hexamethyldisiloxane

This chemical does not meet GHS hazard criteria for 0.1% (5 of 3584) of reports.

H225 (98.8%): Highly Flammable liquid and vapor [Danger Flammable liquids]

H400 (92.6%): Very toxic to aquatic life [Warning Hazardous to the aquatic environment, acute hazard]

H410 (19.3%): Very toxic to aquatic life with long lasting effects [Warning Hazardous to the aquatic environment, long-term hazard]

H411 (71.3%): Toxic to aquatic life with long lasting effects [Hazardous to the aquatic environment, long-term hazard]

P210, P233, P240, P241, P242, P243, P273, P280, P303+P361+P353, P370+P378, P391, P403+P235, and P501 (click each P-code to see the statement)

Aggregated GHS information provided per 3584 reports by companies from 24 notifications to the ECHA C&L Inventory. Each notification may be associated with multiple companies.

Reported as not meeting GHS hazard criteria per 5 of 3584 reports by companies.

There are 23 notifications provided by 3579 of 3584 reports by companies with hazard statement code(s).

Information may vary between notifications depending on impurities, additives, and other factors. The percentage value in parenthesis indicates the notified classification ratio from companies that provide hazard codes. Only hazard codes with percentage values above 10% are shown. For more detailed information, please visit ECHA C&L website.

H225: Highly Flammable liquid and vapor [Danger Flammable liquids]

H315: Causes skin irritation [Warning Skin corrosion/irritation]

H332: Harmful if inhaled [Warning Acute toxicity, inhalation]

H372: Causes damage to organs through prolonged or repeated exposure [Danger Specific target organ toxicity, repeated exposure]

H400: Very toxic to aquatic life [Warning Hazardous to the aquatic environment, acute hazard]

H410: Very toxic to aquatic life with long lasting effects [Warning Hazardous to the aquatic environment, long-term hazard]

P210, P233, P240, P241, P242, P243, P260, P261, P264, P270, P271, P273, P280, P302+P352, P303+P361+P353, P304+P340, P317, P319, P321, P332+P317, P362+P364, P370+P378, P391, P403+P235, and P501 (click each P-code to see the statement)

P210, P233, P240, P241, P242, P243, P280, P303+P361+P353, P370+P378, P403+P235, and P501 (click each P-code to see the statement)

SRP: The most favorable course of action is to use an alternative chemical product with less inherent propensity for occupational exposure or environmental contamination. Recycle any unused portion of the material for its approved use or return it to the manufacturer or supplier. Ultimate disposal of the chemical must consider: the material's impact on air quality; potential migration in soil or water; effects on animal, aquatic, and plant life; and conformance with environmental and public health regulations.

11 [ppm]

120 [ppm]

700 [ppm]

Liquid; Liquid; Other Solid; CBI; Dry Powder

Liquid; [HSDB] Colorless liquid; [MSDSonline]

101 °C @760 [mm Hg]

In water, 0.93 mg/L at 25 °C

0.93 mg/L @ 25 °C (exp)

0.7638 at 20 °C

0.76 @ 20°C

42.0 [mmHg]

log Kow = 4.2

Henry's Law constant = 4.53X10-2 atm-cu m/mol at 25 °C

When heated to decomposition it emits acrid smoke, fumes.

0.51 centistokes at 20 °C

Index of Refraction: 1.3774 at 20 °C

FREEZING POINT: -68 °C

Insoluble in most solvents, water repellent, resistant towards oxidation and chemical attack /Silicone resins/

Hydroxyl radical reaction rate constant = 1.38X10-12 cu cm/molec-sec at 24 °C

29Si nuclear magnetic resonance spectrum

Schoenflies notation

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Occupational hepatotoxin - Secondary hepatotoxins: the potential for toxic effect in the occupational setting is based on cases of poisoning by human ingestion or animal experimentation.

