Propylene Glycol (PG) is a petroleum derivative that acts as a solvent and wetting agent. It also happens to be one of the main ingredients in anti-freeze as well as automatic brake fluid. It’s also used in pet food to keep it moisturized. It’s commonly used in cosmetics and skin formulas because it penetrates the skin easily and acts as a moisture absorbent, preventing it from escaping. Or put quite simply, it helps skin from drying out. Some of the unfortunate side effects of PGs include skin irritation and dry skin.