I don't know what it is that smells, but there is a smell so strong that I have had to throw them out.  I washed them, and the smell was just as bad.  Maybe it's the material the glitter things are made of, or the glue that keeps them on, I don't know, but I bought three of these caps, wore them maybe once, loved them, but then they started to smell and out they went.  Total waste of money.