LC50 (rat) = 15,956 ppm/4h

LD50 Rat oral > 5000 mg/ kg

LD50 Mouse ip 4500 mg/kg

... Hexamethyldisiloxane (HMDS) coadministered with ethinyl estradiol (EE) did produce a small, but statistically significant reduction in uterine weight compared to EE alone.

Basic Treatment: Establish a patent airway (oropharyngeal or nasopharyngeal airway, if needed). Suction if necessary. Encourage patient to take deep breaths. Watch for signs of respiratory insufficiency and assist ventilations if necessary. Administer oxygen by nonrebreather mask at 10 to 15 L/min. Monitor for pulmonary edema and treat if necessary ... . Monitor for shock and treat if necessary ... . Anticipate seizures and treat if necessary ... . For eye contamination, flush eyes immediately with water. Irrigate each eye continuously with 0.9% saline (NS) during transport ... . Do not use emetics. For ingestion, rinse mouth and administer 5 ml/kg up to 200 ml of water for dilution if the patient can swallow, has a strong gag reflex, and does not drool ... . /Irritating materials/

Advanced treatment: Consider orotracheal or nasotracheal intubation for airway control in the patient who is unconscious, has severe pulmonary edema, or is in severe respiratory distress. Early intubation at the first sign of upper airway obstruction may be necessary. Positive-pressure ventilation techniques with a bag valve mask device may be beneficial. Consider drug therapy for pulmonary edema ... . Monitor cardiac rhythm and treat arrhythmias if necessary ... . Start IV administration of D5W /SRP: "To keep open", minimal flow rate/. Use 0.9% saline (NS) or lactated Ringer's if signs of hypovolemia are present. For hypotension with signs of hypovolemia, administer fluid cautiously. Watch for signs of fluid overload ... .Treat seizures with diazepam or lorazepam ... Use proparacaine hydrochloride to assist eye irrigation ... . /Irritating materials/

/HUMAN EXPOSURE STUDIES/ /Hexamethyldisiloxane/ was submitted to determine its ability to sensitize the skin of normal volunteer subjects using a repeated insult patch test. One-hundred subjects completed the study. Patches were applied to the infrascapular area of the back. The entire study extended over a six week period. It involved three phases: induction, rest, and challenge. The induction phases consisted of nine consecutive applications and subsequent evaluations of the test sites. The rest period lasted fourteen days. The challenge phase used identical patches applied to sites previously unexposed to the test material. Under the conditions employed in this study, there was no evidence of sensitization to the test substance.

/SIGNS AND SYMPTOMS/ Hexamethyldisiloxane has been noted to give off vapors which are transiently irritating to the conjunctiva, but no injury has been observed.

/OTHER TOXICITY INFORMATION/ Vapors have a very low toxicity for ... humans. ... /it/ irritates conjunctiva but does not attack the cornea.

/LABORATORY ANIMALS: Acute Exposure/ Guinea pig maximization test: not sensitizing

/LABORATORY ANIMALS: Acute Exposure/ Skin irritation: Rabbit: not irritating. A repeated skin irritation study was conducted. ... The procedure followed was based on (modified) methods of the CTFA Safety Testing Guidelines, Toiletries and Fragrances Association (Sept 27, 1985( and Preclinical Safety Evaluation of Materials used in Medical Devices HIMA report 85-1 "Skin Irritation." The test substance appears to cause no irritation in repeated, occluded contacts. However, irritation did develop outside of the application site which later spread to the site.

/LABORATORY ANIMALS: Acute Exposure/ Skin irritation: Rabbit: not irritating. The procedure followed was the modified method as required by the EPA Toxic Sustances Control Act ... Health Effects Testing Guidelines, "Primary Skin Irritation."

/LABORATORY ANIMALS: Acute Exposure/ A group of 10 albino rabbits was tested with a single dose of 2000 mg/kg undiluted test substance, no deaths or untoward behavioral reactions were noted ... Several studies show dermal LD50 in rabbit to be greater, but mortality was seen at 10,000 mg/kg. Toxic effects at 10,000 mg/kg in all animals included gross pathological findings (lungs, kidney, bladder, heart) while clinical findings (altered activity, ataxia, gasping and eschar formation) occurred in small numbers of rabbits. In contrast to rabbits, dermal exposure in rats did not produce mortality or signs of toxicity at the dose tested.

