Bioaccumulation of decamethylpentacyclosiloxane (D5): A review

Low Dietary Uptake Efficiencies and Biotransformation Prevent Biomagnification of Octamethylcyclotetrasiloxane (D4) and Decamethylcyclopentasiloxane (D5) in Rainbow Trout

With octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) being considered for evaluation under the UN Stockholm Convention on Persistent Organic Pollutants, which specifically acknowledges risks of biomagnification of persistent organic pollutants in traditional foods, a study into the mechanism of the biomagnification process of D4 and D5 in Rainbow trout was conducted by combining the absorption-distribution-metabolism-excretion for bioaccumulation (ADME-B) approach to determine intestinal and somatic biotransformation rates and radiochemical analyses to identify metabolite formation. High rates of intestinal biotransformation of D4 and D5 (i.e., 2.1 (0.70 SE) and 0.88 (0.67 SE) day-1, respectively) and metabolite formation [i.e., 52.0 (17 SD)% of D4 and 56.5% (8.2 SD)% of D5 were metabolized] were observed that caused low dietary uptake efficiencies of D4 and D5 in fish of 15.5 (2.9 SE)% and 21.0 (6.5 SE)% and biomagnification factors of 0.44 (0.08 SE) for D4 and 0.78 (0.24 SE) kg-lipid·kg-lipid-1 for D5. Bioaccumulation profiles indicated little effect of growth dilution on the bioaccumulation of D4 and D5 in fish and were substantially different from those of PCB153. The study highlights the importance of intestinal biotransformation in negating biomagnification of substances in organisms and explains differences between laboratory tests and field observations of bioaccumulation of D4 and D5.

Cyclic volatile methyl siloxanes (D4, D5, and D6) as the emerging pollutants in environment: environmental distribution, fate, and toxicological assessments

Cyclic volatile methyl siloxanes (cVMS) have now become a subject of environmental contamination and risk assessment due to their widespread use and occurrence in different environmental matrices. Due to their exceptional physio-chemical properties, these compounds are diversely used for formulations of consumer products and others implying their continuous and significant release to environmental compartments. This has captured the major attention of the concerned communities on the grounds of potential health hazards to human and biota. The present study aims at comprehensively reviewing its occurrence in air, water, soil, sediments, sludge, dusts, biogas, biosolids, and biota and their environmental behavior as well. Concentrations of cVMS in indoor air and biosolids were higher; however, no significant concentrations were observed in water, soil, and sediments except for wastewaters. No threat to the aquatic organisms has been identified as their concentrations do not exceed the NOEC (maximum no observed effect concentration) thresholds. Mammalian (rodents) toxicity hazards were not very evident except for the occurrence of uterine tumors in very rare cases under long-term chronic and repeated dose exposures in laboratory conditions. Human relevancy to rodents were also not strongly enough established. Therefore, more careful examinations are required to develop stringent weight of evidences in scientific domain and ease the policy making with respect to their production and use so as to combat any environmental consequences.

Chemical Recycling of High-Molecular-Weight Organosilicon Compounds in Supercritical Fluids

The main known patterns of thermal and/or catalytic destruction of high-molecular-weight organosilicon compounds are considered from the viewpoint of the prospects for processing their wastes. The advantages of using supercritical fluids in plastic recycling are outlined. They are related to a high diffusion rate, efficient extraction of degradation products, the dependence of solvent properties on pressure and temperature, etc. A promising area for further research is described concerning the application of supercritical fluids for processing the wastes of organosilicon macromolecular compounds.

A review of bioaccumulation of volatile methylsiloxanes in aquatic ecosystems

Volatile methylsiloxanes (VMSs) are found in a broad range of industrial and consumer products. They are categorized as "high production volume chemicals" by the U.S. Environmental Protection Agency and listed as candidates of substances of very high concern in 2018, by the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). Industrial wastewater and treated effluents may contain VMSs in different amounts, which can be discharged in the receptor media and may lead to environmental contamination. This can result in direct exposure to aquatic receptors in the water column or to benthic invertebrates from contact and/or ingestion of sediments, and indirect exposures through the aquatic food chain. The possible toxicological effects of VMSs for the aquatic biota and human ecology are not very well known since published information regarding this topic is scarce. VMSs have been subjected to regulatory scrutiny for environmental concerns and have already been screened to determine their environmental risk and ecological harm. This paper aims to assess VMSs bioaccumulation and potential biomagnification on food webs, using several bioaccumulation metrics. The result is a high-level overview of all the collected data, comparing the findings and the experimental conditions applied during the assessments. Several studies present conflicting results regarding the bioaccumulation categorization. Some aquatic organisms demonstrated a high bioconcentration and bioaccumulation of these contaminants. Trophic magnification factors (TMFs) have been suggested as the most reliable tool to assess a chemical behaviour in food webs. However, bioaccumulation studies in food webs provided mixed information, with some studies indicating trophic dilution and others presenting a potential of trophic biomagnification of VMSs. Efforts should be directed to obtain field-based levels of VMSs at different trophic levels and a wider range of linear VMSs should be analysed, since most studies focused on D4, D5 and D6.

