Breath control is a pretty common fetish, so I felt that I should post something about the risks that it involves. If people want to explore it, this post will hopefully help them understand the very considerable risks and whether they are worth it.
Breath play is any form of play to restricts the sub's ability to breathe. Serious breath play (which is what I want to address here) is any form of play that temporarily prevents a sub from breathing. Autoerotic asphyxiation (the so-called 'hanging game') is a common (and exceptionally lethal) form of breath play. Many forms of gagging fall into this category, as does choking. Some forms of bondage, such as those involving duct tape or plastic wrap over the sub's face, fall into this category (usually the sub is given a breathing tube).
Most people who do breath play assume that the primary risk involved is asphyxiation. They assume that as long as they make sure to release whatever is constricting the sub's breathing before the sub passes out (or when the sub hits the point of unconsciousness), then breathe play is safe. Certainly asphyxiation is a threat. In fact, it's probably the single-most common way that people die during bdsm (for example, the dom wraps the sub in plastic with a breathing tube and then leaves the sub in the closet. The breathing tube slips out and the immobilized sub asphyxiates). If you explore this sort of play, never, ever leave a sub in this situation alone. One major problem with the idea that you can remove the risk by stopping before unconsciousness is that there is no easy way to determine when a sub is about to fall unconscious. Even experienced medical personnel cannot always figure out when unconsciousness is imminent. And loss of air to the brain can produce brain damage.
However, asphyxiation is not actually the primary danger of breath play. The real danger is a heart attack. When the body begins to be deprived of oxygen, the heart automatically starts producing additional contractions; the lower the oxygen supply, the more erratic contractions the heart produces. If one of these contractions occurs during the wrong phase of the heart's normal pumping cycle, it can produce a heart attack. The more erratic contractions, the greater the odds of a heart attack. To make matters worse, the kind of heart attack that happens is a type of heart attack that people rarely survive, regardless of their age or relative health. (It's not the kind where the heart isn't beating; it's the kind where the heart is beating without normal coordination, making the contractions ineffectual for purposes of pushing blood through the body.) To makes matters worse still, the erratic contractions begin fairly soon after oxygen deprivation starts and continues for several hours after the deprivation ends. So, when you engage in breath play, you're essentially risking a fatal heart attack not only while the breath play is going on, but for quite some time after the breath play ends. Nor is there any easy way to watch for symptoms of erratic contractions.
There are other risks of breath play, which include brain damage, crushed windpipes, strokes, seizures, and more. Some people try to avoid the risk of asphyxiation by pressing on the carotid artery, under the impression that this is somehow less dangerous. In fact, it's even more dangerous. This can trigger a heart attack in as little as 5 seconds. This is the reason that police departments have stopped doing certain types of choke-holds, because they can easily kill someone they're trying to subdue.
Medical experts who are also bdsm players have been unable to identify a way to do breath play that does not trigger the risk of a heart attack. The risk of a heart attack is simply an inherent element of serious breath control. If, after understanding the risks, you wish to continue exploring it, by all means do so. Just understand that the sub is risking death and permanent brain damage, and the dom is risking a police investigation and possible criminal charges. My own feeling is that breath play is simply not worth the risks, but you may disagree.
If you want more information about the risks of breath play, check out Jay Wiseman's page on the subject, which has a number of articles. Wiseman is one of the most respected experts on bdsm, and has considerable medical training.
http://www.jaywiseman.com/SEX_BDSM_BreathPlayMain.html
According to Wiseman, US Law enforcement estimates that about 1000 people die in the US each year from various forms of breath play. So this is a form of play that has very substantial risks. But obviously not every person who does it dies, and there are many people who find it enjoyable for one reason or another.
Again, remember that bdsm needs to be safe, sane, and consensual. Doms have an obligation to understand the risks of any form of play they wish to engage in, and subs have a right to know the risks they are undergoing. Doing breath play without explaining the real risks to the sub means it's not truly consensual.