Methyl Siloxanes Research Articles

Comment on \u201cBioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China\u201d

We have reviewed a paper titled “Bioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China” by Xue et al., which was published in the Archives of Environmental Contamination and Toxicology in 2019. In the paper, the authors presented and discussed the measured bioconcentration factors (BCFs), biomagnification factors (BMFs), and trophic magnification factors (TMFs) of selected volatile methylsiloxanes in Shuangtaizi estuary in China. Although we appreciate their efforts for sample collection and data analysis, we have identified significant errors in calculations of BCFs, TMFs, and BMFs, as well as animal welfare issues and food web trophic level assumptions. Based on the data, we have attempted to correct some of the analysis and offered a more complete and robust interpretation of the related data, when possible. Collectively, these errors would likely lead to very different conclusions than yours in the paper.

Lung cell exposure to secondary photochemical aerosols generated from OH oxidation of cyclic siloxanes.

To study the fate of cyclic volatile methyl siloxanes (cVMS) undergoing photooxidation in the environment and to assess the acute toxicity of inhaled secondary aerosols from cVMS, we used an oxidative flow reactor (OFR) to produce aerosols from oxidation of decamethylcyclopentasiloxane (D5). The aerosols produced from this process were characterized for size, shape, and chemical composition. We found that the OFR produced aerosols composed of silicon and oxygen, arranged in chain agglomerates, with primary particles of approximately 31 nm in diameter. Lung cells were exposed to the secondary organosilicon aerosols at estimated doses of 54-116 ng/cm2 using a Vitrocell air-liquid interface system, and organic gases and ozone exposure was minimized through a series of denuders. Siloxane aerosols were not found to be highly toxic.

A review of contamination status, emission sources, and human exposure to volatile methyl siloxanes (VMSs) in indoor environments.

Siloxanes are organo-silicon compounds containing Si-O-Si linkages and methyl branches. Depending on the structure, siloxanes can be divided into cyclic and linear compounds. Methyl siloxanes with small and medium molecular weights (molecular weights less than 500 g mol-1), are volatile under normal conditions, and hence are referred to as volatile methyl siloxanes (VMSs). VMSs are additive ingredients in many products such as plastics, rubber, personal care products, and household items. This review provides information on the distribution of VMSs in consumer products, indoor air and dust, and their implications for human exposure. VMSs have been used in personal care products and household items at concentrations on the order of hundreds to thousands of micrograms per gram which are the main sources of contamination in the indoor environments. VMSs have been found widely in indoor air and dust. A significant correlation existed between VMS concentrations in indoor air and dust. Among typical VMSs, dodecamethylcylcopentasiloxane (D5) is the major compound found in indoor environments. The human exposure doses to VMSs through dermal absorption, dust ingestion, and inhalation were compiled; Inhalation is a dominant pathway of exposure to VMSs, especially in indoor environments of occupational settings like hair salons. The human exposure doses were higher in children than in adults.

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.

Physical properties of secondary photochemical aerosol from OH oxidation of a cyclic siloxane.

Cyclic volatile methyl siloxanes (cVMS) are high-production chemicals present in many personal care products. They are volatile, hydrophobic, and relatively long-lived due to slow oxidation kinetics. Evidence from chamber and ambient studies indicates that oxidation products may be found in the condensed aerosol phase. In this work, we use an oxidation flow reactor to produce ~ 100 μgm-3 of organosilicon aerosol from OH oxidation of decamethyl-cyclopentasiloxane (D5) with aerosol mass fractions (i.e., yields) of 0.2-0.5. The aerosols were assessed for concentration, size distribution, morphology, sensitivity to seed aerosol, hygroscopicity, volatility and chemical composition through a combination of aerosol size distribution measurement, tandem differential mobility analysis, and electron microscopy. Similar aerosols were produced when vapor from solid antiperspirant was used as the reaction precursor. Aerosol yield was sensitive to chamber OH and to seed aerosol, suggesting sensitivity of lower-volatility species and recovered yields to oxidation conditions and chamber operation. The D5 oxidation aerosol products were relatively non-hygroscopic, with an average hygroscopicity kappa of ~ 0.01, and nearly non-volatile up to 190 °C temperature. Parameters for exploratory treatment as a semi-volatile organic aerosol in atmospheric models are provided.

Uncertainty and equifinality in environmental modelling of organic pollutants with specific focus on cyclic volatile methyl siloxanes.