For more Non-Human Toxicity Excerpts (Complete) data for HEXAMETHYLDISILOXANE (18 total), please visit the HSDB record page.

The mutagenicity of hexamethyldisiloxane (HMDS) was evaluated in bacterial Salmonella tester strains TA98, TA100, TA1535, TA1537 and TA1538 and yeast Saccharomyces tester strain D4, both in the presence and absence of added metabolic activation by Aroclor-induced rat liver S9 fraction. HMDS, diluted in ethanol, was tested for mutagenicity at concentrations up to 5.0 ul/plate using the plate incorporation method. The mutagenicity of HMDS was also tested in bacterial E. coli indicator organisms, although the concentrations used and numerical results were not reported. HMDS did not cause a positive response in any of the bacterial or yeast tester strains, either with or without metabolic activation.

The ability of hexamethyldisiloxane to induce specific locus mutations at the TK locus in cultured L5178Y mouse lymphoma cells (Mouse Lymphoma Mutagenesis Assay) was evaluated in the presence and absence of Aroclor-induced rat liver S9 metabolic activation. Based on preliminary toxicity determinations, both nonactivated and S9 activated cultures were treated with 0.0125, 0.0250, 0.0500, 0.1000, or 0.2000 ul/ml which produced a range of 104 - 32% total growth for nonactivated cultures and from 107 - 0.3% total growth for S9-activated cultures. None of the nonactivated or activated cultures produced mutant frequencies significantly greater than the solvent controls (ethanol).

Hexamethyldisiloxane was examined for DNA modifying activity in Escherichia coli strains W3110 polA+ and P3478 polA- (DNA repair deficiency assay) with and without metabolic activation provided by Aroclor-induced rat liver S9 fraction. The test article did not produce zones of inhibition in either strain when spotted on plates at volumes ranging from 0.001 to 5 ul in the presence or absence of metabolic activation.

The frequency of sister chromatid exchange (SCE) was determined in L5178Y mouse lymphoma cells exposed in vitro to hexamethyldisiloxane with and without metabolic activation provided by mouse liver S9 fraction (induction of liver enzyme activity was not reported). The test article was administered at concentrations of 0.025, 0.05, 0.1 and 0.2 ul/ml in the absence of activation, and at concentrations of 0.025, 0.05 and 0.1 ul/ml in the presence of activation. A statistically significant (students t-test) increase in the mean number of SCE's/cell was observed at 0.05 ul/ml (p < 0.01) in the activated assays, and at 0.025 ul/ml (p < 0.01) and 0.1 ul/ml (p < 0.05) in the nonactivated assays. Statistically significant increases in the mean number of chromosomes/cell with SCE's, and in the mean number of SCE's/chromosome in chromosomes with SCE's were not observed under any condition. The test article was reported to be toxic to cells at concentrations greater than 0.2 ul/ml.

For more TSCA Test Submissions (Complete) data for HEXAMETHYLDISILOXANE (6 total), please visit the HSDB record page.

LC50 Oncorhynchus mykiss (rainbow trout) ca 3.02 mg/L/96 hr, flow-through bioassay

