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TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 6.4(SRC), determined from a structure estimation method(2), indicates that methyl thioglycolate is expected to have very high mobility in soil(SRC). Volatilization of the undissociated form of methyl thioglycolate from moist soil surfaces is expected to be an important fate process(SRC) given an estimated Henry's Law constant of 6.22X10-6 atm-cu m/mole(SRC), using a fragment constant estimation method(3). However, a pKa of 8.08(5) indicates methyl thioglycolate will exist mostly in the ionized form at pH values above 8 and therefore at these pHs, volatilization from moist surfaces is not expected to be an important fate process. The potential for volatilization of methyl thioglycolate from dry soil surfaces may exist(SRC) based upon an estimated vapor pressure of 7.5 mm Hg(SRC), determined from a fragment constant method(4).

AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 6.4(SRC), determined from an estimation method(2), indicates that methyl thioglycolate is not expected to adsorb to suspended solids and sediment in water(SRC). Volatilization from water surfaces is expected(3) based upon an estimated Henry's Law constant of 6.22X10-6 atm-cu m/mole(SRC) for the unionized form, developed using a fragment constant estimation method(4). Using this Henry's Law constant and an estimation method(3), volatilization half-lives for a model river and model lake are 4 and 48 days, respectively(SRC). A pKa of 8.08(8) indicates methyl thioglycolate will exist mostly in the ionized form at pH values above 8 and therefore volatilization from water surfaces at these pHs is not expected to be an important fate process. According to a classification scheme(5), an estimated BCF of 3.1(SRC), from an estimated log Kow of 0.32(6) and a regression-derived equation(7), suggests the potential for bioconcentration in aquatic organisms is low.

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), methyl thioglycolate, which has an estimated vapor pressure of 7.5 mm Hg at 25 °C(2), is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase methyl thioglycolate 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 11 hrs(SRC), calculated from its rate constant of 3.5X10-13 cu cm/molecule-sec at 25 °C, determined using a structure estimation method(3).

Based on the available data, the CIR Expert Panel concluded that ... Methyl Thioglycolate is safe for use in hair straighteners, permanent waves, tonics, dressings, and so forth, wave sets, other non-coloring hair products, and hair dyes and colors, as described in this safety assessment, at concentrations up to 15.2% (as Thioglycolic Acid). Hairdressers should avoid skin contact and minimize consumer skin exposure.

Safe for use in cosmetics, with qualifications

Using a structure estimation method based on molecular connectivity indices(1), the Koc for methyl thioglycolate can be estimated to be 6.4(SRC). According to a classification scheme(2), this estimated Koc value suggests that methyl thioglycolate is expected to have very high mobility in soil. The pKa of methyl thioglycolate is 8.08(3), indicating that this compound will exist in the dissociated form in the environment at basic conditions and anions generally do not adsorb to organic carbon and clay more strongly than their neutral counterparts.

Methyl mercaptoacetate (CAS # 2365-48-2) was evaluated for acute dermal toxicity in guinea pigs (strain unspecified) administered occluded applications (dosage, volume unspecified) to depilated abdominal skin for 24 hours. Treatment was associated with signs of "moderate" dermal irritation in 2/2 animals including slight to moderate erythema and edema, necrosis at 24 hours, and scars at 14 days post-exposure. In 6 guinea pigs, unspecified dermal doses administered by the same method produced mortality (4/6) consistent with a dermal LD50 of 0.5-1 ml/kg. No further information regarding method or results was provided.

Methyl mercaptoacetate (CAS # 2365-48-2) was evaluated for subchronic oral toxicity in groups of 5 male rats (strain unspecified) fed 10, 100, and 300 mg/kg doses in corn oil by oral gavage on 5 days/week over a period of 15 days (11 treatments). After 2 doses of 300 mg/kg, 3/5 rats died, while the remaining 2 were sacrificed due to poor body condition. Clinical signs of toxicity included dehydration, hypoactivity, weakness, hypothermia, and poor body condition. No overt toxicity was noted among rats receiving 10 or 100 mg/kg doses, save a solitary 100 mg/kg male which exhibited transient inactivity after an initial dose only. In 100 mg/kg rats, the rate of weight gain during Days 1-4 and the terminal mean weight gain were significantly lower than those of controls. The 2 sacrificed rats of 300 mg/kg doses demonstrated depressed white cell and glucose levels, while aspartate aminotransferase, alanine aminotransferase, lactic dehydrogenase, and urea nitrogen values were elevated. Both hematological and clinical chemistry values were comparable to those of controls in rats fed 100 mg/kg doses. Upon necropsy of 300 mg/kg rats, gross lesions included hemorrhagic stomachs, unilateral testicular atrophy, adipose atrophy, discolored liver and spleen, and corneal opacity. Cellular changes were characterized by thymic lymphocytic necrosis, lipoid degeneration, hypertrophic hepatocytes and nuclear changes in the liver; glomerular casts, cytoplasmic vacuolization, and degeneration of the proximal convoluted tubules in the kidney; serous atrophy of adipose tissue; degenerated spermatids or atrophied seminiferous epithelium of the testicle, and degenerated spermatids in the epididymis. Terminal necropsy of lower dosed rats identified only eosinophilic cytoplasmic changes in the liver at 100 mg/kg (4/5). Rats dosed with 10 mg/kg showed no toxicity by the criteria and methods of this study.

