By Donovan Schafer
The Colorado School of Public Health (CSPH) at the University of Colorado recently published an article in Science of the Total Environment presenting results from a study on air pollution due to oil and gas development (including hydraulic fracturing or “fracking”) in Garfield County.
Benzene and other ”potentially toxic” chemicals were found at concentrations potentially hazardous to human health, the article said. But before this study, or any similar study, can be taken as a basis for alarm, several questions should be answered: What are these “potentially toxic” chemicals? Where do they come from? And how dangerous are they really?
To answer these questions, it will be helpful to focus on just one chemical, benzene, which is the chemical most often associated oil and gas development, and in the case of the CSPH study, “the major contributor to lifetime excess cancer risk.”
Benzene is inextricably linked to oil and gas development because it is a natural hydrocarbon- like methane, propane, octane, and the hundreds of other chemicals in the mixtures we call “crude oil” and ”natural gas.”
Consequently, benzene also is found in gasoline, diesel fuel, and engine exhaust. And, because benzene is a powerful solvent, small amounts (in a mixture called “petroleum distillate”) are added to fracking fluids to make it easier for the other chemicals to dissolve.
Also, benzene is used as a feedstock in manufacturing other products, including nylon, plastics, polymers, resins, and adhesives.
Given these uses, it should not be surprising that benzene can be found everywhere, not just near oil and gas development.
“Benzene is ubiquitous in the atmosphere,” according to the U.S. Department of Health and Human Services. Not only does it come from tailpipes, but also cigarettes, volcanoes, forest fires, and even campfires.
Government agencies, however, are not alarmed by the benzene in our daily lives, because they recognize that the mere presence of a substance (the fact that a laboratory can physically detect it) does not automatically pose a threat to public health. It is equally important to determine what concentrations can actually do harm.
At what level, then, does benzene become a problem? The truth is, we don’t know. The EPA does the best it can to estimate the risks at various levels; however, its data is limited. In the case of benzene, the EPA uses a 1987 study, in which workers were exposed to concentrations of benzene literally thousands of times higher than the levels the EPA is trying to estimate.
The EPA then extrapolates (i.e. draws a best-fit line) from the data to estimate a concentration that will result in 10 additional cancer cases (not necessarily deaths) per million people exposed.
This is essentially the same as increasing the cancer risk of an individual by one-thousandth of a percent (0.001%).
For benzene, the EPA estimates that a 0.001% increase in cancer risk corresponds to exposures of 0.4 parts per billion (ppb) in the air and 10 ppb in drinking water. For even greater caution, the actual limit on drinking water, enforceable under the Safe Drinking Water Act, has been set at 5 ppb.
While a 0.001% risk may seem small to begin with, there are two critical assumptions built into these estimates that need to be remembered: First, the estimated concentrations represent the lowest concentrations within wide ranges of uncertainty.
As explained by the EPA, there is an “equal scientific plausibility” that the real levels of benzene that cause a 0.001% increase in cancer risk are 3-times higher than the current estimates.
Second, the estimates assume a person will be exposed to the same concentrations during their entire lifetime. This is especially unlikely in the case of benzene, which is biodegradable and does not last long in the air. (The CSPH study does make adjustments for the second assumption; however, this is often not the case in other studies, or at least, in how they are presented to the public.)
Understanding the assumptions built into EPA estimates is essential to evaluating studies like the CSPH one, because these studies almost always express their findings in relation to EPA estimates. Without such an understanding, the studies can give an exaggerated perception of the risks involved.
It is equally important to consider a study’s findings in context. The CSPH study calculated an increased cancer risk of 10 cases per million people living near oil and gas development.
However, according to the EPA’s National-Scale Air Toxics Assessment (NATA), the average increased cancer risk nationwide due to air pollution is 50 cases per million. The risk in Denver is even higher (80 per million) simply because it is an urban environment.
In Garfield County, where the CSPH study was conducted, the NATA risk is 20 per million.
Thus, a person living near oil and gas development in Garfield County will experience a total increased cancer risk of roughly 30 per million, far below the national average, and less than half the risk from living in an urban environment.
While benzene and other air pollutants should not be ignored when discussing oil and gas development, it is important for the public to recognize that estimates and limits set by the EPA represent very small -though perhaps not insignificant-risks, and that these risks are comparable to the risks associated with automobile emissions, urban living, and industrial activities in general.
This article originally appeared in the Denver Business Journal, June 22, 2012.