Hexamethyldisiloxane's production and use as a chemical intermediate in the synthesis of silicone fluids, elastomers and fluorosilicone oils as well as in photolithography, personal care products and cosmetics may result in its release to the environment through various waste streams. If released to air, a vapor pressure of 42.1 mm Hg at 25 °C indicates hexamethyldisiloxane will exist solely as a vapor in the atmosphere. Vapor-phase hexamethyldisiloxane will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 18 days. Methyl siloxanes are transparent to UV radiation >290 nm, and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation. If released to soil, hexamethyldisiloxane is expected to have slight mobility based upon an estimated Koc of 4,600. Volatilization from moist soil surfaces is expected to be an important fate process based upon a Henry's Law constant of 0.0453 atm-cu m/mole. Hexamethyldisiloxane may volatilize from dry soil surfaces based upon its vapor pressure. However, adsorption to soil is expected to attenuate volatilization. No biodegradation data regarding hexamethyldisiloxane were found; however dimethyl siloxanes in general are highly resistant to biodegradation. As a member of this class, biodegradation is not expected to be an important environmental fate process. Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble, indicating that soil mediated hydrolysis of hexamethyldisiloxane may be an important environmental fate process for hexamethyldisiloxane. If released into water, hexamethyldisiloxane is expected to adsorb to suspended solids and sediment based upon the estimated Koc. Volatilization from water surfaces is expected to be an important fate process based upon this compound's Henry's Law constant. Estimated volatilization half-lives for a model river and model lake are 1.3 hours and 5 days, respectively. However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered. An estimated BCF of 340 suggests the potential for bioconcentration in aquatic organisms is high. Occupational exposure to hexamethyldisiloxane may occur through inhalation and dermal contact with this compound at workplaces where hexamethyldisiloxane is produced or used. The most likely pathway by which the general public is exposed to hexamethyldisiloxane is by inhalation and dermal contact when personal care products and cosmetics containing this substance are used. (SRC)

Hexamethyldisiloxane's production and use as a chemical intermediate in the synthesis of silicone fluids, elastomers and fluorosilicone oils as well as in photolithography(1), personal care products and cosmetics(2) may result in its release to the environment through various waste streams(SRC).

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to have slight mobility in soil(SRC). Volatilization of hexamethyldisiloxane from moist soil surfaces is expected to be an important fate process(SRC) given a estimated Henry's Law constant of 0.0453 atm-cu m/mole(4). Hexamethyldisiloxane is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 42.1 mm Hg(5). However, adsorption to soil is expected to attenuate volatilization(SRC). No biodegradation data regarding hexamethyldisiloxane were found; however, dimethyl siloxanes in general are highly resistant to biodegradation(6). As a member of this class biodegradation is not expected to be an important fate process.

TERRESTRIAL FATE: Poly(dimethylsiloxane) (PDMS) fluids in intimate contact with many soils undergo siloxane redistribution and hydrolysis, resulting in the formation of low molecular weight cyclic and linear oligomers. Low molecular weight hydroxy-functional hydrolysis products are water soluble, and the cyclics and trimethylsiloxy-end-blocked oligomers are volatile, thus providing materials which can partition from the soil to the water and atmospheric environmental compartments(1). /Poly(dimethylsiloxane)/

AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon a Henry's Law constant of 0.0453 atm-cu m/mole(4). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 1.3 hours and 5 days, respectively(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered(5). According to a classification scheme(6), an estimated BCF of 340(SRC), from its log Kow(2) and a regression-derived equation(7), suggests the potential for bioconcentration in aquatic organisms is high(SRC). Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble(8), indicating that hydrolysis of hexamethyldisiloxane may be an important environmental fate process leading to products that will predominantly partition into the atmosphere. No biodegradation data regarding hexamethyldisiloxane were found; however dimethyl siloxanes in general are highly resistant to biodegradation(9). As a member of this class biodegradation is not expected to be an important fate process.

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hexamethyldisiloxane, which has a vapor pressure of 42.1 mm Hg at 25 °C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase hexamethyldisiloxane is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 18 days(SRC), calculated from its rate constant of 1.4x10-12 cu cm/molecule-sec at 25 °C(3). Methyl siloxanes are transparent to UV radiation >290 nm(4) and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation(SRC).

AEROBIC: No biodegradation data regarding hexamethyldisiloxane were found(SRC, 2006); however dimethyl siloxanes in general are highly resistant to biodegradation(1). As a member of this class biodegradation is not expected to be an important fate process.