Multi-media fate and transport models (MFTMs) are invaluable tools in understanding and predicting the likely behaviour of organic pollutants in the environment. However, some parameters describing the properties of both the environmental system and the chemical pollutant under consideration are uncertain and or variable in space and time. Furthermore, model performance is often evaluated using sparse data sets on chemical concentrations in different media. This can result in equifinality - the phenomenon in which several different combinations of model parameters can result in similar predictions of environmental concentrations. We explore this idea for MFTMs for the first time using, as examples, three cyclic volatile methyl siloxanes (cVMS: D4, D5 and D6) and the QWASI lake model applied to Tokyo Bay. Monte Carlo simulation was employed with parameters selected from probability distributions representing estimated uncertainty in a large number of iterations. This generated distributions of predicted chemical concentrations in water (CW) and sediment (CS) which represent the aleatory uncertainty envelope but which also demonstrate significant equifinality. For all three compounds, the uncertainty implied in the CW was lower (coefficient of variation, CV, of the order of 20%) than for CS (CV ca. 45%), reflecting the propensity of cVMS compounds to sorb to sediment and the sensitivity of the model to KOC. Confidence intervals were particularly high for the persistence of D5 and D6 in sediment which both ranged between approximately 1.7 years and approximately 26 years for Tokyo Bay. Predicted concentration distributions matched observations well for D5 and D6 not for D4. Equifinality could be reduced by better constraining acceptable parameter sets using additional measured data from different environmental compartments.

Distribution and evaluation of the fate of cyclic volatile methyl siloxanes in the largest lake of southwest China

Cyclic volatile methyl siloxanes (cVMS) used in personal care products are released to aquatic environments through wastewater effluent. cVMS are persistent, toxic, bioaccumulative, and have potential to cause ecological harm. In this study, the environmental behavior of octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) were evaluated in the largest lake of southwest China, Dian Lake. Air, water and sediment samples were measured for three cVMS compounds in the winter (January) and summer (July) of 2017. In air, D5 exhibited the highest measured mean concentration among the three cVMS, which were 18.4 ± 8.0 ng·m−3 in winter and 5.78 ± 3.61 ng·m−3 in summer. In water and sediment, D6 was the cVMS with the highest measured mean concentration. The mean concentrations in water of D6 were 20.8 ± 5.8 ng·L−1 in winter and 20.4 ± 5.8 ng·L−1 in summer. The mean concentrations in sediment of D6 were 281 ± 45.8 ng·g−1 dw in winter and 270 ± 31.3 ng·g−1 dw in summer. A fugacity-based mass balance chemical fate model for lakes (QWASI) was used for Dian Lake to compare measurements and explore the behavior of cVMS. D6 was predicted to have the highest water column and sediment concentrations. Modeling results showed that most of the D5 and D6 partitioned into sediment and could persist for several years. Persistence was significantly influenced by the high rate of sediment burial. In an analysis of the impact of physicochemical properties and environmental parameters, KOC was identified as a key parameter for predicting cVMS behavior. This study illustrates the importance of cVMS in sediments and the potential aquatic risk that they may pose.

Determination of linear and cyclic volatile methyl siloxanes in biogas and biomethane by solid-phase microextraction and gas chromatography-mass spectrometry

A new method based on solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS) was developed for the analysis of seven linear (L2 – L5) and cyclic (D3 – D5) volatile methyl siloxanes (VMS) in biogas and biomethane, directly collected into Tedlar® bags (Tedlar SPME) from anaerobic digesters and wastewater treatment plants. The method was employed to monitor VMS content in biomethane produced by biogas upgrading with a pilot-plant membrane unit and provided adequate limits of quantification (< 0.05 mg m−3) to detect trace siloxane impurities. Tedlar SPME was validated against a standard procedure based on indirect sampling of gas streams with sorbent tubes followed by solvent extraction and GC-MS. Method precision (RSD) on total and individual VMS concentrations was lower than 10%, while RSD values of the standard procedure were higher than 20%. Tedlar SPME suitably revealed high VMS levels, expressed as total volatile silicon (> 1 mgSim−3), in wastewater biogas and provided a more efficient sampling of heavier VMS in comparison to the sorbent tubes method. At low values (< 0.1 mgSim−3) typical of wood waste biogas and biomethane, no statistically significant differences were observed between the two methods. Overall, Tedlar SPME simplified the analytical procedure by reducing the procedural steps, avoiding the use of solvents and demonstrated its applicability for testing the quality of biomethane as advanced biofuel.

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).

Bioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China.

