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Description of key information

D5 exhibits a lower potential for bioaccumulation.

Key value for chemical safety assessment

Additional information

Assessment of bioaccumulation is typically limited to water-breathing aquatic organisms e.g., fish, however, it is important to consider bioaccumulation in air-breathing organisms (non-aquatic organisms) such as marine and terrestrial mammals and birds. It has been demonstrated that the bioaccumulation behaviour of neutral hydrophobic organic substances in air-breathing organisms is often related to the octanol–air partition coefficient (KOA) of the substance (Kelly et al, 2007). Substances with low octanol–air partition coefficients can be exhaled quickly and hence exhibit a lower potential for bioaccumulation.

 

A 2008 study (Andersen et al.,2008) found that D5 was quickly depurated in rats and humans by exhalation as a result of D5's high vapour pressure and relatively low KOA. Another study (Varaprath et al. 2006) showed extensive biotransformation of D5 in Fisher 344 rats that were intravenously and orally exposed to D5. The high rate of depuration of D5 through exhalation and biotransformation indicates that D5 does not have a potential for biomagnification in air-breathing organisms or terrestrial food webs.

In published papers discussing the related substance D4 (octamethylcyclotetrasiloxane, CAS 556-67-2), it is concluded that the bioaccumulation of D4 in the body is unlikely, due to the effective elimination through metabolism and exhalation (Plotzke et al., 2000), (Andersen et al., 2008). In addition, the results suggest a dose-dependency in the kinetics of D4 that is likely related to a combination of factors, such as saturation of uptake/transport mechanisms and nonlinear storage and metabolism. It is also well demonstrated that D4 is readily absorbed and reaches systemic circulation rapidly following exposure (Dobrev et al., 2008). The results also indicate that D4 is unlikely to bioaccumulate in the fat, even though it is lipophilic, primarily due to three excretion mechanisms: ready clearance as water soluble metabolites from the tissues via the urine; exhalation of parent D4; and to a lesser extent, excretion of parent D4 in the faeces. These findings are also applicable to D5.

Analysis of cyclic volatile methylsiloxanes (cVMS) in fat, muscle, and liver tissue of American mink (Mustela vison) obtained from Lake Pepin, USA, found that the highest cVMS concentrations were in mink fat. The average lipid-adjusted residues in mink fat for D5 was 44 ng/g-lipid (LOD 0.19 ng, MDL 2.23 ng/g). Concentrations in mink fat were reported to be significantly lower that lipid-adjusted cVMS residues in fish and benthic invertebrates comprising part of the mink diet on Lake Pepin. These data indicate that trophic magnification processes for D5 are not occurring in this terrestrial predator species in the Lake Pepin food web. The potential reasons for the observed biodilution of lipid-normalized D5 residues in mink muscle, fat, and liver tissue, compared to whole fish residues from Lake Pepin, are numerous. First of all, TMF calculations make the assumption that all species data included are comparable. For example, it is assumed that the chemical assimilation efficiencies and metabolism rates are similar for all the organisms included in the TMF regression analysis (Broman et al., 1992, Rolff et al., 1993); clearly, this may not be the case. Cold- and warm-blooded animals (poikilotherms and homeotherms, respectively) have different energy requirements and biotransformation abilities (Sibly and Calow 1986), and combining these organisms in an trophic magnification evaluation tends to exaggerate TMF values to higher levels compared to using only species within a poikilothermic energy regime (Hop et al., 2002). Secondly, food web and carbon/nitrogen flows may undergo significant seasonal changes, and this may account for the disconnect in carbon flow between Lake Pepin fish/invertebrates and mink (Vezina and Savenkoff, 1999); in Lake Pepin, the benthic invertebrates/fish samples were collected during the summer months, while mink were collected in late fall (November). Lastly, Kelly et al. (2007) have suggested that lipophilic substances (log Kow >5) with low octanol/air partition coefficients (log Koa <6) may not accumulate in terrestrial mammals due to an elevated rate of respiratory elimination from tissues to air. The cVMS materials all have log Kow values >5 and log Koa values <6, indicating that respiration or “exhalation” may be a significant loss mechanism for terrestrial mammals, thereby contributing to loss of these materials in terrestrial mammals in a food web system. This exhalation process has been experimentally confirmed as a significant loss process for D5 in rats and humans (Reddy et al., 2008).

 

References:

 

Andersen ME, Reddy MB, Plotzke KP. 2008. Are highly lipophilic volatile compounds expected to bioaccumulate with repeated exposures? Toxicol Lett 179:8592.

 

Kelly BC, Ikonomou MG, Blair JD, Morin AE, Gobas FAPC. 2007. Food web-specific biomagnification of persistent organic pollutants. Science 317:236239.

 

Varaprath S, McMahon JM, Plotzke KP. 2003. Metabolites of hexamethyldisiloxane and decamethylcyclopentasiloxane in Fischer 344 rat urine: A comparison of a linear and a cyclic siloxane. Drug Metabol Dispos 31:206214.

Dow Corning Corporation, 2009. Analysis of cyclic volatile methylsiloxanes (cVMS) in fat, muscle, and liver tissue of American mink (Mustela vison) obtained from Lake Pepin, Minnesota, USA. Testing laboratory: Dow Corning Corporation, Health and Environmental Sciences, 2200 West Salzburg Road, Auburn, MI 48611. Owner company: CES. Study number: 11216 -108. Report date: 2009 -11 -25

Broman D, Näf C, Rolff C, Zebuhr Y, Fry B and Hobbie J (1992) Using ratios of stable nitrogen isotopes to estimate bioaccumulation and flux of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in two food chains from the northern Baltic. Environ Toxicol Chem 11:331-45.

Hop, H., Pearson, T., Hegseth, E. N., Kovacs, K. M., Wiencke, C., Kwasniewski, S., Eiane, K., Mehlum, F., Gulliksen, B., Wlodarska-Kowalczuk, M., Lydersen, C., Weslawski, J. M., Cochrane, S., Gabrielsen, G. W., Leakey, R. J. G., Lønne, O. J., Zajaczkowski, M., Falk-Petersen, S., Kendall, M., Wängberg, S.-Å., Bischof, K., Voronkov, A. Y., Kovaltchouk, N. A., Wiktor, J., Poltermann, M., di Prisco, G., Papucci, C. and Gerland, S. (2002), The marine ecosystem of Kongsfjorden, Svalbard.Polar Research, 21: 167–208. doi:10.1111/j.1751-8369.2002.tb00073.x

Kelly, B. C., Ikonomou, M. G., Blair, J. D., Morin, A. E. and Gobas, F. A. P. C. (2007). Food web-specific biomagnification of persistent organic pollutants. Science 317: 236-239.

Rolff, C., Broman, D., Näf, C., and Zebühr, Y., Potential biomagnification of PCDD/Fs – New possibilities for quantitative assessment using stable isotop trophic position.Chemosphere,27, 461-468, 1993.

Sibly R. M. & Calow P.1986. Physiological ecology of animals: an evolutionary approach. Blackwell Scientific Publications, Oxford.

Vézina, A.F., Savenkoff, C., 1999. Inverse modeling of carbon and nitrogen flows in the pelagic food web of the northeast subarctic Pacific. Deep Sea Research Part II: Topical Studies in Oceanography 46, 2909e2939.