The rate constant for the vapor-phase reaction of hexamethyldisiloxane with photochemically-produced hydroxyl radicals has been reported as 1.4X10-12 cu cm/molecule-sec at 25 °C(1). This corresponds to an atmospheric half-life of about 18 days at an atmospheric concentration of 5X10+5 hydroxyl radicals per cu cm(1). Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble(2), indicating that hydrolysis of hexamethyldisiloxane may be an important environmental fate process leading to products that will predominantly partition into the atmosphere. Methyl siloxanes are transparent to UV radiation >290 nm(3) and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation(SRC).

An estimated BCF of 340 was calculated for hexamethyldisiloxane(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is high(SRC), provided the compound is not metabolized by the organism(SRC).

The Koc of hexamethyldisiloxane is estimated as 4,600(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this estimated Koc value suggests that hexamethyldisiloxane is expected to have slight mobility in soil.

The Henry's Law constant for hexamethyldisiloxane is 0.0453 atm-cu m/mole(1). This Henry's Law constant indicates that hexamethyldisiloxane is expected to volatilize rapidly from water surfaces(2). Based on this Henry's Law constant, the volatilization half-life from a model river (1 m deep, flowing 1 m/sec, wind velocity of 3 m/sec)(2) is estimated as 1.3 hours(SRC). The volatilization half-life from a model lake (1 m deep, flowing 0.05 m/sec, wind velocity of 0.5 m/sec)(2) is estimated as 5 days(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered(3). Hexamethyldisiloxane's Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). Hexamethyldisiloxane is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 42.1 mm Hg(4).

Hexamethyldisiloxane was detected in two separate domestic waste sites in Munich Germany at concentrations of 0.01 and 0.31-0.45 mg/cu m(1). Hexamethyldisiloxane was also detected in the respective sewage treatment plants at concentrations of 0.14-0.17 and 0.18-0.20 mg/cu m, respectively(1).

SOURCE DOMINATED: Hexamethyldisiloxane was detected but not quantified in volatile chemical emissions from sponge rubber, bonded urethane and prime urethane carpet cushions(1).

Hexamethyldisiloxane was detected but not quantified in volatile chemical emissions from sponge rubber, bonded urethane and prime urethane carpet cushions(1). Hexamethyldisiloxane was detected but not quantified in emissions from new carpet vapors(2).

... SILICONES OF A LOW DEGREE OF CONDENSATION CAN PRESENT PROBLEMS IN INDUSTRIALTOXICOLOGY ... HEXAMETHYLSILOXANE /IS A/ ... SUBSTANCE OF RELATIVELY LOW BOILING POINT.

NIOSH (NOES Survey 1981-1983) has statistically estimated that 14,234 workers may be exposed to hexamethyldisiloxane in the US(1). Occupational exposure to hexamethyldisiloxane may occur through inhalation and dermal contact with this compound at workplaces where hexamethyldisiloxane is produced or used(SRC). The most likely pathway by which the general public is exposed to hexamethyldisiloxane is by inhalation and dermal contact when personal care products and cosmetics containing this substance are used(SRC).

LC50 Oncorhynchus mykiss (rainbow trout) ca 3.02 mg/L/96 hr, flow-through bioassay

