According to a study published in the International Journal of Toxicology, PEGs (including PEG 100 Stearate) can contain harmful impurities, including: Ethylene Oxide, known to increase the incidences of uterine and breast cancers and of leukemia and brain cancer, according to experimental results reported by the National Toxicology Program; 1,4-dioxane, a known carcinogen; PAHs, known to increase the risk of breast cancer; lead; iron; and arsenic (source).