Perfluorooctane sulfonate in the environment: chapter 2
Background on PFOS
PFOS is a synthetic fluorocarbon that contains eight carbon atoms in which all of the carbon-hydrogen (C-H) bonds are replaced by carbon-fluorine (C-F) bonds (Figure 1). The C-F bond is thermodynamically the strongest bond known, which makes this compound extremely persistent in the environment. PFOS also contains a reactive sulfonyl group (SO3H), which, together with the fluorocarbon chain, imparts the ability to repel both oil and water. PFOS can exist in anionic, acid and salt forms. However, under normal environmental conditions of approximately neutral pH, the anionic form (i.e., C8F17SO3-) dominates, resulting in very low volatility and high water solubility. PFOS also belongs to a larger family of fluoro-organic compounds that includes perfluorooctanoic acid (PFOA) as well as more volatile precursor compounds that are commonly used in commercial products and ultimately degrade to compounds such as PFOS and PFOA.
Figure 1. Molecular structure of PFOS.
Owing to its chemical and physical properties, PFOS is typically found at higher concentrations in water compared with air, and can travel long distances by oceanic currents. In contrast, PFOS precursors are more volatile and can be transported through air to areas far from initial release, where they are subsequently degraded to PFOS. PFOS is bioaccumulative and readily taken up by aquatic and terrestrial animals. However, in contrast to well-known legacy contaminants such as polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT), which are stored predominantly in the fatty tissues of organisms, PFOS binds to proteins in blood and liver.
There are no known natural sources of PFOS, its salts or its precursors. PFOS and its precursors were never manufactured in Canada, but instead were imported as raw chemicals, products, formulations and in manufactured articles. In 2002, the primary manufacturer of PFOS phased out worldwide production. Prior to 2002, PFOS and its precursors were released into the Canadian environment during the manufacture and use of products such as paper, packaging, carpets and fabrics, where PFOS and its precursors were applied to repel water, oil, soil and grease. Currently, PFOS and its precursors are released into the Canadian environment during the use and disposal of products, such as firefighting foam, that are exempt from regulations until May 2013. PFOS or its precursors may also enter the Canadian environment through long-range transport from foreign sources.
Through risk assessment activities under the Canadian Environmental Protection Act, 1999 (CEPA 1999), the Government of Canada concluded that PFOS, its salts, and precursors pose a risk to the environment but not to human health.18,19 Based on these conclusions, PFOS, its salts and precursors were added to Schedule 1 of CEPA 1999, the List of Toxic Substances. PFOS and its salts were also added to the Virtual Elimination List compiled under CEPA1999, which demonstrates the Government of Canada’s commitment to virtually eliminate PFOS and to meet the requirements of the Perfluorooctane Sulfonate Virtual Elimination Act.20
A use pattern survey published in 2005 indicated that, with the exception of approximately three tonnes of PFOS present in stockpiles of aqueous film-forming foam, most supplies in Canada had been depleted.21 Regulations published in 2008 in Canada prohibit the manufacture, use, sale and import of PFOS, its salts and its precursors. Under these Regulations, five-year exemptions ending in 2013 were provided for aqueous film-forming foam and PFOS-based fume suppressants used in the metal plating industry to allow for the transition to alternatives. Ongoing exemptions in niche applications allow for use in photolithography processes, photographic materials and aviation hydraulic fluids. In the U.S., the Environmental Protection Agency requires that manufacturers and importers provide notification at least 90 days before new manufacture or import of these substances. Internationally, PFOS has been added to several agreements, such as the Stockholm Convention on Persistent Organic Pollutants, which aim to reduce production and use of PFOS on an international scale. Long-term monitoring studies and surveillance work are being conducted to determine PFOS levels in various environmental media over time.
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