Hexamethyldisiloxane's production and use as a chemical intermediate in the synthesis of silicone fluids, elastomers and fluorosilicone oils as well as in photolithography, personal care products and cosmetics may result in its release to the environment through various waste streams. If released to air, a vapor pressure of 42.1 mm Hg at 25 °C indicates hexamethyldisiloxane will exist solely as a vapor in the atmosphere. Vapor-phase hexamethyldisiloxane will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 18 days. Methyl siloxanes are transparent to UV radiation >290 nm, and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation. If released to soil, hexamethyldisiloxane is expected to have slight mobility based upon an estimated Koc of 4,600. Volatilization from moist soil surfaces is expected to be an important fate process based upon a Henry's Law constant of 0.0453 atm-cu m/mole. Hexamethyldisiloxane may volatilize from dry soil surfaces based upon its vapor pressure. However, adsorption to soil is expected to attenuate volatilization. No biodegradation data regarding hexamethyldisiloxane were found; however dimethyl siloxanes in general are highly resistant to biodegradation. As a member of this class, biodegradation is not expected to be an important environmental fate process. Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble, indicating that soil mediated hydrolysis of hexamethyldisiloxane may be an important environmental fate process for hexamethyldisiloxane. If released into water, hexamethyldisiloxane is expected to adsorb to suspended solids and sediment based upon the estimated Koc. Volatilization from water surfaces is expected to be an important fate process based upon this compound's Henry's Law constant. Estimated volatilization half-lives for a model river and model lake are 1.3 hours and 5 days, respectively. However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered. An estimated BCF of 340 suggests the potential for bioconcentration in aquatic organisms is high. Occupational exposure to hexamethyldisiloxane may occur through inhalation and dermal contact with this compound at workplaces where hexamethyldisiloxane is produced or used. The most likely pathway by which the general public is exposed to hexamethyldisiloxane is by inhalation and dermal contact when personal care products and cosmetics containing this substance are used. (SRC)

Hexamethyldisiloxane's production and use as a chemical intermediate in the synthesis of silicone fluids, elastomers and fluorosilicone oils as well as in photolithography(1), personal care products and cosmetics(2) may result in its release to the environment through various waste streams(SRC).

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to have slight mobility in soil(SRC). Volatilization of hexamethyldisiloxane from moist soil surfaces is expected to be an important fate process(SRC) given a estimated Henry's Law constant of 0.0453 atm-cu m/mole(4). Hexamethyldisiloxane is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 42.1 mm Hg(5). However, adsorption to soil is expected to attenuate volatilization(SRC). No biodegradation data regarding hexamethyldisiloxane were found; however, dimethyl siloxanes in general are highly resistant to biodegradation(6). As a member of this class biodegradation is not expected to be an important fate process.

TERRESTRIAL FATE: Poly(dimethylsiloxane) (PDMS) fluids in intimate contact with many soils undergo siloxane redistribution and hydrolysis, resulting in the formation of low molecular weight cyclic and linear oligomers. Low molecular weight hydroxy-functional hydrolysis products are water soluble, and the cyclics and trimethylsiloxy-end-blocked oligomers are volatile, thus providing materials which can partition from the soil to the water and atmospheric environmental compartments(1). /Poly(dimethylsiloxane)/

AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 4,600(SRC), determined from a log Kow of 4.2(2) and a regression-derived equation(3), indicates that hexamethyldisiloxane is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is expected(3) based upon a Henry's Law constant of 0.0453 atm-cu m/mole(4). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 1.3 hours and 5 days, respectively(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered(5). According to a classification scheme(6), an estimated BCF of 340(SRC), from its log Kow(2) and a regression-derived equation(7), suggests the potential for bioconcentration in aquatic organisms is high(SRC). Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble(8), indicating that hydrolysis of hexamethyldisiloxane may be an important environmental fate process leading to products that will predominantly partition into the atmosphere. No biodegradation data regarding hexamethyldisiloxane were found; however dimethyl siloxanes in general are highly resistant to biodegradation(9). As a member of this class biodegradation is not expected to be an important fate process.

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), hexamethyldisiloxane, which has a vapor pressure of 42.1 mm Hg at 25 °C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase hexamethyldisiloxane is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals(SRC); the half-life for this reaction in air is estimated to be 18 days(SRC), calculated from its rate constant of 1.4x10-12 cu cm/molecule-sec at 25 °C(3). Methyl siloxanes are transparent to UV radiation >290 nm(4) and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation(SRC).

AEROBIC: No biodegradation data regarding hexamethyldisiloxane were found(SRC, 2006); however dimethyl siloxanes in general are highly resistant to biodegradation(1). As a member of this class biodegradation is not expected to be an important fate process.

