Ecological screening assessment report on perfluorooctane sulfonate, salts and precursors: chapter 6

6. Discussion

There are special concerns about highly persistent and bioaccumulative substances. Although current science is unable to accurately predict the ecological effects of these substances, they are generally acknowledged to have the potential to cause serious, irreversible impacts. Assessments of such substances must therefore be performed using a protective, preventative and precautionary approach to ensure that such harm does not occur.

Evidence that a substance is persistent and bioaccumulative may itself be a significant indication of its potential to cause environmental harm. Persistent substances remain in the environment for long periods of time, increasing the probability and the duration of exposure. Persistent substances that are subject to long-range transport are of particular concern because they can result in low-level, regional or global contamination. Releases of small amounts of persistent and bioaccumulative substances may lead to relatively high concentrations in organisms over wide areas. Bioaccumulative and persistent substances may also biomagnify through the food chain, resulting in internal exposures for top predators. Since they are widespread, several different persistent and bioaccumulative substances may be present simultaneously in the tissues of organisms, increasing the likelihood and potential severity of harm.

Other information can increase concerns regarding the potential for persistent and bioaccumulative substances to cause environmental harm. For example, there is a particular concern for substances that, based on laboratory toxicity tests, have the potential to harm organisms at low concentrations, and/or have modes of toxic action beyond narcosis. A substance which does not naturally occur in the environment may also have an elevated potential to cause harm as organisms may not have evolved specific strategies for mitigating exposures and effects. Monitoring studies indicating that a substance is widespread in the environment and/or that concentrations have been increasing over time may be an indicator of elevated exposure potential. A substance that is used in Canada in moderate to large quantities (e.g., greater than 1000 kg/yr) in a variety of locations, and/or if use quantities are increasing, may also be taken as an indicator of elevated exposure potential.


While certain data gaps and uncertainties exist, there is nonetheless a substantial body of information on PFOS and its precursors. For example, while the mechanism of transport of PFOS and its precursors to the Arctic is not clear, they appear to be mobile in some form, as PFOS has been measured in biota throughout the Canadian Arctic, far from known sources. Environmental pathways of PFOS to biota are not well understood because information on degradation is lacking, and there are relatively few monitoring data on concentrations of various precursors in air, water, effluents and sediment in Canada. While mechanisms of toxic action of PFOS are not well understood, a range of toxicological effects have been reported in a variety of species. Currently, there is limited information on the toxicology of PFOS precursors and the potential for combined or synergistic effects with PFOS.


The weight of evidence on the persistence of PFOS, the degradation of precursors to PFOS, and the volatilization and atmospheric transport of the precursors to PFOS, indicate that PFOS has the potential to move in the environment.

PFOS is resistant to hydrolysis, photolysis, microbial degradation and metabolism by vertebrates and is persistent. PFOS is present in biota, notably in vertebrates, throughout the world, including in a range of fish, birds and mammals in remote sites, including the Canadian Arctic, far from known sources or manufacturing facilities of PFOS and its precursors. This indicates that PFOS and/or its precursors may undergo long-range transport. The precursor POSF is persistent in air, with an atmospheric half-life of 3.7 years (US EPA OPPT AR226-1030a104). In water, PFOS persisted over 285 days in microcosms under natural conditions (Boudreau et al. 2003b). While the vapour pressure of PFOS is similar to those of other globally distributed compounds (e.g., PCBs, DDT), its water solubility indicates that PFOS itself is less likely to partition to and be transported in air (Giesy and Kannan 2002). Although PFOS itself has low volatility, several PFOS precursors are considered volatile, including N-EtFOSE alcohol, N-MeFOSE alcohol, N-MeFOSA and N-EtFOSA (US EPA OPPT AR226-0620). When present in residuals in products, these PFOS precursors could evaporate into the atmosphere when the products containing them are sprayed and dried (US EPA OPPT AR226-0620). Therefore, precursors to PFOS, in addition to contributing to the ultimate loading of this persistent and bioaccumulative substance, also contribute to its widespread occurrence.

