October 07, 2005
It's been found in salmon, polar bears and dolphins. It's been found in the Great Lakes, the Arctic and the Mediterranean. It's been found in apples, green beans, bread and ground beef. It's been found in the bloodstreams of people worldwide.
It's perfluorooctanoic acid (PFOA), a persistent, bioaccumulative compound that has come under scrutiny from the Environmental Protection Agency because of its as yet unknown potential for toxicity in human beings. The EPA issued a preliminary risk assessment regarding PFOA in 2003, but there are still more questions than answers about the chemical's effects on human beings and how it has come to be so pervasive in the global environment.
In an attempt to answer some of those questions, University of Illinois atmospheric scientist Donald Wuebbles uses computing resources at NCSA to investigate the chain of events that leads to the presence of potentially toxic PFOA in the environment.
"Here's this substance that's found in nature," Wuebbles said. "So the science question arises, "Where is this stuff coming from?"
Since the early 1950s, PFOA (C8HF15O2) has been used in the manufacture of fluoropolymers, fluorine-containing plastics that are components of non-stick cookware (such as DuPont's Teflon), water-repellant fabrics, and other industrial products. Scientists have also theorized that PFOA can be formed as a result of the breakdown of fluorotelomer alcohols (FTOHs), chemicals that are widely used in fabrics and carpets (to provide stain resistance), in fast-food packaging (to help paper wrappers and cardboard boxes stand up to grease), and in some paints (to enhance dispersal). Millions of kilograms of fluorotelomer alcohols are produced around the world each year.
In recent years, PFOA has been observed in flora, fauna and human beings around the world, leading to concerns about potential health risks. Studies in rats and rabbits have demonstrated some adverse affects from PFOA exposure, ranging from reduced birth weights in offspring to enlarged kidneys and livers and even death. The results of studies of workers who have been exposed to PFOA are mixed, however, with no clear-cut picture of the chemical's danger or safety yet emerging. What is clear is that PFOA and its chemical kin are not metabolized in the body and that they are bioaccumulative, meaning they hunker down in human and animal tissue and their levels continue to mount year after year.
Wuebbles set out to test the hypothesis that the PFOA observed in the environment is a by-product of the breakdown of these fluorotelomer alcohols and to explain why it is so widely observed. He decided to focus on 8:2 FTOH (C8F17CH2CH2OH) because of its importance in the $500 million annual fluorotelomer alcohol market.
The atmospheric oxidation mechanism for 8:2 FTOH, constructed from data in prior publications, was integrated into two atmospheric chemistry models: a two- dimensional model from the University of Illinois and the three-dimensional IMPACT model developed at Lawrence Livermore National Laboratories and the University of Michigan. These models calculate the chemistry of the breakdown of the initial compound and the physics that control how the resulting products are carried through the atmosphere.
Using estimates of the amount of 8:2 FTOH present in the atmosphere and factoring in locations where the chemical was being generated, Wuebbles ran calculations using both models on Copper, NCSA's IBM p690 high-performance computing system. These simulations mapped the concentration and global dispersal of PFOA over time.
The simulation results showed molar yields of PFOA in the range of 1-to-10 percent, depending on the location and season. This yield corresponds well with what has actually been observed in nature. The distribution of PFOA shown by the simulations also correlated closely with environmental observations. In the simulation results, PFOA was ubiquitous in the Northern Hemisphere, as it is in reality.
Also correlating closely with real-world observations, the simulated concentration of PFOA was higher at locations far from its source, with the highest PFOA levels over the Atlantic and Pacific oceans, North Africa, and the Arctic during the summer. This somewhat counterintuitive result is explained by the fact that the reaction that forms PFOA competes with another reaction involving nitrogen oxides; larger concentrations of nitrogen oxides favor the competing reaction and result in less PFOA being formed. The sources of fluorotelomer alcohol emissions are also typically associated with high concentrations of nitrogen oxides, so less PFOA is formed close to the source of its parent compound, and more PFOA is formed farther away.
Wuebbles also noted how concentrations of PFOA in the simulation fluctuated throughout the year. In the Arctic, the PFOA concentration is high during the summer, falling by an order of magnitude during the winter. In fact, the simulation showed lower levels of PFOA during the winter throughout the Northern Hemisphere. This is due to the fact that photochemical activity slows during the winter months, creating a lull in the production of PFOA in the atmosphere.
Because the results of the simulation accorded so well with what has been observed in the environment, Wuebbles believes the study presents persuasive evidence that 8:2 FTOH degrades in the atmosphere to form PFOA and other perfluorocarboxylic acids and that these chemicals are then dispersed around the world.
Wuebbles said he would like to carry out finer-grained simulations to tease out in more detail the reactions and transport mechanisms that lead to the globally ubiquitous PFOA pollution.
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