Having a sheet of rubber blocking your airway can take some getting used to, but for most people who can breathe through their noses, it isn't too uncomfortable. Those with conditions inhibiting nose-breathing, considered mouth-breathers in medical terms, may have considerable discomfort if fitted with a dental dam. Because a dam is attached either by special clamps, with adhesive or with some twining of dental floss over teeth -- similar to tying a boat line to a post on a dock or securing a tent by stretching it across stakes -- dentists and hygienists can fit the barrier tightly, preventing much exchange between the mouth cavity and the area being worked. Small holes are punched into the dam to match the opening size and placement of teeth needing exposure, but other than those cuts, the dam seals off the rest of the mouth [source: Perrine].
Some may find this setup claustrophobic, and there may be some gagging or difficulty breathing, but generally it is not a problem or major source of discomfort for patients. Precautions do need to be taken for those patients with latex allergies, but non-latex varieties of dental dams are available. Patients can refuse to wear dams with or without medical reasons such as mouth-breathing issues or fears of dental procedures.
Although some studies suggest that many dentists are foregoing the use of dental dams, most professional associations, including the American Dental Association (ADA), advocate their use and make a point of including them in training dental professionals. When used for root canals, for instance, not only do dental dams keep the open root area dry and better exposed so the dentist can focus on the work, they also keep bacteria and saliva away from an area where infection is being cleaned and where it needs to remain clean before closing up the tooth.
After opening wide and getting a dental diagnosis, it doesn't hurt to ask your dental care provider about when and if a dam will improve the quality of care and finished work and whether it's considered mandatory for the procedure.
More information and links on oral care and dental dams follows.