Getting the smell out is not the problem. Having it revisit on a daily basis is.
Before, it was the foam in the earmuffs on the other hats that got aromatic. That foam makes up a very small portion of the helmet, so it took awhile (couple of weeks of hard work maybe) before things got out of hand.
But the Kasks - solid foam on the interior of the helmet, with a heater coil of some sort in there I'm certain, and I believe they come pre-innoculated with odor causing bacteria...geez...petri dish. Day #2 and I'm looking for my other helmet. What's real nice is early in the morning, toward the end of a long week, when you put your Kask on and lower the earmuffs down tight and you get a big squirt of fermented sweat from the days before running down your face and yes...in your mouth. MMmmmmmmmm.