Concentrations in Biota

The worldwide and widespread occurrence of PFOS in wildlife and in the Canadian polar bear where high PFOS concentrations have been detected (Martin et al. 2004a, Smithwick et al. 2005a,b,c) have significant bearing on the conclusions of this assessment. Indications of high concentrations in top predators are of concern. While the sample sizes for the Canadian polar bears are small, the PFOS levels in polar bear liver are corroborated by samples from 6 other circumpolar locations. Eastern Greenland polar bear livers were in the same order of magnitude but had higher concentrations. Since PFOS is known to partition to liver, the availability of field measured concentrations of PFOS in liver tissue which can be compared to toxicological effects in liver at certain liver concentrations are particularly relevant to this assessment and reduces some of the uncertainties that are typical for persistent and bioaccumulative substances. However, risks could still be underestimated if steady state conditions were not achieved in exposed wildlife or in laboratory toxicity tests.

There are no known local sources of PFOS at the sampling site of the South Hudson Bay polar bear and there are no PFOS manufacturing sites in the area. While accidental release from sources such as fire fighting foams cannot be entirely ruled out, it is noted that mean liver PFOS concentrations in polar bears from 7 circumpolar locations were within an order of magnitude of each other, varying only by a factor of 3-4. Most importantly, it is expected that, given the very large home range of polar bear, concentrations in these mammals may reflect integration of exposure over large geographic areas. It is also noted that the concentrations of PFOS in polar bear are 5-10 times higher than the concentration of all other perfluoroalkyl substances. The PFOS concentrations in polar bear liver were also higher than any other previously reported concentrations of persistent organochlorine chemicals (e.g., PCBs, chlordane or hexachlorocyclohexane) in polar bear fat (Martin et al., 2004a).


In vertebrates, PFOS preferentially partitions to proteins in liver and blood. The bioaccumulation potential of PFOS may not be related to the typical mechanisms associated with bioaccumulation in lipid-rich tissues. The weight of evidence considered for bioaccumulation includes both laboratory and field-based BAFs, BCFs, BMFs (avian and aquatic), and data on elimination half-lives in a range of species. Whole-body laboratory BCFs in fish ranged from 690 to 2796 and are below 5000. Tissue-based field BAFs in Canadian biota ranged from 6 300 to 125 000. The bioaccumulative tendencies of PFOS, suggested by the BCF/ BAF values, are confirmed by tissue-based field BMF studies (BMF values ranged from 0.4 - 20) indicating the potential for biomagnification. In addition to information on PFOS, estimated BCFs for the precursors n-EtFOSEA and n-MeFOSEA were 5 543 and 26 000, respectively.

In Canada, PFOS has been detected in higher trophic level biota and predators such as fish, piscivorous birds (double-crested cormorant), mink, and Arctic biota (polar bear) far from known sources or manufacturing facilities. In Canadian Arctic biota, PFOS concentrations in liver ranged from 20 µ (mink) to > 4000 µ (polar bear). Also, predator species such as eagles have been shown to accumulate higher PFOS concentrations than birds from lower trophic levels. Chronic and acute effects of PFOS have been observed in laboratory studies with mallard, Japanese quail, and northern bobwhite quail. Effects noted in the chronic reproductive studies for mallards and bobwhite quail include reduced testicular size in quails and mallards (including altered spermatogenesis), increased liver weight in female quails and reduced 14 day survivability in quail chicks as a percentage of eggs set. The testicular regression is accompanied by a histologically-visible effect on spermatogenesis in the mallard. Effects of PFOS on thyroid function have also been reported. Even with reductions in manufacturing of PFOS by some North American manufacturers, wildlife such as birds can continue to be exposed to persistent and bioaccumulative substances such as PFOS by virtue of their persistence and long-term accumulation. Therefore, the weight of evidence is sufficient to conclude that PFOS and its salts are bioaccumulative.

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