Why not wash it to begin with? Lots of garments are treated with some pretty scary chemicals that don't even have to be listed on the labels. (That's how permanent press fabrics are made, just for one example.) They'll stay in the fabric for the life of the garment, but obviously the off-gassing is greatest when the garment is new, which is one of the reasons why many garments have a label that specifically says to wash before wearing.