Potential trade-off between water consumption and water quality: life cycle assessment of nonaqueous solvent dyeing

Fashion industry consumes over 60% of global fibers and attracts increasing attentions due to its environmentally polluting supply chain. In addition to natural fibers cultivation, wet processes of textile manufacturing are also important contributors to water-related impacts due to their large freshwater consumption and the production of chemicals containing wastewater. Despite of efforts made in improving efficiency of water use and wastewater treatment in textile industry, innovative 'water-free' technologies, such as nonaqueous dyeing technology using organic solvent, have been developed and demonstrated to reduce water consumption significantly. However, the potential impact on water quality by organic solvents induced in supply chain of this emerging technology remains unassessed, posing an unknown risk of its promotion. Hence, in the present study, a comprehensive life cycle assessment is applied to evaluate its full environmental impacts, including those on ecosystem and human health caused by decamethylcyclopentasiloxane (D5) as the solvent used. Further, the nonaqueous dyeing system is compared with traditional aqueous dyeing technology from both environmental and economic perspectives. Results indicate that nonaqueous dyeing system is advanced in most of environment categories except for abiotic depletion potential (ADP) and Ecotoxicity. However, scenarios analysis reveal that these findings are influenced by the loss fraction of D5 during the solvent recovery process. It is suggested that the loss fraction should be controlled below 2% o.w.f. for the nonaqueous dyeing technology to be advanced throughout all environmental categories. Nonaqueous D5 dyeing could reduce water consumption by 61.30%-79.95% and greenhouse gas emissions by 43.70% compared to the traditional system, delivering a promising contribution to China's 2060 carbon neutrality ambition. Sensitivity and uncertainty analyses are also conducted to investigate the effects of the key parameters (incl. inventory data and USEtox model inputs) and demonstrate the robustness of our assessment.

Comparative study of the percutaneous permeation and bioaccumulation of the cyclic siloxane using frozen-thawed and nonfrozen ex vivo human skin

Literature shows contradictory information regarding the effect of freezing the excise skin ex vivo on the diffusion of substances into the skin. Few studies indicate that storing the human or animal skin in a frozen state decreases the barrier properties after thawing. Therefore, to understand the properties of frozen skin, we evaluated the effect of storage of ex vivo human skin (2 weeks at -20 °C) on the penetration of stratum corneum and permeation into deeper skin layers (epidermis, and dermis) as well as to the receptor fluid by octamethylcyclotetrasiloxane (D4) a representative test compound of cyclic siloxanes. The main research were preceded by checking the integrity of nonfrozen ex vivo human skin in comparison to the frozen-thawed one by using the Electrical Resistance technique (ER) and the fluorescence microscopy. Samples collected in the skin absorption experiment were analyzed by gas chromatography equipped with a flame ionization detector (GC-FID). The results of this study demonstrated that freezing of excised ex vivo human skin at -20 °C for up to 14 days does not alter the permeability of D4 in a statistically significant manner. Thus, our results confirmed the validity of using skin storage conditions for testing the penetration and permeation of xenobiotics recommended by the OECD, EMA, and WHO guidelines.

Influence of wastewater effluents on the bioaccumulation of volatile methylsiloxanes in the St. Lawrence River

The bioaccumulation of cyclic volatile methylsiloxanes (D3 to D6) as well as linear siloxanes (L3 to L5) was studied in a food web in the St. Lawrence River downstream of the effluent of the municipal wastewater treatment plant in Montreal, Canada. For most species, differences in δ¹⁵N in fish and prey showed a clear separation of individual fish feeding in food webs influenced by the wastewater plume from those feeding outside the plume. Cyclic siloxanes were detected in all biotic samples from the individuals identified as feeding in the effluent plume. Siloxane D5 accounted for more than 80% of the total siloxanes. Linear siloxane L5 was also abundant in walleye and gull eggs. Total siloxane concentrations in suspended sediments were 17.3 times higher than in surface sediments in the region of the river in the effluent plume. Caged freshwater mussels (Elliptio sp.), placed in the effluent plume, bioaccumulated 43 times more total siloxanes than PBDEs in 30 days, demonstrating how readily siloxanes are taken up in biota and what a significant component of the contaminant body burden they can account for. The sediment-biota accumulation factors (BSAF) for total siloxanes (∑ D3 to D6 and L3 to L5) showed values of 65.4, 27.8, 9.9 and 6.4 for walleye, northern pike, yellow perch and round goby respectively.

