Stoddard solvent (white spirit/mineral spirit) is the most widely used solvent in the paint industry. It is used as a dry cleaning agent; as an extraction, cleaning, and degreasing solvent; and as a solvent in aerosols, paints, wood preservatives, asphalt products, lacquers, and varnishes. Stoddard solvent IIC was nominated by the International Union, United Auto Workers, for carcinogenicity testing because of the large volume used in industrial and other settings. Male and female F344/N rats and B6C3F1 mice were exposed to Stoddard solvent IIC (greater than 99% pure) by inhalation for 2 weeks, 3 months, or 2 years. Genetic toxicology studies were conducted in Salmonella typhimurium and mouse peripheral blood erythrocytes.
Two-week study in rats
Groups of five male and five female rats were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 138, 275, 550, 1,100, or 2,200 mg/m3, 6 hours per day, 5 days per week for 16 days. All rats survived to the end of the study, and mean body weights of all exposed groups were similar to those of the chamber controls. Liver weights of males exposed to 550 mg/m3 or greater and of females exposed to 275 mg/m3 or greater were increased. Minimal diffuse cytoplasmic vacuolization of hepatocytes of the liver occurred in all females exposed to 2,200 mg/m3.
Two-week study in mice
Groups of five male and five female mice were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 138, 275, 550, 1,100, or 2,200 mg/m3, 6 hours per day, 5 days per week for 17 days. All mice survived to the end of the study, and mean body weights of all exposed groups were similar to those of the chamber controls. Liver weights of males and females exposed to 275 mg/m3 or greater were significantly increased. Cytomegaly of the liver occurred in all males and females exposed to 2,200 mg/m3.
Three-month study in rats
Groups of 10 male and 10 female rats were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 138, 275, 550, 1,100, or 2,200 mg/m3, 6 hours per day, 5 days per week for 14 weeks. All rats survived to the end of the study, and the final mean body weight of females exposed to 275 mg/m3 was greater than that of the chamber controls. The relative kidney, liver, and testis weights of all exposed groups of males and the absolute kidney weights of males exposed to 550 mg/m3 or greater were increased. The sperm motility of 550 mg/m3 or greater males was significantly decreased. The incidences of renal tubule granular casts were significantly increased in males exposed to 550 mg/m3 or greater, and the severities of renal tubule hyaline droplet accumulation, granular casts, and regeneration increased with increasing exposure concentration in males. The incidences of goblet cell hypertrophy of the nasal respiratory epithelium in males and females exposed to 2,200 mg/m3 were significantly increased. Sperm motility was decreased in males exposed to 550 mg/m3 or greater.
Three-month study in mice
Groups of 10 male and 10 female mice were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 138, 275, 550, 1,100, or 2,200 mg/m3, 6 hours per day, 5 days per week for 14 weeks. Mean body weights of exposed groups were similar to those of the chamber controls, but liver weights of males exposed to 2,200 mg/m3 were significantly increased. The sperm motility of 2,200 mg/m3 males was significantly decreased. This reduction in sperm motility, while statistically significant, is probably of modest importance as studies in mice have found that fertility is unaffected by motility decreases of less than 40%. The incidences of hematopoietic cell proliferation of the spleen in all exposed groups of females were greater than that in the chamber controls.
Two-year study in rats
Groups of 50 male and 50 female rats were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 138 (males), 550, 1,100, or 2,200 (females) mg/m3, 6 hours per day, 5 days per week for 104 to 105 weeks. Additional groups of 10 males and 10 females were exposed to the same concentrations for 3 months for renal toxicity analyses. Survival in the top exposure concentration groups of males and females was significantly less than that of the chamber controls. Mean body weights of exposed males and females were similar to those of the chamber controls.
Cell proliferation analyses were performed in the left kidney of males and females after 3 months of exposure. The mean numbers of labeled cells and the labeling indices in males exposed to 550 and 1,100 mg/m3 were significantly increased. The amount of α 2u-globulin in the right kidney of males increased with increasing exposure concentration. Also, the incidences of granular casts and cortical tubule degeneration and regeneration were generally increased in exposed males, as was the severity of hyaline droplets. These effects did not occur in females.
At 2 years, the incidences of benign and benign or malignant pheochromocytoma (combined) of the adrenal medulla occurred with positive trends in males, and the incidences in the 550 and 1,100 mg/m3 groups were significantly increased. Due to increased incidences of renal tubule hyperplasia in males at 2 years, extended kidney evaluations were conducted; a slightly increased incidence of renal tubule adenoma occurred in the 1,100 mg/m3 group. Nonneoplastic lesions related to Stoddard solvent IIC exposure occurred in the kidney of males.
Two-year study in mice
Groups of 50 male and 50 female mice were exposed to Stoddard solvent IIC by inhalation at concentrations of 0, 550, 1,100, or 2,200 mg/m3, 6 hours per day, 5 days per week for 105 weeks. Survival of exposed mice was similar to that of the chamber controls. Mean body weights of exposed females were greater than those of the chamber controls. The incidences of hepatocellular adenoma occurred with a positive trend in females, and the incidence of multiple hepatocellular adenoma in females exposed to 2,200 mg/m3 was significantly increased. However, the incidences of hepatocellular adenoma or carcinoma (combined) and hepatocellular carcinoma alone in exposed males and females were not significantly increased.
Genetic toxicology
Stoddard solvent IIC was tested for mutagenicity in Salmonella typhimurium strains TA97, TA98, TA100, and TA1535, with and without S9 metabolic activation enzymes; all results were negative. In vivo, the frequency of micronucleated erythrocytes was assessed in peripheral blood samples from male and female B6C3F1 mice after 3 months of inhalation exposure to Stoddard solvent IIC, and results were negative.
Conclusions
Under the conditions of these 2-year inhalation studies, there was some evidence of carcinogenic activity of Stoddard solvent IIC in male F344/N rats based on increased incidences of adrenal medulla neoplasms; the slightly increased incidences of renal tubule adenoma may have been related to Stoddard solvent IIC exposure. There was no evidence of carcinogenic activity of Stoddard solvent IIC in female F344/N rats exposed to 550, 1,100, or 2,200 mg/m3. There was no evidence of carcinogenic activity of Stoddard solvent IIC in male B6C3F1 mice exposed to 550, 1,100, or 2,200 mg/m3. There was equivocal evidence of carcinogenic activity of Stoddard solvent IIC in female B6C3F1 mice based on increased incidences of hepatocellular adenoma; this slight increase was associated with increased body weight in exposed females.
Exposure of male rats to Stoddard solvent IIC resulted in nonneoplastic lesions of the kidney characteristic of α 2u-globulin accumulation.