When we don't have enough oxygen in our system though exhaustion from physical exercise we slow down and/or stop. In extreme cases some may pass out due to lack of oxygenated blood supply to their brain.

Wearing a mask restricts air flow and so will bring about exhaustion faster. To counter this your diaphragm will have to work harder and so over continued use this will lead to development of the diaphragmatic muscle. This my be more helpful for deep sea diving or similar breath activities but any data on this is anecdotal.

There are few studies as to the effectiveness of the mask and I cannot find any scientific data but there are a number of personal reports on the effects of wearing these masks.

##Effectiveness of Masks

Key Points

• Wearing the ETM during a 6-week high-intensity cycle ergometer training program may improve performance variables, such as VO2max, PPO, VT, PO at VT, RCT and PO at RCT.
• Wearing the ETM did not improve lung function, inspiratory muscle strength, or stimulate changes in hemoglobin or hematocrit levels.
• The ETM does not simulate altitude, but works more like an respiratory training device.

Porcari, J. P., Probst, L., Forrester, K., Doberstein, S., Foster, C., Cress, M. L., & Schmidt, K. (2016). Effect of Wearing the Elevation Training Mask on Aerobic Capacity, Lung Function, and Hematological Variables. Journal of Sports Science & Medicine, 15(2), 379–386.

User /u/elguiri on reddit.com/r/running provided the following great analogy to support the "train low, live high" principle of training at low altitude and resting at high altitude over using a mask to simulate this:

First - you have to picture your ability to see ALL the gasses in the air (which would be pretty nuts). When you are at sea level (Say Boston, where I live) there is 20.9% effective oxygen in the air you breathe. This means, whenever you breathe in, 20.9% of the gas is basically oxygen, 78% is nitrogen and the rest is argon, CO2, methane. At sea level, we have higher air pressure. Why? Because there are more gasses on top, pressing down on you. If you could see all of the gas, there would be a lot above you at sea level.

Now - take yourself to Leadville, Colorado, at 10,000ft above sea level. At this point, you have 10,000ft less gas pushing down on the you. That means, the gasses have more room to spread out because of less air pressure. Because the gas can spread out with less air pressure, the air contains 14.3% effective oxygen. Every breath you take contains 6% less oxygen overall, which is 25% less oxygen (20.9% vs 14.3%) than at sea level.

The masks simply restrict the flow of oxygen to your lungs, however it cannot change the effective oxygen % of the air you take in. Even when breathing through a straw, you are still getting 20.9% relative oxygen at sea level. Even with the mask on, it's still 20.9%. That doesn't change, but yet effective oxygen is exactly the variable which needs to be changed to truly see impact for the reasons of wearing one.

A bigger rundown of the effectiveness can be found here: http://dirtinyourskirt.com/life/musings/stop-wearing-the-mask-it-just-makes-you-look-stupid/

###Anecdotal Evidence of Detrimental Effects:

I fell for the Training Mask gimmick about 2 years back. Thought it would help strengthen my weak ass lungs but what I actually was doing was creating micro-tears in the lining of my lungs, I'd cough up blood. Went to the doctor, told me I was a dumbass as the lungs aren't muscles but bags of air, using this fake product was creating unreasonable stress on my lungs because I was sucking air through a straw.

/u/kamikazeska - reddit.com/r/running

###Anecdotal Evidence of Positive Effects:

...When you take it off, you keep that same steady deep breath pattern versus doing choppy short breaths. I noticed that even when I got tired while running without the mask, I was able to tell myself "deep breaths" and I picked back up a rhythm, before the mask, that was never a thing.

Tiahnaon - amazon.com review

##Summary:

At best using a mask is a good way to help develop breathing technique when running. At worst it will cause significant damage to your lungs. There appears to be no scientifically-backed training benefit to using one over not and altitude athletes use different means to simulate working at low oxygen concentrations.