An estimated BCF of 3.2 was calculated for methyl thioglycolate(SRC), using an estimated log Kow of 0.32(1,SRC) and a regression-derived equation(2). According to a classification scheme(3), this BCF suggests the potential for bioconcentration in aquatic organisms is low.

The Henry's Law constant for methyl thioglycolate is estimated as 6.22X10-6 atm-cu m/mole(SRC) for the undissociated form using a fragment constant estimation method(1). This Henry's Law constant indicates that methyl thioglycolate is expected to volatilize 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 4 days(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 48 days(SRC). A pKa of 8.08(4) indicates methyl thioglycolate will exist mostly in the ionized form at pH values above 8 and therefore volatilization from water surfaces at these pHs is not expected to be an important fate process. Methyl thioglycolate's Henry's Law constant(1) indicates that volatilization from moist soil surfaces may occur(SRC). The potential for volatilization of methyl thioglycolate from dry soil surfaces may exist(SRC) based upon an estimated vapor pressure of 7.5 mm Hg(SRC), determined from a fragment constant method(3).

The rate constant for the vapor-phase reaction of methyl thioglycolate with photochemically-produced hydroxyl radicals has been estimated as 3.5X10-13 cu cm/molecule-sec at 25 °C(SRC) using a structure estimation method(1). This corresponds to an atmospheric half-life of about 11 hours at an atmospheric concentration of 5X10+5 hydroxyl radicals per cu cm(1). A base-catalyzed second-order hydrolysis rate constant of 1.7 L/mole-sec(SRC) was estimated using a structure estimation method(2); this corresponds to half-lives of 48 and 5 days at pH values of 7 and 8, respectively(2). Methyl thioglycolate is not expected to undergo hydrolysis in the environment due to the lack of hydrolyzable functional groups(3) nor to directly photolyze due to the lack of absorption in the environmental UV spectrum (>290 nm).

Methyl thioglycolate's production and use as chemical intermediate may result in its release to the environment through various waste streams. If released to air, an estimated vapor pressure of 7.5 mm Hg at 25 °C indicates methyl thioglycolate will exist solely as a vapor in the ambient atmosphere. Vapor-phase methyl thioglycolate 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 11 hrs. If released to soil, methyl thioglycolate is expected to have very high mobility based upon an estimated Koc of 6.4. Volatilization from moist soil surfaces is expected to be an important fate process based upon an estimated Henry's Law constant of 6.22X10-6 atm-cu m/mole for the undissociated form. The pKa of methyl thioglycolate is 8.08 indicating that this compound will exist in the dissociated form in the environment at basic conditions and anions generally do not adsorb to organic carbon and clay more strongly than their neutral counterparts. Methyl thioglycolate may volatilize from dry soil surfaces based upon its vapor pressure. If released into water, methyl thioglycolate is not expected to adsorb to suspended solids and sediment in water based upon the estimated Koc. Volatilization from water surfaces is expected to be an important fate process based upon this compound's estimated Henry's Law constant. A pKa of 8.08 indicates methyl thioglycolate will exist mostly in the ionized form at pH values above 8 and therefore at these pHs, volatilization from water surfaces is not expected to be an important fate process. Estimated volatilization half-lives for a model river and model lake are 4 and 48 days, respectively. An estimated BCF of 3.2 suggests the potential for bioconcentration in aquatic organisms is low. Occupational exposure to methyl thioglycolate may occur through inhalation of this compound at workplaces where methyl thioglycolate is produced or used. (SRC)

LD50 Rat oral 209 mg/kg

LD50 Rat ip 252 mg/kg

LD50 Mouse ip 100 mg/kg