Bioaccumulation of dodecamethylcyclohexasiloxane (D6) in fish

To investigate the bioaccumulation behavior of dodecamethylcyclohexasiloxane (D6, CAS number: 540-97-6) in fish, an OECD-305 style dietary bioaccumulation study of D6 in rainbow trout was conducted in the presence of non-metabolizable reference chemicals. The dietary uptake absorption efficiency of D6 was 14 (3 SE) % and lower than that of the reference chemicals which ranged between 22 (2 SE) to 60 (8 SE) %. The concentration of D6 in the body of the fish showed a rapid 40% drop during the first day of the depuration phase, followed by a slower decline during the remainder of the depuration period. The overall depuration rate constant of D6 was 0.016 (0.0026 SE) d^-1 and significantly greater than those of PCB153 and PCB209, which were not significantly different from zero. During the depuration phase, when fish body weight did not significantly change over time, depuration of D6 appears to be almost entirely due to biotransformation in the body of the fish. The biomagnification factor of D6 in rainbow trout was 0.38 (0.14 SE) kg-lipid kg-lipid^-1, indicating a lack of biomagnification. The bioconcentration factor (BCF) of D6 in Rainbow trout was estimated at 1909 (483 SE) L kg^-1 wet for natural waters of mostly oligotrophic lakes in Northern Canada with an average concentration of total organic carbon of 7.1 mg L^-1. Comparing the bioaccumulation profile of D6 to that of 238 similar profiles for 166 unique chemicals indicates that the bioaccumulation capacity of D6 is markedly less than that of many very hydrophobic organochlorines.

Ex Vivo Human Skin is not a Barrier for Cyclic Siloxanes (Cyclic Silicones): Evidence of Diffusion, Bioaccumulation, and Risk of Dermal Absorption Using a New Validated GC-FID Procedure

Cyclic methylsiloxanes D4, D5, D6 (also called cyclic silicones) are widely used in various dermatological products and cosmetics, both for children and adults. As a result of their unique physicochemical properties, the production of cyclic methylsiloxanes has greatly increased over the last few years, which has resulted in increased exposure to mankind. The validated quantitative for gas chromatography-flame ionization detector (GC-FID) analysis with using the transdermal diffusion system with vertical Franz cells demonstrated that ex vivo human skin is not a barrier to cyclic siloxanes. D4, D5, and D6 have a specific affinity to stratum corneum (SC) (especially D6), and can even diffuse into the deeper layers of the skin (epidermis (E) and dermis (D)), or into the receptor fluid as well. An important achievement of this work was the observation of the characteristic ratio partitioning D4, D5, and D6 in skin layers and receptor fluid (RF). The studies have shown that, in order to thoroughly understand the mechanism, it is important to determine not only the differences in the amounts of cumulated doses in total in all skin layers and receptor fluid, but also the mutual ratios of analyte concentrations existing between matrices. For example, in the case of the stratum corneum, the cumulative doses of D4, D5, and D6 were 27.5, 63.9, and 67.2 µg/cm2/24 h, respectively, and in the epidermis, they were 6.9, 29.9, and 10.7 µg/cm2/24 h, respectively, which confirmed the highest affinity of D6 to stratum corneum as the amount diffused into the epidermis was 2.8 times smaller compared to D5. The calculated epidermis-to-stratum corneum ratios of analyte concentrations also confirm this. The largest ratio was identified for D5 (E/SC = 47), followed by D4 (E/SC = 25), and finally by D6 (E/SC = 16). The analysis of the next stage of diffusion from epidermis to dermis revealed that in dermis the highest cumulative dose was observed for D5 (13.9 µg/cm2/24 h), while the doses of D4 and D6 were similar (5.1 and 5.3 µg/cm2/24 h). Considering the concentration gradient, it can be concluded that the diffusion of D5 and D6 occurs at a similar level, while D4 diffuses at a much higher level. These observations were also confirmed by the dermis-to-epidermis concentration ratios. The final stage of diffusion from dermis to the receptor fluid indicated that D4 was able to permeate easily, while D5 exhibited a difficult diffusion and the diffusion of D6 was limited. The receptor fluid-to-dermis concentration ratios (RF/D) were calculated for D4, D5, and D6: 80, 53, and 17, respectively. Our results also revealed the increased risk of D4 and D5 absorption into the blood and lymphatic systems, whereas D6 demonstrated the lowest risk. Therefore, we can argue that, among the three tested compounds, D6 is the safest one that can be used in dermatological, cosmetic, and personal care products. This study demonstrates that the stratum corneum, epidermis, and dermis can be also considered reservoirs of cyclic methylsiloxanes. Therefore, these compounds can demonstrate potential long-term bioaccumulation, and can be absorbed to the bloodstream in a long-term and uncontrolled process.

