I'm Not Sure That These Are a Good Idea:

Lately I've been getting a lot of ads for "elevation training masks," which essentially appear to be gas masks that increase the effort needed to breath, somehow increasing aerobic fitness and simulating training at altitude.  I've spent time in gas masks, and they do increase the effort needed to breath (just watch any SWAT competition), but I'm really doubtful that these will be of any benefit in training to function at altitude.  Some athletes would sleep in airlocks that were sort of reverse-Gamow bags, in which the oxygen content was lowered, causing an increase in the oxygen carrying capacity of the blood and speeding acclimatization.  That makes a lot more sense then essentially  breathing through a dense filter, you could stick cigarette butts up your nostrils and tape your mouth shut and get the same effect.  But these guys still use rubber sauna suits, so I'm not sure that there is a whole lot of exercise physiology science going into these products.