Elevation masks, also known as Altitude masks are designed to simulate the effects of training at altitude. The reason for this is that training at altitude can improve your aerobic performance, and as a result many endurance athletes train at altitude or sleep in hypoxic tents.
If elevation masks do work then they could save athletes (and regular gym-goers) a lot of time, effort, and money whilst producing fantastic results.
To understand why altitude training is so effective you need to understand about a hormone called Erythropoietin, which is more commonly known as EPO.
When your body releases EPO it causes red blood cell production to increase, more red blood cells leads to a greater oxygen capacity, which means that you can exercise for longer, or at a higher intensity.
It is for this reason that EPO is a banned substance for professional athletes, as is blood doping.
Athletes looking for an edge use blood doping as an easy way to improve aerobic performance. Take a quantity of blood out of your body and freeze it, then train as normal with much lower levels of blood.
This will cause EPO to be released which will increase red blood cell production, after a certain period of time the athlete will add the frozen blood back into their bloodstream and all of a sudden they have more red blood cells then they had previously.
Altitude training is a popular and more importantly legal way of stimulating EPO.
People who live at high altitude have higher levels of EPO than people who live closer to sea-level. So for a sea-level dwelling athlete to ensure that they also have high levels they must spend at least some time at altitude.
Interestingly, the majority of athletes tend to live at high altitude (when training for an event) but actually perform their training at low-altitude. This is because training at high altitude actually leads to poor performances . The philosophy is known as live high/train low.
If they can't do this, or don't want to leave home they can simulate the effects of living at high altitude by sleeping in a hypoxic tent. This is a tent where the oxygen levels have been altered to simulate high altitude.
Some up-market gyms have hypoxic chambers where their members can train at high altitude-like conditions but this misses the point. As mentioned earlier, you train at low-level and live at high altitude for best results.
Do elevation masks work?
While it would be impractical to assess all elevation masks, the most well known ones suffer from two problems.
1) They are trying to recreate training at high-altitude which even the professionals don't do, it's similar to running with your laces tied together. It just lowers your performance in the hope that it will toughen you up next time you run without your laces tied together (or when running at low altitude).
2) They fail at recreating altitude training! Most elevation masks work by restricting the amount of air that you can breathe in at one time, they reason that the less air you breathe in results in less oxygen. This reasoning is flawed, when you're at high altitude your ability to breathe in air isn't limited at all, it is the percentage of oxygen in the air that is affected.
What most elevation masks do is prevent you from breathing properly, this actually causes you to alter your breathing technique. This is not a good thing and will actually negatively affect your performance, rather than improving it.
As mentioned in point 1, athletes don't even train in high altitude they live at high altitude and train at low altitude. Meaning that if elevation masks were to successfully recreate live high/train low, you would need to be wearing them all the time except when training.
This is why athletes use hypoxic tents to sleep in, rather than train inside a hypoxic tent. So you can avoid your gym's brand new hypoxic chamber as well as the elevation masks.
So what should you do?
One question that you should ask yourself is whether altitude training is really necessary, endurance athletes use it to give them that last 5%. But you're not an endurance athlete, nobody is paying for you to travel to Mexico City or Denver Colorado because you don't need to focus on 5% you need to focus on the 95%.
Athletes already train twice per day up to seven times per week before they even consider altitude training or hypoxic tents. You could get more benefits by spending an extra hour running then you would from an elevation mask.
If you have money to burn and want to see some improvements then spend your money on healthy food or supplements, or some decent running/cycling gear that will have a guaranteed effect on your performance.
 McArdle, W., Katch, F. & Katch, V. 2007 Exercise Performance and Environmental Stress 6th Edition. Maryland: Lippincott Williams & Wilkins