The rate constant for the vapor-phase reaction of hexamethyldisiloxane with photochemically-produced hydroxyl radicals has been reported as 1.4X10-12 cu cm/molecule-sec at 25 °C(1). This corresponds to an atmospheric half-life of about 18 days at an atmospheric concentration of 5X10+5 hydroxyl radicals per cu cm(1). Polydimethylsiloxane fluids undergo siloxane bond rearrangement to form low molecular weight linear and cyclic oligomers that are water soluble(2), indicating that hydrolysis of hexamethyldisiloxane may be an important environmental fate process leading to products that will predominantly partition into the atmosphere. Methyl siloxanes are transparent to UV radiation >290 nm(3) and therefore hexamethyldisiloxane is not expected to undergo direct photolytic degradation(SRC).

An estimated BCF of 340 was calculated for hexamethyldisiloxane(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is high(SRC), provided the compound is not metabolized by the organism(SRC).

The Koc of hexamethyldisiloxane is estimated as 4,600(SRC), using a log Kow of 4.2(1) and a regression-derived equation(2). According to a classification scheme(3), this estimated Koc value suggests that hexamethyldisiloxane is expected to have slight mobility in soil.

The Henry's Law constant for hexamethyldisiloxane is 0.0453 atm-cu m/mole(1). This Henry's Law constant indicates that hexamethyldisiloxane is expected to volatilize rapidly from water surfaces(2). Based on this Henry's Law constant, the volatilization half-life from a model river (1 m deep, flowing 1 m/sec, wind velocity of 3 m/sec)(2) is estimated as 1.3 hours(SRC). The volatilization half-life from a model lake (1 m deep, flowing 0.05 m/sec, wind velocity of 0.5 m/sec)(2) is estimated as 5 days(SRC). However, volatilization from water surfaces is expected to be attenuated by adsorption to suspended solids and sediment in the water column. The estimated volatilization half-life from a model pond is 30 days if adsorption is considered(3). Hexamethyldisiloxane's Henry's Law constant indicates that volatilization from moist soil surfaces may occur(SRC). Hexamethyldisiloxane is expected to volatilize from dry soil surfaces(SRC) based upon a vapor pressure of 42.1 mm Hg(4).

Hexamethyldisiloxane was detected in two separate domestic waste sites in Munich Germany at concentrations of 0.01 and 0.31-0.45 mg/cu m(1). Hexamethyldisiloxane was also detected in the respective sewage treatment plants at concentrations of 0.14-0.17 and 0.18-0.20 mg/cu m, respectively(1).

SOURCE DOMINATED: Hexamethyldisiloxane was detected but not quantified in volatile chemical emissions from sponge rubber, bonded urethane and prime urethane carpet cushions(1).

Hexamethyldisiloxane was detected but not quantified in volatile chemical emissions from sponge rubber, bonded urethane and prime urethane carpet cushions(1). Hexamethyldisiloxane was detected but not quantified in emissions from new carpet vapors(2).

... SILICONES OF A LOW DEGREE OF CONDENSATION CAN PRESENT PROBLEMS IN INDUSTRIALTOXICOLOGY ... HEXAMETHYLSILOXANE /IS A/ ... SUBSTANCE OF RELATIVELY LOW BOILING POINT.

NIOSH (NOES Survey 1981-1983) has statistically estimated that 14,234 workers may be exposed to hexamethyldisiloxane in the US(1). Occupational exposure to hexamethyldisiloxane may occur through inhalation and dermal contact with this compound at workplaces where hexamethyldisiloxane is produced or used(SRC). The most likely pathway by which the general public is exposed to hexamethyldisiloxane is by inhalation and dermal contact when personal care products and cosmetics containing this substance are used(SRC).

SRP: The most favorable course of action is to use an alternative chemical product with less inherent propensity for occupational exposure or environmental contamination. Recycle any unused portion of the material for its approved use or return it to the manufacturer or supplier. Ultimate disposal of the chemical must consider: the material's impact on air quality; potential migration in soil or water; effects on animal, aquatic, and plant life; and conformance with environmental and public health regulations.