A polybutene safety assessment published in the Journal of the American College of Toxicology defines this common ingredient as an isotactic polymer of isobutene and n-butene. Polymers are substances made of macro (or large) molecules, and isotactic means that the polymer's molecular structure uniformly repeats itself. Other names for polybutene include butene, polybutylene, and butene polymer.
The JACT report used skin irritation, oral toxicity, and pulmonary toxicity tests on animals and found polybutene to cause mild to no irritation when applied to the skin and eyes, ingested, or inhaled. The assessment cites limited human clinical data, but from what was available, polybutene seemed to cause little to no irritation during skin sensitivity, photosensitization, and phototoxicity tests.
Polybutene's limited effects on the skin are largely due to its macromolecule makeup, which board-certified dermatologist Dr. Carmen Castilla told Byrdie makes it impossible for the synthetic polymer to penetrate the skin. That's a lot of scientific jargon to say, cosmetically speaking, polybutene is harmless. But if it can't penetrate or notably affect the skin, why is it found in so many beauty products?
