At the height of the Texas summer I discovered that my bottom of the line Bell helmet was getting particularly, um, ripe. On a whim, remembering what I learned working as a young dishwasher in a fancy restaurant, I place the helmet in the dishwasher with no soap, but with a few ounces of vinegar running the machine on rinse.

Worked like a charm. I only wish I thought to throw it in at the end of the day so it could dry overnight. I did so as I was starting my day, and it wound up a very antsy not riding day. Putting it upside-down, outside in the Texas sun dried it in several hours. YMMV.