A new study finds that pregnant women living near a fracking well are more likely to have children with severe congenital abnormalities. The question is why?

Around 20 million Americans live within 1 km of one of these – a fracking well.

Hydraulic fracturing – fracking- began in the 50s, but came into its heyday in the 90s, when the technique started being applied broadly to create fissures in deep shale deposits, liberating the natural gas and oil within. If the outcome of interest is the extraction of hydrocarbons, fracking has been an astonishing success – leading the US to its current position as a dominant force in world oil markets.

But if the outcome of interest is public health, the story is not so rosy.

There have been multiple studies suggesting a link between fracking and a variety of adverse health outcomes, including one – appearing in Nature Energy - which found an increased risk of death among elderly individuals living downwind of fracking sites.

But there have been signals of harm on the other end of the life cycle as well, with this Pennsylvania study suggesting that a pregnant woman’s proximity to a fracking well is associated with adverse birth outcomes.

We got some validation of those Pennsylvania findings this week, with this study, appearing in JAMA Pediatrics, which examined the outcomes of over 30,000 pregnancies in Alberta, Canada.

The analytic setup was fairly straight-forward, given that Canada’s universal health system captures data on all pregnancies. Women were classified, based on postal code, as living within 10km of a fracking well or not – you can see the density here along the line of shale deposits.

The question, then, is whether pregnant women who live close to fracking wells would have a higher risk of adverse birth outcomes.

The authors examined five adverse outcomes: sponataneous preterm birth, indicated preterm birth, small for gestational age, major congenital abnormalities, and severe neonatal morbidity or mortality.

After adjustment for multiple factors including, importantly, socioeconomic status, there was still a statistically significant increase in small for gestational age infants and infants with major congenital abnormalities.

To be fair, the mechanism here is not entirely clear. Fracking uses a slew of chemicals, many of which are classified as toxic to humans, but they are injected deep, deep underground – well below the level of aquifers and water sources. Groundwater contamination, when it occurs, is often due to spills on the surface.

The researchers also note that with a fracking well comes trucking. Trucking brings pollution, which also might affect pregnancies.

Whether the observed association is causal is still unclear, though the evidence is certainly mounting. To channel Bradford Hill for a minute, I can point out that the best evidence of causality – experiment- is not going to happen. No one will be randomized to living near a fracking well.

Hill would have liked to see a dose-response effect, whereby individuals living closer to wells would have worse outcomes than those living farther away. The study found no such effect when distance to the closest well was used as the exposure of interest.

Of course dose is not just distance. The authors did find that living near more wells is worse than living near fewer wells, with those living near greater than 100 wells at the highest risk of adverse pregnancy outcomes.

So no, this single study is not a smoking gun. In the context of prior studies, though, the gun is a bit smokier. The problem is that the political and economic forces supporting fracking are dramatically more powerful than those supporting the people who live near the wells, who are often quite disadvantaged – particularly in Canada where mineral rights default to the crown instead of the landowner. Until those individuals start to wield real political power, there will be no fracking ban. It’s not so different in the US.

If change will come, it will be by understanding mechanism. The next round of studies need to drill down, pardon the pun, to figure out if these associations are real, what drives them. Is it toxins in the water, or the air, or the soil? Or something else. Until the science is undismissable, I’m afraid it will be dismissed.

A version of this commentary first appeared on Medscape.com.