In a rather interesting article from Fox News, Marine SGT Jeremy Soles ran the MCM this year with a gas mask that increases the work of respiration by at least 30%. He described it as if he was running while breathing through a snorkel. He managed to rig a water pack such that he could get some hydration and Gu during the run, but otherwise didn't take the thing off during the whole race.
Why, you might ask? He has a nonprofit (TEAM X-T.R.E.M.E.)which supports wounded veterans by performing feats of physical prowess, such as running a marathon in a gas mask. That and I'm guessing because he is a bad-ass, based on his appearance in the photo accompanying the article.