Despite the high production volume and widespread use of methyl siloxanes, limited studies have been conducted to investigate the bioconcentration, biomagnification, and trophic magnification potentials of these substances. In the present study, bioconcentration factors (BCFs) of methyl siloxanes were determined with common carp exposed at environmental relevant levels for 32days. BCF of octamethylcyclotetrasiloxane (D4) was estimated as 6197L/Kg, indicating strong bioconcentration potential in the common carp. To assess the food chain transfer of methyl siloxanes, 12 aquatic invertebrates and vertebrates species were collected from a food web in Shuangtaizi estuary in northeastern China and concentrations of methyl siloxanes in these species were determined with gas chromatography mass spectrometry. Trophic magnification factors (TMFs) of decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and linear siloxanes (L7-10) were significantly greater than 1 in one food chain selected, which suggest trophic magnification potentials of these methyl siloxanes. Biomagnification factors of D4-D6 and L7-L10 from planktons to Japanese snapping shrimp were greater than 1, indicating biomagnification potentials of these methyl siloxanes from the prey to predator. This is the first study to investigate the bioaccumulation behaviors of methyl siloxanes by coupling BCF and BMF with TMF.

Headspace passive dosing of volatile hydrophobic chemicals – Aquatic toxicity testing exactly at the saturation level

It is challenging to conduct aquatic tests with highly hydrophobic and volatile chemicals while avoiding substantial sorptive and evaporative losses. A simple and versatile headspace passive dosing (HS-PD) method was thus developed for such chemicals: The pure liquid test chemical was added to a glass insert, which was then placed with the open end in the headspace of a closed test system containing aqueous test medium. The test chemical served as the dominating partitioning donor for establishing and maintaining maximum exposure levels in the headspace and aqueous solution, without direct contact between the donor and the test medium. The HS-PD method was cross validated against passive dosing with a saturated silicone elastomer, using headspace gas chromatography as analytical instrument and saturated vapors as reference. The HS-PD method was then applied to control the exposure in algal growth inhibition tests with the green algae Raphidocelis subcapitata. The model chemicals were C9-C14 n-alkanes and the cyclic volatile methyl siloxanes octamethyltetracyclosiloxane (D4) and decamethylpentacyclosiloxane (D5). Growth rate inhibition at the solubility limit was 100% for C9-C13 n-alkanes and 53 ± 31% (95% CI) for tetradecane. A moderate inhibition of 11 ± 4% (95% CI) was observed for D4, whereas no inhibition was observed for D5. The present study introduces an effective method for aquatic toxicity testing of a difficult-to-test group of chemicals and provides an improved experimental basis for investigating toxicity cut-offs.

Atmospheric Concentrations of New Persistent Organic Pollutants and Emerging Chemicals of Concern in the Group of Latin America and Caribbean (GRULAC) Region.

A special initiative was run by the Global Atmospheric Passive Sampling (GAPS) Network to provide atmospheric data on a range of emerging chemicals of concern and candidate and new persistent organic pollutants in the Group of Latin America and Caribbean (GRULAC) region. Regional-scale data for a range of flame retardants (FRs) including polybrominated diphenyl ethers (PBDEs), organophosphate esters (OPEs), and a range of alternative FRs (novel FRs) are reported over 2 years of sampling with low detection frequencies of the novel FRs. Atmospheric concentrations of the OPEs were an order of magnitude higher than all other FRs, with similar profiles at all sites. Regional-scale background concentrations of the poly- and perfluoroalkyl substances (PFAS), including the neutral PFAS (n-PFAS) and perfluoroalkyl acids (PFAAs), and the volatile methyl siloxanes (VMS) are also reported. Ethyl perfluorooctane sulfonamide (EtFOSA) was detected at highly elevated concentrations in Brazil and Colombia, in line with the use of the pesticide sulfluramid in this region. Similar concentrations of the perfluoroalkyl sulfonates (PFAS) were detected throughout the GRULAC region regardless of location type, and the VMS concentrations in air increased with the population density of sampling locations. This is the first report of atmospheric concentrations of the PFAAs and VMS from this region.

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.