Retrospective analysis of cyclic volatile methylsiloxanes in archived German fish samples covering a period of two decades

Cyclic volatile methylsiloxanes (cVMS) are widely applied chemicals used as intermediates in the production of silicon polymers or as ingredients in personal care products. cVMS are under scrutiny due to their environmental properties and their potential for long-range atmospheric transport, persistence and food web magnification. In 2018, the cVMS octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) were identified as Substances of Very High Concern (SVHC) under the European REACH regulation. To obtain current data on the presence of cVMS in German waters, the spatial and temporal occurrence of D4, D5 and D6 in fillets of bream from major rivers archived in the German Environmental Specimen Bank (ESB) was analyzed with a GC-ICP-MS/MS coupling method. The spatial comparison of 17 sites for the year 2017 revealed that highest cVMS burdens occurred in samples from the Saar river (near to the French/German border). cVMS levels in fish from a lake in northern Germany did not exceed the limits of detection. For selected sites, time series covering the period from 1995 to 2017 were investigated. In most years D5 concentrations in fish were clearly higher than the observed D4 and D6 concentrations. Overall maximum D4 and D5 concentrations (about 320 and 7600 ng g^-1 wet weight, respectively) were found at one Saar site in 2009. In three of five analyzed time series D5 concentrations peaked 2007-2011. In recent years, cVMS levels in fish decreased at almost all sites. To allow an assessment of the relevance of the detected cVMS fish concentrations these were compared to environmental quality standards (EQS) for D4 and D5 which were recently enacted in the context of the Swedish implementation of the European Water Framework Directive (WFD). The D5 EQS in fish was exceeded at four sites in several years in the investigated period and in the Saar even till 2017.

The consequences of overcoming the human skin barrier by siloxanes (silicones) Part 1. Penetration and permeation depth study of cyclic methyl siloxanes

Dynamic production of cyclic siloxanes: octamethylcyclotetrasiloxane D4, decamethylcyclopentasiloxane D5 and dodecamethylcyclohexasiloxane D6 increases their concentrations in environment. It is considered that both environmental pollution and the usage of personal care products and cosmetics containing cyclic siloxanes can be the main source of the human exposure by transdermal route. The aim of the study was to verify the possibility to overcome the skin barrier by cyclic siloxanes (ATR-FTIR and GC-FID), evaluation of diffusion pathway to stratum corneum SC (Fluorescence microscopy), and determination of depth of permeation to deeper skin layers: epidermis and dermis (ATR-FTIR) and also of potential interaction with SC lipids and proteins (Fluorescence microscopy, ATR-FTIR) and the cytotoxicity studies against HaCaT cells (MTT test). The results show that D4, D5 and D5 can penetrate to SC and permeate into the deeper layers of the skin: epidermis and dermis. The quantitative analysis (GC-FID) showed that total cumulative doses for D4, D5 and D6 were: 42.50; 95.37 and 77.19 μg/cm2/24 h, respectively. The microscopic analysis proved, transepidermal route through the lipid matrix as well as through the canyons (intercluster spaces) were a diffusion pathway to the SC as well as disruption of human SC lipid structure by: D4 (the most), D5 and D6 (the least). The cytotoxicity studies demonstrated that the tested range of concentrations of D5 and D6 (up to 300 mM, 111 300 mg and 133 500 mg respectively) did not impaired the HaCaT growth, while D4 had IC50 value of 40 098 mM ± 7.94 (10 906 ± 872,5 mg).

