I was recently asked this question for the 10th time in the past few months so I thought I would address it. So the short answer is probably not for the reasons you see most people wearing the masks. I see a lot of “hard core” guys wearing gas masks and doing all sorts of crazy intense workouts in them. So the first thing is, if your are a fire fighter that may find yourself in an extreme hypoxic situation and want to get comfortable with being uncomfortable than this may have a practical purpose. Thinking that your body will adapt and create more red blood cells is probably unlikely.
There has been lots of research on training at high altitudes and competing at low altitudes. There is reasoning behind this type of training. At high altitudes the atmospheric pressure is lower, so the body will receive less oxygen with each breath(the % of O2 in the air is the same though). Over time the body will adapt and create more red blood cells to try and attach more oxygen. Upon returning to lower elevation the athlete will then have more red blood cells with the ability to now hold more Oxygen with the increased atmospheric pressure at the lower elevation. The problem is it takes the body weeks to months to fully adapt to high altitude to create more red blood cells. Wearing a mask for short bursts of training is not going to change the atmospheric pressure. Wearing a mask will restrict breathing and increase carbon dioxide in the blood which will increase the respiratory rate(breathing more frequently). Some argue that the difficulty in breathing through the mask will strengthen the diaphragm, but in many cases I think the difficulty in breathing will lead to more rapid and shallow chest breathing which is completely opposite of what happens in diaphragmatic breathing.
A mask is something I would never put on a client to train. A really well balanced conditioning program that has anaerobic and some aerobic conditioning involved is the best bet. In May I have a group of clients and friends getting ready for a StrongFirst kettlebell certification in Salt Lake City. SLC has an elevation of 4200 ft. This isn’t even really considered high elevation, but some are concerned. Tucson is about 2600 ft. Will there be a difference? Maybe a bit. So how am I training them. Well I want people to be able to do the snatch test in 4minutes and 20 seconds prior to going to the cert. So a solid conditioning program should not have too much of an effect. I am suggesting that they go to Mount Lemmon at a about 4500-5000 feet and attempting the snatch test at least once prior to the certification. This isn’t to attempt to create an adaptation, just to have a understanding of how they will feel at that elevation prior to the trip.