The SM community (as was) used to accept choking/strangulation/suffocation was incredibly dangerous - but now young women report it's standard to be choked - and you won't be asked first.
this is from 1997, from a bloke who wrote S&M 101:
We have discussed how breath control might be done in a way that is not life-threatening, and come up blank. We have discussed how the risk might be significantly reduced, and come up blank. We have discussed how it might be determined that an arrest is imminent, and come up blank.
Indeed, so far not one (repeat, not one) single physician, nurse, paramedic, chiropractor, physiologist, or other person with substantial training in how a human body works has been willing to step forth and teach a form of breath control play that they are willing to assert is acceptably safe -- i.e., does not put the recipient at imminent, unpredictable risk of dying. I believe this fact makes a major statement.
www.telecomassociation.com/pubs/chokinggamereport/files/aea3.htm