Trophic transfer of cyclic methyl siloxanes in the marine food web in the Bohai Sea, China

Trophic transfer of cyclic methyl siloxanes (CMS) in aquatic ecosystems is an important criterion for assessing its environmental risks. This study researched the trophic transfer of four CMS (octamethylcyclotetrasiloxane (D₄), decamethylcyclopentasiloxane (D₅), dodecamethylcyclohexasiloxane (D₆), and tetradecamethylcycloheptasiloxane (D₇)) in marine food web from zooplankton up to seabirds in the Chinese Bohai Sea. In the zooplankton-invertebrate-fish-seabird based food web, the significant trophic magnifications were found for D₄ to D₆ (D₄: R² = 0.040, p < 0.05, D₅: R² = 0.26, p < 0.0001, D₆: R² = 0.071, p < 0.001), and the significant trophic dilution was found for D₇ (R² = 0.026 and p < 0.05). The trophic magnification factors (TMF) for D₄ to D₇ were 1.7 (95% confidence interval: 1.1-2.6), 3.5 (2.5-5.0), 1.8 (1.3-2.6), and 0.63 (95% CI: 0.40-0.99) respectively. In the zooplankton-invertebrate-fish based food web, both significant trophic magnification for D₅ (R² = 0.16, p < 0.0001, TMF = 3.0) and significant trophic dilution for D₇ (R² = 0.073, p < 0.01, TMF = 0.4) were found, but for D₄ and D₆, the trophic magnifications were not significant (D₄: R² = 0.010, p = 0.23, D₆: R² = 0.010, p = 0.23). The trophic transfer of the legacy contaminant BDE-47 and BDE-99 were also conducted as the benchmark chemicals and significant positive correlation was found. As far as we know, this is the first research on the trophic transfer of CMS in the zooplankton-invertebrate-fish-bird food chain which provided new insight of these compounds in the area.

Siloxane in baking moulds, emission to indoor air and migration to food during baking with an electric oven

Linear and cyclic volatile methylsiloxanes (l-VMS and c-VMS) are man-made chemicals with no natural source. They have been widely used in cosmetics, personal care products, coatings and many other products. As a consequence of their wide use, VMS can be found in different environmental media, as well as in humans. We bought 14 new silicone baking moulds and 3 metallic moulds from the market and used them in different baking experiments. Four of the silicone baking moulds were produced in Germany, two in Italy, four in China, and for the other moulds were no information available. The metal forms were all produced in Germany. VMS were measured in the indoor air throughout the baking process and at the edge and in the center of the finished cakes using a GC/MS system. Additionally, the particle number concentration (PNC) and particle size distribution were measured in the indoor air. The highest median concentrations of VMS were observed immediately following baking: 301 μg/m³ of D7, 212 μg/m³ of D6, and 130 μg/m³ of D8. The silicone moulds containing the highest concentrations of c-VMS corresponded with distinctly higher concentrations of the compounds in indoor air. Using a mould for more than one baking cycle reduced the indoor air concentrations substantially. Samples collected from the edge of the cake had higher concentrations relative to samples from the center, with a mean initial concentration of 6.6 mg/kg of D15, 3.9 mg/kg of D9, 3.7 mg/kg of D12, and 4.8 mg/kg of D18. D3 to D5 were measured only at very low concentrations. Before starting the experiment, an average PNC of 7300 particles/cm³ was observed in the room's air, while a PNC of 140,000 particles/cm³ was observed around the electric stove while it was baking, but this PNC slowly decreased after the oven was switched off. Baking with 4 of the moulds exceeded the German indoor precaution guide value for c-VMS, but the health hazard guide value was not reached during every experiment. Compared to other exposure routes, c-VMS contamination of cake from silicone moulds seems to be low, as demonstrated by the low concentrations of D4 and D6 measured. For less volatile c-VMS > D6 the results of the study indicate that food might play a more important role for daily intake. As a general rule, silicone moulds should be used only after precleaning and while strictly following the temperature suggestions of the producers.

Fluorotelomer alcohols (FTOHs), brominated flame retardants (BFRs), organophosphorus flame retardants (OPFRs) and cyclic volatile methylsiloxanes (cVMSs) in indoor air from occupational and home environments

