relatively few sibling pairs with an exposed and an unexposed sibling even when we are examining all Pennsylvania births.
Abstract
The development of hydraulic fracturing (“fracking”) is considered the biggest change to the global energy production system in the last half-century. However, several communities have banned fracking because of unresolved concerns about the impact of this process on human health. To evaluate the potential health impacts of fracking, we analyzed records of more than 1.1 million births in Pennsylvania from 2004 to 2013, comparing infants born to mothers living at different distances from active fracking sites and those born both before and after fracking was initiated at each site. We adjusted for fixed maternal determinants of infant health by comparing siblings who were and were not exposed to fracking sites in utero. We found evidence for negative health effects of in utero exposure to fracking sites within 3 km of a mother’s residence, with the largest health impacts seen for in utero exposure within 1 km of fracking sites. Negative health impacts include a greater incidence of low–birth weight babies as well as significant declines in average birth weight and in several other measures of infant health. There is little evidence for health effects at distances beyond 3 km, suggesting that health impacts of fracking are highly local. Informal estimates suggest that about 29,000 of the nearly 4 million annual U.S. births occur within 1 km of an active fracking site and that these births therefore may be at higher risk of poor birth outcomes.
Introduction
The growth in unconventional gas production involving hydraulic fracturing (“fracking”) has transformed the energy landscape, reducing energy prices, decreasing conventional air pollution by displacing coal in electricity generation, disrupting international energy trading arrangements, and increasing the prospects for energy self-sufficiency for the United States.
At the same time, continuing concerns about the possible local health effects of hydraulic fracturing have led some states and communities to ban the practice altogether. The absence of a systematic evaluation of fracking’s health effects has complicated the decision process for those governments around the world who are debating whether to allow hydraulic fracturing.
Hydraulic fracturing could affect human health through several channels, including water and air pollution. In the fracking process, water and other chemicals are forced into shale rock to fracture it and allow the gas or petroleum trapped in the shale to be tapped. Whereas much of the previous research has focused on water pollution (1–3), several recent studies address the possible effects of chemicals that have been found in both “fracturing fluid” (the fluid that is forced into the shale in order to fracture it) and in air emissions near fractured gas wells (4–6). One study measured various air pollutants weekly for a year surrounding the development of a newly fractured gas well and detected nonmethane hydrocarbons, methylene chloride (a toxic solvent), and polycyclic aromatic hydrocarbons, which have been shown to affect fetal outcomes (7).
There are at least two reasons to focus particularly on infant health in probing the health effects of exposure to hydraulic fracturing. First, there is increasing evidence that the fetus is vulnerable to a range of maternal pollution exposures (8–13). Second, because the fetus is in utero for at most 9 months, it is possible to pinpoint the timing of potential exposure. This is not the case with other possible health effects, such as cancer, that develop over long periods of time. Moreover, birth data are available with precise information on mothers’ residential locations, permitting researchers to examine the effects of proximity to fracturing sites on the health of newborns.
This paper provides evidence for impacts of hydraulic fracturing on human health, based on a large-scale analysis of vital statistics records from more than 1.1 million births in Pennsylvania during the period 2004–2013. Our empirical approach compares infants born to mothers living at different distances from hydraulically fractured well sites, both before and after hydraulic fracturing was initiated at the well site. In addition, we probe the robustness of the results by adjusting the estimates for maternal fixed effects to include comparisons of siblings who were exposed to fracking with those who were not. Further, we explore the relationship between infant health outcomes and residential distance from fracturing sites, comparing birth data from mothers residing at increasing 1-km intervals from the fracturing sites to investigate whether there is a gradient in the effects of exposure.
The results of our analysis suggest that the introduction of fracking reduces health among infants born to mothers living within 3 km of a well site during pregnancy. For mothers living within 1 km, we find a 25% increase in the probability of low birth weight (birth weight < 2500 g) and significant declines in average birth weight and in an index of infant health. There are also reductions in infant health for mothers living within 1 to 3 km of a fracking site, but the estimates are about one-third to one-half of the size of those within the 0- to 1-km band. There is little evidence of health effects at further distances, suggesting that health impacts are highly local.
This paper addresses four problems that have plagued the previous literature (14–16). First, the sample size of this analysis is much larger than that used in previously published work. Second, in addition to examining low–birth weight status, which is the most commonly used measure of infant health in the literature, we use an index of infant health outcomes informed by the literature on multiple hypothesis testing (17, 18) to incorporate the many other measures of infant health that are available in the vital statistics data. Third, we test for effects at various distances of maternal residence from fracking sites, rather than imposing one arbitrary assumption about the distance where health impacts may become apparent, or about the functional form of the distance gradient.
An additional innovation is that our models control for mother fixed effects. Estimates of fracking-independent aspects of maternal health in these models are controlled by comparing the health of fracking-exposed and unexposed siblings born to the same mother. In principle, this comparative technique controls for all the unobserved time invariant characteristics of the mother such as race that could confound conventional difference-in-differences estimates (that is, before and after comparisons of places with and without fracking). However, in practice, the mother fixed effects estimates are imprecise because there are relatively few sibling pairs with an exposed and an unexposed sibling even when we are examining all Pennsylvania births.
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