Commonly-used ingredient that has multiple functions in cosmetics, including as a texture enhancer. It’s similar to propylene glycol, but has a lighter texture.
The Cosmetic Ingredient Review board has evaluated several toxicology tests and other research concerning butylene glycol and has determined it is safe as used in cosmetics products. The U.S. Food and Drug Administration (FDA) has even determined that butylene glycol is safe as a food additive.
References for this information:
The Journal of Dermatology, September 2014, pages 815-816
International Journal of Toxicology, May 2011, Supplement, pages 5S-16S
Journal of the American College of Toxicology, 1985, issue 2, pages 223-248