Indoor air samples were collected from private homes and various occupational indoor environments using passive air sampler and analysed for fluorotelomer alcohols (FTOHs), brominated flame retardants (BFRs), organophosphorus flame retardants (OPFRs) and cyclic volatile methyl siloxanes (cVMSs). The aim was to investigate their occurrence in indoor air, factors that may affect their presence and human daily exposure dose (DED) via inhalation. In general, levels of cVMSs were 3-4 orders of magnitude greater than the other compound classes. OPFRs concentration was found significantly higher than BFRs in indoor air. The most abundant compounds in each chemical class were 8:2 FTOH, 2,4,6-TBP, TNBP and TCEP and decamethylcyclopentasiloxane (D5). Home samples contained higher level of FTOHs, BFRs and cVMSs than occupational environments, whereas concentration of OPFRs in office samples were higher. BFRs concentrations were significantly correlated with building age and with the number of electronic/electrical devices at the sampling sites. Moreover, significantly lower levels of FTOHs and cVMSs were observed in rooms with forced-ventilation system. Estimated DED via inhalation was significantly higher at home than in office and the total DED was on average 3-5 orders of magnitude lower than the reference value.

Use of multiple lines of evidence to provide a realistic toxic substances control act ecological risk evaluation based on monitoring data: D4 case study

D4 (octamethylcyclotetrasiloxane) is a high-production-volume cyclic volatile methyl siloxane with a wide range of industrial and consumer applications. This study conducted a robust ecological risk evaluation for D4 using exposure data collected under a nation-wide environmental monitoring program facilitated under the Toxic Substances Control Act (TSCA). This ecological risk evaluation was conducted consistent with the principles outlined in the U.S. Environmental Protection Agency's (EPA's) Guidance to Assist Interested Persons in Developing and Submitting Draft Risk Evaluations under TSCA (U.S. EPA 2017a). The evaluation examined multiple lines of evidence (LoEs) to determine the potential risks from D4 to aquatic receptors in rivers and streams in the United States from municipal wastewater treatment plant (WWTP) discharges and discharges from manufacturing, processing, and/or formulating (MPF) facilities after onsite wastewater treatment. The LoEs consisted of comparing D4 concentrations measured in water and sediment to toxicity thresholds derived from laboratory studies; comparing D4 concentrations measured in biota tissue to critical target lipid body burdens (CTLBBs); comparing fugacity-based chemical activities between toxicity thresholds and measured environmental concentrations; and assessing benthic macroinvertebrate community structure and habitat suitability. The approach taken moves beyond a standard deterministic hazard quotient approach to incorporate more advanced methods for risk prediction, using distributions rather than conservative point estimates of exposure to obtain a realistic view of the probability of harm, consistent with EPA's stated intent to "strive to utilize probabilistic approaches for exposure assessments included in a risk evaluation" (U.S. EPA 2017b). The risk evaluation concluded there is negligible risk to water column and sediment receptors from D4 discharged from MPF facilities after onsite wastewater treatment or from municipal WWTPs that may treat a mix of industrial and consumer wastewater.

Comprehensive atmospheric modeling of reactive cyclic siloxanes and their oxidation products

Cyclic volatile methyl siloxanes (cVMSs) are important components in personal care products that transport and react in the atmosphere. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and their gas-phase oxidation products have been incorporated into the Community Multiscale Air Quality (CMAQ) model. Gas-phase oxidation products, as the precursor to secondary organic aerosol from this compound class, were included to quantify the maximum potential for aerosol formation from gas-phase reactions with OH. Four 1-month periods were modeled to quantify typical concentrations, seasonal variability, spatial patterns, and vertical profiles. Typical model concentrations showed parent compounds were highly dependent on population density as cities had monthly averaged peak D5 concentrations up to 432ngm-3. Peak oxidized D5 concentrations were significantly less, up to 9ngm-3, and were located downwind of major urban areas. Model results were compared to available measurements and previous simulation results. Seasonal variation was analyzed and differences in seasonal influences were observed between urban and rural locations. Parent compound concentrations in urban and peri-urban locations were sensitive to transport factors, while parent compounds in rural areas and oxidized product concentrations were influenced by large-scale seasonal variability in OH.

Trophic dilution of cyclic volatile methylsiloxanes (cVMS) in the pelagic marine food web of Tokyo Bay, Japan

