Fracking involves drilling horizontally into shale and injecting gallons of water at high pressure to fracture the shale and releasing natural gas.
This finding was obtained following an analysis of southwestern Pennsylvania birth records. The results of the study, published in PLOS ONE and funded by the Heinz Endowments, suggest that further research should be conducted on this association.
"Our work is a first for our region and supports previous research linking unconventional gas development and adverse health outcomes," says co-author Dr. Bruce Pitt. "These findings cannot be ignored. There is a clear need for studies in larger populations with better estimates of exposure and more in-depth medical records."
The term unconventional gas drilling includes the practices of horizontal drilling and high-volume hydraulic fracturing that are commonly referred to as fracking. The process is growing in popularity as it allows companies to access previously inaccessible pockets of natural gas trapped in deposits of shale.
In Pennsylvania's Marcellus Shale formation, only 44 wells were known to be drilled before 2007. From 2007-2010, this number increased rapidly to 2,864 wells.
There have been many concerns about the effects of fracking on the environment, with studies conducted assessing the amount of polluting hydrocarbons in the air near fracking sites and levels of thermogenic methane found in shallow drinking water sources.
Despite these concerns, the authors state that there remains a lack of information linking potential exposures with public health risks.
The researchers gathered data on natural gas wells and births for the Butler, Washington and Westmoreland counties of Pennsylvania for 2007-2010, including a total of 15,451 births. They then divided the birth data into four groups, based on the number and proximity of gas wells within a 10-mile radius of the mothers' homes.
Findings held after adjusting for influential factors such as maternal smoking and prenatal care
After analysis, the researchers found that the group of mothers who lived closest to a high density of fracking wells were 34% more likely to give birth to infants who were small for gestational age - babies with birth weights below the smallest 10% when compared with their peers - than the group of mothers who lived farthest away.
This finding still held after adjusting the results for other factors that could influence birth weight, including prenatal care, race and whether the mother smoked or not.
"Developing fetuses are particularly sensitive to the effects of environmental pollutants," suggests Dr. Pitt. "We know that fine particulate air pollution, exposure to heavy metals and benzene, and maternal stress all are associated with lower birth weight."
Many aspects of the fracking process are believed to be polluting in southwestern Pennsylvania. Waste fluids known as "flowback" can contain benzene, and the flaring of methane gas at well heads can release volatile compounds such as toluene, ethylbenzene and xylene into the air.
A number of limitations with the study must be taken into account. The proximity of living areas to the gas wells is not a precise measure of exposure to pollutants, and there are several unmeasured factors that may also have influenced the results.
Dr. Pitt says it is important to stress that their study does not confirm that these pollutants caused the lower birth weights, however.
"Unconventional gas development is dynamic and varies from site to site, changing the potential for human exposure," he states. "To draw firm conclusions, we need studies that thoroughly assess the exposure of a very large number of pregnant women to not just the gas wells, but other potential pollutants."
The association between proximity to fracking sites and decreased birth weight was small but significant, the authors conclude. Although inconclusive, it represents an interesting starting point for inquiries into this area.
Previously, Medical News Today reported on a study demonstrating that at levels found in typical urban environments, ultrafine airborne particles can affect heart function within minutes of being inhaled.