Lisa McKenzie of the Colorado School of Public Health and other researchers in 2014 came out with a study suggesting a link between higher rates of birth defects and how close mothers lived to natural gas development, the study included some limitations they acknowledged and critics highlighted.

Now McKenzie and colleagues are out with a new study analyzing some of the same potential health impacts, while addressing some of the shortcomings of the past research. And it's indicating an association between congenital heart defects in newborns and proximity to drilling — this time involving both gas and oil wells — that's even greater than their past research suggested.

"It gives us more confidence in the results. We addressed some of the limitations and we actually observe stronger associations," McKenzie, the study's senior author, said Friday.

The new, peer-reviewed research was published Thursday in the journal Environment International and involved researchers at Colorado School of Public Health, at the University of Colorado Anschutz Medical Campus. It found that mothers living near more intense oil and gas development activity have a 40 to 70 percent higher chance of having children with congenital heart defects than those living where there's less intense activity.

The study doesn't demonstrate a causal link between birth defects and oil and gas development, but the researchers say in the study that it "provides further evidence of a positive association" between the two, underscoring the need for further research. It says at least 6 percent of Coloradans live within a mile of an active well site.

The study focused on 3,324 infants born in Colorado from 2005-2011, and looked at infants with several types of congenital heart defects. It looked at mothers living in 34 counties where at least 20 wells were drilled per 10,000 births from 2004-11.

Weld County leads the state in drilling levels and has more than 21,000 active wells. Garfield County ranks second with nearly 12,000 active wells, and Mesa County has more than 1,200 active wells.

"I think given the studies that we've done and the studies that other researchers are doing in Pennsylvania and in Texas, we're starting to build an evidence base that … there might be potential for harm to health, particularly in early childhood pregnancy, for children living near oil and gas sites," McKenzie said.

Dan Haley, president and chief executive officer of the Colorado Oil and Gas Association, said in a prepared statement, "This study is not new. It's a re-examination of her 2014 report using the same old data from 2005 to 2011 — data that has no relevance to current regulations or to the common practices used by today's operators."

McKenzie has been researching possible health impacts of drilling in Colorado for years, and coming under industry criticism for her findings. A 2012 study she led, based on air sampling near well pads in Garfield County, found both non-cancer and cancer risks increase for residents living within a half-mile of wells.

The 2014 research analyzed nearly 125,000 live births in Colorado from 1996 to 2009. Among other things, it found that babies born to mothers living within a mile of at least 125 gas wells experienced a 30 percent greater prevalence of congenital heart defects, compared to those born to mothers living no closer than 10 miles from wells.

That study drew some criticism from the Colorado Department of Public Health and Environment. Dr. Larry Wolk, then the department's director, said at the time that the department disagreed with some of the associations the study identified between birth defects and drilling. He said some of the statistical differences in defects found in the study were minuscule, and pointed to limitations such as not knowing where mothers lived during the first trimester, when most birth defects occur.

The new study specifically focuses on where mothers lived from three months prior to conception through the second month of pregnancy. It also accounted for nearby drilling activity during that time period based on factors such as size of well sites, production volumes, and whether wells were being drilled, hydraulically fractured or producing. The earlier research didn't address varying drilling activities that can potentially affect the level of health impact.

John Putnam, CDPHE's environmental programs director, said in a statement released by the department Thursday, "We are unable speak specifically to this study because we haven't had the opportunity to read through it since it just came out today. This is an extremely important topic to the department.

"With the oil and gas industry, the laws continue to change and evolve, and it's important that our health research keep up and that we look at from the current landscape. Currently, we are pursuing even more assertive strategies to lessen the health and environmental impacts of the oil and gas industry through the implementation of the new oil and gas laws."

Said McKenzie, "What's important to remember is the defects are still pretty rare, which is a good thing."

According to the study, the national rate of congenital heart defects is 8.1 per 1,000 births. However, it says the rate in Colorado is 18.9 per 1,000.

It says congenital heart defects are the most common birth defect in the country and the leading cause of infant mortality due to birth defects, at a rate of 41.46 deaths per 100,000 live births.

It says fewer than a fifth of congenital heart defects are attributed to genetics, and animal models show such defects can occur due to a single environmental exposure during early gestation.

Haley said, "Interestingly, this study says particulate matter from oil and gas operations could lead to these health effects, but that contradicts the conclusions of another McKenzie study published just last month that found particulate matter levels near Colorado oil and gas operations were three times lower than EPA national air standards.

"Bottom line, the data is old and no air samples were taken. However, air samples that have been taken by Colorado's health department, for many years now, are conclusive. After thousands of thousands of air samples, many of which have been collected near oil and gas operations, not one exceeds state or federal protective health guidelines. Dr. McKenzie's studies have been called 'misleading' in the past, and this seems to be par for the course."

McKenzie said particulate matter isn't the only thing emitted from oil and gas sites, and the paper points to hazardous air pollutants such as benzene that she said are emitted from well sites at levels of concern. Those pollutants are suspected of playing a role in birth defects.

She acknowledged that over the years many laws and regulations have been passed in the state aimed at reducing oil and gas emissions, "which is great," she said. But she said research shows levels of some air pollutants remain at concerning levels, and intermittent problems such as malfunctioning valves result in "pretty high emissions."

With Colorado regulators starting to undertake a number of oil and gas rulemakings following passage of Senate Bill 181 this year, McKenzie said those efforts likely will focus on new drilling, and she suggested regulators also consider the hundreds of thousands of people living near existing oil and gas sites. She also thinks some thought should be given to zoning codes where housing is being built in relation to oil and gas facilities.

The new research continues to have some limitations. It doesn't consider the potential influence of traffic pollution and small-scale fixed pollution sources such as gas stations, or account for time mothers spent away from their homes, such as while working.

McKenzie said researchers were able to take into account large sources of pollution listed with the Environmental Protection Agency, and agricultural pollution.

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