Characterisation and cleaning of biogas from sewage sludge for biomethane production

This study investigates the conversion of sewage sludge from wastewater treatment plants (WWTP) into biomethane for automotive fuel or grid injection. A prototype plant was monitored in Northern Italy, based on vacuum swing adsorption (VSA) on synthetic zeolite 13×: this biogas upgrading method is similar to pressure swing adsorption (PSA) and commonly used for other kinds of biomass. Measurements of biogas inlet, biomethane outlet and off-gas were performed including CH4, CO2, CO, H2, O2, N2, HCl, HF, NH3, H2S and volatile organic compounds (VOCs). Critical levels were observed in the biogas for of H2S and HCl, whose concentrations were 1570 and 26.8 mg m−3, respectively. On the other hand, the concentration of halogenated VOCs (including tetrachloroethylene and traces of perfluoroalkilated substances, PFAS) and mercaptans were relatively low. A simultaneous and reversible adsorption on 13× zeolite was achieved for H2S and CO2, and carbon filters played a minor role in desulfurisation. The presence of HCl is due to clarifying agents, and its removal is necessary in order to meet the required biomethane characteristics: an additional carbon-supported basic adsorbent was successfully used to remove this contaminant. This study also highlights the interference of CO2 towards HCl if sampling is performed in compliance with the new EU standard for biomethane. High total volatile silicon (TVS) was confirmed in sewage sludge biogas, with a major contribution of siloxane D5: the suitability of this compound as an indicator of total siloxanes is discussed. Results demonstrate that volatile methyl siloxanes (VMS) do not represent a critical issue for the VSA upgrading methodology.

Atmospheric concentrations and trends of poly- and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network

Poly- and per-fluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) were monitored at 21 sites in the Global Atmospheric Passive Sampling (GAPS) Network. Atmospheric concentrations previously reported from 2009 were compared to concentrations measured at these sites in 2013 and 2015, to assess trends over 7 years of monitoring. Concentrations of the fluorotelomer alcohols (FTOHs) and fluorinated sulfonamides and sulfonamidoethanols (FOSAs and FOSEs) were stable at these sites from 2009 to 2015 with no significant difference (p > 0.05) in concentrations. Elevated concentrations of all the neutral PFAS were detected at the urban sites as compared to the polar/background sites. The perfluorosulfonic acids (PFSAs), meanwhile, saw a significant increase (p < 0.001) in concentrations from 2009 to 2015. The perfluorocarboxylic acids (PFCAs) had elevated concentrations in 2015, however, the difference was not statistically significant (p > 0.05). Concentrations of the PFSAs and the PFCAs were similar at all location types, showing the global reach of these persistent compounds. Concentrations of the cyclic VMS (cVMS) were at least an order of magnitude higher than the linear VMS (lVMS) and the PFAS. Octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) saw a weak significant increase in concentrations from 2009 to 2013 (p < 0.05), however, hexamethylcyclotrisiloxane (D3) had a strong significant decrease in concentrations from 2009 to 2015 (p < 0.01).

A critical assessment of the environmental fate of linear and cyclic volatile methylsiloxanes using multimedia fugacity models.

We apply multimedia models to systematically evaluate the fate profile of cyclic volatile methyl siloxanes (VMS) D4, D5 and D6, and the linear VMS L4 and L5 using recently reported measurements of their partition ratios between organic carbon and water (KOC), their salting out constants (Ks), and their enthalpy of sorption to organic carbon (ΔHOC). Our assessment follows a multi-stage strategy where the environmental fate of the chemicals is explored in generic regional models with increasing fidelity to the real system and in a region-specific model. Modeled emissions of VMS to air remained in air and were degraded or advected out of the system with overall residence times ranging from 2.4 to 2.5 days, while emissions to water resulted in accumulation in sediment and longer residence times ranging from 29.5 to 1120 days. When emitted to water the modeled residence times of VMS in the sediment exceeded the REACH criterion for persistence in freshwater sediments. Reported KOC measurements for D5 differ by 1 log unit, which results in a 500-day difference in the overall residence times calculated in the generic regional modeling. In the specific-region modeling assessment for Adventfjorden, Svalbard in Norway, the different KOC measurements of D5 resulted in a 200-day difference in overall residence times. Model scenarios that examined combinations of previously published ΔHOC or enthalpy of phase change between octanol and water (ΔHOW) for D5 in combination with the range of the KOC measurements resulted in 1100-days difference in overall residence times. Our results demonstrate that residence times of VMS in aquatic systems are highly sensitive to their degree of sorption to organic carbon, and that residence times of VMS likely exceed several persistence criteria and therefore they cannot be considered as non-persistent.

