When it comes to skin care and makeup, many of us probably have a list of ingredients we would never put on our faces — some lists longer than others. But one ingredient with some heavily skewed opinions is polyaminopropyl biguanide, also known as polyhexamethylene biguanide or PHMB. But what is PHMB and is it bad for you? The ingredient is used in some pretty popular products in the USA but is actually banned from use in cosmetics in other countries. So what gives? To find out a little more about this controversial ingredient and whether you should avoid it, I emailed with a few experts.