Bioaccumulation and trophic transfer of cyclic volatile methylsiloxanes (cVMS), specifically octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), and dodecamethylcyclohexasiloxane (D6), were evaluated in the pelagic marine food web of Tokyo Bay, Japan. Polychlorinated biphenyl (PCB) congeners that are "legacy" chemicals known to bioaccumulate in aquatic organisms and biomagnify across aquatic food webs were used as a benchmark chemical (CB-180) to calibrate the sampled food web and as a reference chemical (CB-153) to validate the results. Trophic magnification factors (TMFs) were calculated from slopes of ordinary least-squares (OLS) regression models and slopes of bootstrap regression models, which were used as robust alternatives to the OLS models. Various regression models were developed that incorporated benchmarking to control bias associated with experimental design, food web dynamics, and trophic level structure. There was no evidence from any of the regression models to suggest biomagnification of cVMS in Tokyo Bay. Rather, the regression models indicated that trophic dilution of cVMS, not trophic magnification, occurred across the sampled food web. Comparison of results for Tokyo Bay to results from other studies indicated that bioaccumulation of cVMS was not related to type of food web (pelagic vs demersal), environment (marine vs freshwater), species composition, or location. Rather, results suggested that differences between study areas was likely related to food web dynamics and variable conditions of exposure resulting from non-uniform patterns of organism movement across spatial concentration gradients.

Quantitative weight-of-evidence analysis of the persistence, bioaccumulation, toxicity, and potential for long-range transport of the cyclic volatile methyl siloxanes

Cyclic volatile methyl siloxanes (cVMSs) are highly volatile and have an unusual combination of physicochemical properties, which are unlike those of halocarbon-based chemicals used to establish criteria for identification of persistent organic pollutants (POPs) that undergo long-range transport (LRT). A transparent quantitative weight of evidence (QWoE) evaluation was conducted to characterize their properties. Measurements of concentrations of cVMSs in the environment are challenging, but currently, concentrations measured in robust studies are all less than thresholds of toxicity. The cVMSs are moderately persistent in air with half-lives ≤11 d (greater than the criterion of 2 d) but these compounds partition into the atmosphere, the final sink. The cVMSs are rapidly degraded in dry soils, partition from wet soils into the atmosphere, and are not classifiable as persistent in soils. Persistence in water and sediment is variable, but the greatest concentrations in the environment are observed in sediments. Based upon the measurements that have been made in the environment, cVMSs should not be classified as persistent. Studies in food webs support a conclusion that the cVMSs do not biomagnify, a conclusion that is consistent with results of toxicokinetic studies. Concentrations in air in remote locations are small and deposition has not been detected. Taken together, evidence indicates that traditional measures of persistence and biomagnification used for legacy POP are not suitable for cVMS. Refined approaches used here suggest that cVMSs are not classifiable as persistent, bioaccumulative, or toxic. Further, these chemicals do not undergo LRT in the sense of legacy POPs.

Evaluating the roles of biotransformation, spatial concentration differences, organism home range, and field sampling design on trophic magnification factors

Trophic magnification factors (TMFs) are field-based measurements of the bioaccumulation behavior of chemicals in food-webs. TMFs can provide valuable insights into the bioaccumulation behavior of chemicals. However, bioaccumulation metrics such as TMF may be subject to considerable uncertainty as a consequence of systematic bias and the influence of confounding variables. This study seeks to investigate the role of systematic bias resulting from spatially-variable concentrations in water and sediments and biotransformation rates on the determination of TMF. For this purpose, a multibox food-web bioaccumulation model was developed to account for spatial concentration differences and movement of organisms on chemical concentrations in aquatic biota and TMFs. Model calculated and reported field TMFs showed good agreement for persistent polychlorinated biphenyl (PCB) congeners and biotransformable phthalate esters (PEs) in a marine aquatic food-web. Model testing showed no systematic bias and good precision in the estimation of the TMF for PCB congeners but an apparent underestimation of model calculated TMFs, relative to reported field TMFs, for PEs. A model sensitivity analysis showed that sampling designs that ignore the presence of concentration gradients may cause systematically biased and misleading TMF values. The model demonstrates that field TMFs are most sensitive to concentration gradients and species migration patterns for substances that are subject to a low degree of biomagnification or trophic dilution. The model is useful in anticipating the effect of spatial concentration gradients on the determination of the TMF; guiding species collection strategies in TMF studies; and interpretation of the results of field bioaccumulation studies in study locations where spatial differences in chemical concentration exist.