Design and analysis of siloxanes removal by adsorption from landfill gas for waste-to-energy processes

Separation of volatile methyl siloxanes from landfill gas using fixed adsorption beds was modeled with the objective of identifying appropriate technology and the economics associated with this purification step. A general adsorption model assuming plug flow and radial symmetry was developed and used to conduct a parametric sweep of 162 unique cases. The varied parameters were adsorbent type (activated carbon and silica gel), bed height (3.05–9.15 m/10–30 ft), inlet siloxane concentration (5–15 mg/m3), moisture content (0–100% relative humidity at STP or RH), and siloxane tolerance limit (0.094–9.4 mg/m3) that correlated to three distinct energy conversion technologies (electricity production using engines or fuels cells or catalytic conversion to liquid hydrocarbon fuels). Due to the detrimental effect of RH on siloxane absorption, the maximum allowable moisture content of LFG before purification is 50% RH and moisture removal processes are also required.The design calculations using a selected case study show that the adsorption bed height required needed for 6 months minimum breakthrough time for catalytic fuel production is twice that for engine applications. Fuel cell applications require 3 times the bed height compared to engine applications. However, the purification costs amounted to 94%, 16% and 52% of recovered product value for engine, liquefaction, and fuel cell applications, respectively indicating the need for a high value product to justify purification costs. The approaches and conclusions can be extended to specific process conditions for landfill gas purification and to other processes that use biogas produced from waste as a feedstock.

Methyl siloxanes in barbershops and residence indoor dust and the implication for human exposures

Methyl siloxanes (MSs) have been widely added as emollients or solvents in various personal care products (PCPs) such as soap, shampoo and hair conditioner. A considerable percentage of MSs are released into the atmosphere directly from the use of PCPs in indoor environments. Indoor dust is a major reservoir of MSs, due to their particle-binding affinity. Dust ingestion could be one exposure pathway for special populations (hairdressers, for example). Though the toxicity of MSs is known, there is no information regarding the occurrence of MSs in barbershop dusts and the exposure of hairdressers through indoor dust ingestion. In the present study, the levels of three cyclic siloxanes (D4–D6) and 13 linear siloxanes (L4–L16) were measured in indoor dust samples collected from barbershops, and some other microenvironment including bathhouses, dormitories and households for comparison. The concentrations of ∑MSs in barbershop dusts were one to two orders of magnitude higher than those from the other three indoor microenvironments. The daily intakes of ∑MSs through indoor dust ingestion were estimated using the model of worst-case exposure (95th percentile concentration) and high dust ingestion, For the hairdressers in workplaces (barbershops), the exposure rates were 14.3ng/kg-bw/day, and for the general population, college students and toddlers (1–3yr), the corresponding exposure rates in living spaces were 3.43, 2.00 and 222ng/kg-bw/day, respectively. Such high exposure levels of MSs through dust ingestion indicate that we should not overlook the potential health risks for occupational groups and toddlers.

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.

Metabolism and disposition of [14C]-methylcyclosiloxanes in rats

Octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) are low molecular weight cyclic volatile methyl siloxanes (cVMSs) primarily used as intermediates or monomers in the production of high molecular weight silicone polymers. The use of D4 as a direct ingredient in personal care products has declined significantly over the past 20 years, although it may be present as a residual impurity in a variety of consumer products. D5 is still used as an intentional ingredient in cosmetics, consumer products and in dry cleaning. Persons who may be exposed include occupational exposure for workers, and potential inhalation or dermal exposure for consumers and the general public. Because of the diverse use, especially of D5, and the potential for human exposure, a comprehensive program was undertaken to understand the kinetics, metabolism, enzyme induction and toxicity of D4 and D5 in rats following relevant routes of exposure. Physiologically based pharmacokinetic (PBPK) models utilizing these studies have been reported for D4 and D5 in the rat and human following dermal and inhalation exposures, with the oral uptake component of the model being limited in its description. Data from high dose oral studies in corn oil and simethicone vehicles and neat were used in the D4/D5 harmonized PBPK model development. It was uncertain if the inability to adequately describe the oral uptake was due to unrealistic high doses or unique aspects of the chemistry of D4/D5. Low dose studies were used to provide data to refine the description of oral uptake in the model by exploring the dose dependency and the impact of a more realistic food-like vehicle. Absorption, distribution, metabolism and elimination (ADME) of D4 and D5 was determined following a single low oral gavage dose of 14C-D4 and 14C-D5 at 30 and 100mg/kg body weight (bw), respectively, in a rodent liquid diet.Comparison of the low vs. high dose oral gavage administration of D4 and D5 demonstrated dose-dependent kinetic behavior. Data and modeling results suggest differences in metabolism between low and high dose administration indicating high dose administration results in or approaches non-linear saturated metabolism. These low dose data sets were used to refine the D4/D5 multi-route harmonized PBPK model to allow for a better description of the disposition and toxicokinetics of D4/D5 following oral exposure. With a refined oral uptake description, the model could be used in risk assessment to better define the internal dose of D4 and D5 following exposure to D4 and D5 via multiple routes.