Perfluoroalkyl Acids: The Good News with the Bad
Perfluoroalkyl acids are a class of compounds that have been around for more than 60 years. They are used for a wide range of applications including protective coatings for fabrics, paints, cosmetics, paper coatings, insecticides and fire-fighting foams. To let that one hit home, they are used in Gortex, our key to staying dry in the Pacific Northwest, and the key to the Gore compost system. They are also used to keep grease out of paper, as in pizza delivery boxes and microwave popcorn packages. Like PBDEs, PAHs and dioxins, perfluoroalkyl acids come in a wide range of forms, all about equally hard to pronounce. Some examples include perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and perfluoroheptanoic acid (PFHpA). And like all of those other hard to pronounce compounds that come with different initials and different cogeners, you can find them in biosolids. They first hit the press as a contaminant of concern in biosolids a few years back (October 2010 library). The big news back then was related to a particular treatment plant in an area with a plant that produced the PFCs. The biosolids from that plant had very high concentrations of these compounds, and it made the news. As a new compound on the block, we are not really clear how much of each of the versions of this class of compounds is in biosolids. Again from the 2010 library, one article found a range from 5 to152 ng/g (ppm) for total perfluorocarboxylates, and from 55 to 3370 ng/g for total perfluoroalkyl sulfonyl-based compounds. Unlike many of the other initial compounds, these compounds are typically not bound so tightly to soil organic matter and can enter the water transpiration stream. They are also very tough to degrade. While they may be really useful, from an environmental perspective, you are much better off with a little easily degraded PCPP. These compounds are the focus of the library again this month because of a recent article that measured plant uptake of these compounds from biosolids amended soils (Article 1). This was a two part study. The first part was a proof of conceptare these compound plant available? For this part, the scientists looked at lettuce and tomato uptake for plants grown in biosolids in the greenhouse. The first biosolids they tested was a high industrial material added at 10% dry weight to the soil. The second came from long-term Chicago plots where the soil was all biosolids (1654 Mg ha) from long-term applications. They found that these compounds are plant available and that the concentrations in both lettuce and tomato were higher than the concentrations in the soils. In other words, they bioaccumulate. Concentrations got up to 266 ng/g for PFBA (I am not spelling that one out) in lettuce and 211 ng/g for PFBA in tomato. The more complex versions of these chemicals were taken up by smaller quantities than the shorter versions, despite higher soil concentrations of these compounds. This is likely the part of the study that you will hear about. The part that you won’t hear quite so much about was the field portion of the study. Here the researchers planted lettuce and tomato in soils that had gotten just one application of biosolids added at agronomic rates. Most samples were below detection- just like the control. They also sampled corn grain and stover from farmer’s fields that had biosolids applied. All below detection in the grain and either below or close to detection limits in the stover. The rest of the articles in the library provide you background on these compounds, as you may need to answer some questions. The first (2nd article) is a summary of a workshop that was held in Virginia. It goes into details on what we know about the toxicity of these compounds that are ubiquitous in both humans and wild life. There are details about one study indicating that they may be related to lower birth weight in people, and another that had contradictory findings. They talk a lot about what we don’t know about them. The 3rd article talks about trends in human exposure to these compounds. Here human blood samples from representative populations were measured. It turns out that 3M, one of the primary manufacturers, actually stopped making PFOs and so concentrations in blood have decreased significantly. Other versions such as PFOA, although lower than PFO, have stayed the same. The article includes data on averages and ranges of a number of these compounds for different populations. Some are higher in men and there is also an effect of race with lower concentrations in Mexican Americans. The last two articles focus on likely routes of exposure. Article number 4 includes data from a number of publications on indoor air concentrations of the different compounds from homes, offices, and day cares. It also includes similar data for PBDEs and HBCDs (hexabromocyclododecanes). The last article in the library looks at the potential for food chain transfer. This is a study from Canada where researchers measured concentrations of PFCs in different foods. Highest concentrations were seen in fast foods, or prepared foods because of the use of these compounds in packaging material. So the bad news is that these are soluble, persistent compounds that can enter the food chain from biosolids amended soils. They are also everywhere, so the potential for biosolids to be a significant pathway for exposure is minimal. The good news is that there are much less of them being made in response to the recognition that they are problematic. The other good news is that plant uptake from biosolids amended soils was only demonstrated in extreme cases, specifically high contaminant biosolids or extraordinary loading rates.
Sally Brown University of Washington