LETTER pubs.acs.org/est
Letter to the Editor regarding, “Polyfluorinated Compounds: Past, Present, and Future” indstr€om et al.1 have provided a commendable overview of research on perfluoroalkyl acids (PFAAs) including perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), their potential precursors and related substances. We offer the following comments to strengthen the article and bring greater clarity to information presented in it. Use of acronyms (e.g., “PFCs”) that are not well-defined to describe large classes of fluorinated substances and thereafter writing broad statements regarding their properties and behavior leads to factual errors. Unfortunately, “PFC” has been used and defined in many different ways making it ambiguous when writing about PFAAs and their precursors. In fact, “PFCs” was first coined to describe perfluorocarbons, a family of greenhouse gases, in official Kyoto Protocol documents since the adoption of this important international agreement in 1997.2 In a recent critical review,3 we and our coauthors urge the scientific community to adopt “PFASs” (singular PFAS) as an acronym for “perfluoroalkyl and polyfluoroalkyl substances” and suggest that “PFCs” be used exclusively to describe perfluorocarbons (hereafter we use the recommended acronym “PFASs”). Additionally, the review3 presents clear, specific and descriptive nomenclature, recommended acronyms, classification and an organizational hierarchy for PFASs to provide a unified lexicon for use by the global scientific community. PFASs comprise thousands of substances with diverse properties. Even with qualifiers like “many”, which was used in several instances in the Lindstr€om et al. article, it is often misleading to make generalizations. The physical-chemical (e.g., water solubility, acid dissociation constants)4 and biological (e.g., toxicology, bioaccumulation) properties5 of individual homologues in the homologous series of perfluoroalkane sulfonates (PFSAs) and perfluoroalkyl carboxylates (PFCAs) vary greatly with carbon chain length. While PFOS has been designated as a persistent organic pollutant (POP) under the Stockholm Convention, many other
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PFAAs still need to be assessed to determine whether they satisfy the defining characteristics of POPs. There is an even greater diversity in properties between different classes of PFASs. It is therefore crucial to be clear and specific when writing about PFASs. An example of overgeneralization is Lindstr€om et al.’s suggestion that PFASs undergo “global distillation.” PFASs actually have diverse global transport properties and mechanisms. Notably, PFAAs are acidic and are mostly present in their dissociated (nonvolatile) anionic form in the environment.10 Pathways for PFAAs to reach polar regions are (a) atmospheric deposition following the transport and transformation of volatile precursors,6 (b) direct transport following environmental release, mainly through transport in water,7 and (c) transport on marine aerosols.8 PFAAs are not transported by “global distillation”, a process whereby substances are volatilized from warm regions, transported in air and then deposited and trapped in cold regions.9 We thank Lindstr€om et al. for their article, which will raise awareness of PFAS research to a wide audience. We hope that our comments highlight the need to harmonize PFAS terminology to facilitate clear, accurate characterizations of specific PFASs such as PFAAs in the future. Ian T. Cousins† and Robert C. Buck‡,* †
Department of Applied Environmental Science (ITM), Stockholm University, SE 106 91 Stockholm, Sweden
‡
E.I. du Pont de Nemours & Company, Inc., DuPont Chemicals and Fluoroproducts, Wilmington, Delaware, United States
(2) United Nations Kyoto Protocol to the United Nations Framework Convention on Climate Change, 1998. (3) Buck, R. C.; Franklin, J.; Berger, U.; Conder, J. M.; Cousins, I. T.; de Voogt, P.; Jensen, A. A.; Kannan, K.; Mabury, S. A.; van Leeuwen, S. P. J. Perfluoroalkyl and polyfluoroalkyl substances in the environment: Terminology, classification, and origins. Integr. Environ. Assess. Manage. 2011, 7 (4), 513–541. (4) Wang, Z.; MacLeod, M.; Cousins, I. T.; Scheringer, M.; Hungerb€uhler, K. Using COSMOtherm to predict physicochemical properties of poly- and perfluorinated alkyl substances (PFASs). Environ. Chem. 2011, 8 (4), 389–398. (5) Conder, J. M.; Hoke, R. A.; De Wolf, W.; Russell, M. H.; Buck, R. C. Are PFCAs bioaccumulative? A critical review and comparison with regulatory lipophilic compounds. Environ. Sci. Technol. 2008, 42 (4), 995–1003. (6) Young, C. J.; Mabury, S. A. Atmospheric Perfluorinated Acid Precursors: Chemistry, Occurrence, and Impacts. In Reviews of Environmental Contamination and Toxicology, Perfluorinated Alkylated Substances; Whitacre, D. M., DeVoogt, P., Eds.; 2010; Vol. 208, pp 1109. (7) Prevedouros, K.; Cousins, I. T.; Buck, R. C.; Korzeniowski, S. H. Sources, fate and transport of perfluorocarboxylates. Environ. Sci. Technol. 2006, 40 (1), 32–44. (8) McMurdo, C. J.; Ellis, D. A.; Webster, E.; Butler, J.; Christensen, R. D.; Reid, L. K. Aerosol enrichment of the surfactant PFO and mediation of the water—Air transport of gaseous PFOA. Environ. Sci. Technol. 2008, 42 (11), 3969–3974. (9) Simonich, S. L.; Hites, R. A. Global distribution of persistent organochlorine compounds. Science 1995, 269 (5232), 1851–1854. (10) Goss, K. U. The pK(a) values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42 (2), 456–458.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Lindstrom, A. B.; Strynar, M. J.; Libelo, E. L. Polyfluorinated Compounds: Past, Present, and Future. Environ. Sci. Technol. 2011 45, 7954 7961.
r 2011 American Chemical Society
9821
Received: October 5, 2011 Accepted: October 6, 2011 Published: October 28, 2011
dx.doi.org/10.1021/es203530y | Environ. Sci. Technol. 2011, 45, 9821–9821