Fugacity and activity analysis of the bioaccumulation and environmental risks of decamethylcyclopentasiloxane (D5)

As part of an initiative to evaluate commercial chemicals for their effects on human and environmental health, Canada recently evaluated decamethylcyclopentasiloxane (D5; CAS no. 541-02-06), a high-volume production chemical used in many personal care products. The evaluation illustrated the challenges encountered in environmental risk assessments and the need for the development of better tools to increase the weight of evidence in environmental risk assessments. The present study presents a new risk analysis method that applies thermodynamic principles of fugacity and activity to express the results of field monitoring and laboratory bioaccumulation and toxicity studies in a comprehensive risk analysis that can support risk assessments. Fugacity and activity ratios of D5 derived from bioaccumulation measures indicate that D5 does not biomagnify in food webs, likely because of biotransformation. The fugacity and activity analysis further demonstrates that reported no-observed-effect concentrations of D5 normally cannot occur in the environment. Observed fugacities and activities in the environment are, without exception, far below those corresponding with no observed effects, in many cases by several orders of magnitude. This analysis supports the conclusion of the Canadian Board of Review and the Minister of the Environment that D5 does not pose a danger to the environment. The present study further illustrates some of the limitations of a persistence-bioaccumulation-toxicity-type criteria-based risk assessment approach and discusses the merits of the fugacity and activity approach to increase the weight of evidence and consistency in environmental risk assessments of commercial chemicals.

Characterization of ecological risks from environmental releases of decamethylcyclopentasiloxane (D5)

Decamethylcyclopentasiloxane (D5) is used in personal care products and industrial applications. The authors summarize the risks to the environment from D5 based on multiple lines of evidence and conclude that it presents negligible risk. Laboratory and field studies show that D5 is not toxic to aquatic organisms or benthic invertebrates up to its solubility limit in water or porewater or its sorptive capacity in sediment. Comparison of lipid-normalized internal concentrations with measured concentrations in benthos indicates that field-collected organisms do not achieve toxic levels of D5 in their tissues, suggesting negligible risk. Exposure to D5 resulted in a slight reduction of root biomass in barley at test concentrations 2 orders of magnitude greater than measured D5 levels in biosolids-amended soils and more than twice as high as the maximum calculated sorptive capacity of the soil. No effects were observed in soil invertebrates exposed to similar concentrations, indicating that D5 poses a de minimis risk to the terrestrial environment. High rates of metabolism and elimination of D5 compared with uptake rates from food results in biodilution in the food web rather than biomagnification, culminating in de minimis risk to higher trophic level organisms via the food chain. A fugacity approach substantiates all conclusions that were made on a concentration basis.

Decamethylcyclopentasiloxane (D5) environmental sources, fate, transport, and routes of exposure

The environmental sources, fate, transport, and routes of exposure of decamethylcyclopentasiloxane (D5; CAS no. 541-02-6) are reviewed in the present study, with the objective of contributing to effective risk evaluation and assessment of this and related substances. The present review, which is part of a series of studies discussing aspects of an effective risk evaluation and assessment, was prompted in part by the findings of a Board of Review undertaken to comment on a decision by Environment Canada made in 2008 to subject D5 to regulation as a toxic substance. The present review focuses on the early stages of the assessment process and how information on D5's physical-chemical properties, uses, and fate in the environment can be integrated to give a quantitative description of fate and exposure that is consistent with available monitoring data. Emphasis is placed on long-range atmospheric transport and fate in water bodies receiving effluents from wastewater treatment plants (along with associated sediments) and soils receiving biosolids. The resulting exposure estimates form the basis for assessments of the resulting risk presented in other studies in this series. Recommendations are made for developing an improved process by which D5 and related substances can be evaluated effectively for risk to humans and the environment.

Bioconcentration and Aquatic Toxicity of Superhydrophobic Chemicals: A Modeling Case Study of Cyclic Volatile Methyl Siloxanes

Many chemicals in commerce are classified as "superhydrophobic", having log octanol-water partition coefficients (log KOW) approaching or exceeding 7. Examples include long-chain alkanes, halogenated aromatics, and cyclic volatile methylsiloxanes (cVMS). We show that superhydrophobic chemicals present unique assessment challenges because of their sparing solubility in water and difficulties in empirical determinations of bioconcentration factors (BCFs) and aquatic toxicity. Using cVMS as an example, BCFs are considerably lower than expected due to biotransformation. Reviewed aquatic toxicity test data for cVMS in a range of aquatic organisms show little or no toxic effects up to solubility limits in water and sediment. Explanations for this apparent lack of toxicity of cVMS, and by extension to other superhydrophobic chemicals, are explored using a conventional one-compartment uptake model to simulate bioconcentration and toxicity tests using an assumed baseline narcotic critical body residue (CBR) and a range of organism sizes. Because of the low aqueous concentrations, equilibration times are very long and BCFs are sensitive to even very slow rates of biotransformation. Most organisms fail to achieve the assumed CBR during feasible test durations even at the solubility limit. Regulatory evaluation of superhydrophobic substances requires specially designed test protocols addressing biotransformation and dietary uptake.