In an out of air emergency, is it safe to rebreathe air exhaled into a buoyancy compensator? There are a number of considerations involved:
Will rebreathing allow you more time for ascent? Obviously, you can rebreathe your exhaled breath at least once. In mouth-to-mouth resuscitation, there is plenty of oxygen available. Continued rebreathing will naturally reduce the percentage of oxygen, but you are using it instead of exhaling it into the surrounding water. As the carbon dioxide builds up, your hunger for fresh air will increase, but your break point will be surprisingly long in coming. Experiments have shown that when a breath-hold break point is reached, exhaling into a bag and re-inhaling the same air will extend it. This is because when you inflate and deflate your lungs, you stimulate stretch receptors in your chest, thereby reducing air hunger. Another reason is that air in the dead spaces of the airways is moved back into the alveoli where oxygen can be absorbed. The increasing carbon dioxide concentration will eventually cause you to breathe faster. This ensures that breath-holding will be avoided.
Remember also that the air expands as you rise. If you start at 100 feet, you will have four times your original volume to breathe on the way up.
Is there any danger of breathing from a fully inflated BC? As long as the BC flexes, it is not only safe to breathe from, but it is also an exceptionally easy breather. The entire surface serves as a large diaphragm, furnishing air at the pressure of the surrounding water.
However, if the BC is allowed to become turgid from over-inflation so it no longer flexes, the air pressure inside can exceed that of the water. Breathing from it then could possibly cause a lung overpressure accident. Such an event is unlikely, however, during rebreathing. It takes more than five full breaths to fully inflate most BCs. Even at 130 feet (the safe limit for sport diving), the BC will expand about five times on the way up, so you will have plenty of air to breathe, but won’t overinflate the BC. To be certain, instead of exhaling into the BC, you can exhale through your nose occasionally as you rise.
What is the danger of breathing pure carbon dioxide? This is potentially dangerous, but can be avoided by habitually purging the CO2 from the BC following each cartridge inflation. You can flush it out by filling it with air from your tank or your lungs a few times.
If you forget to purge the CO2 and try to breathe it, you will be dramatically warned of its presence. It is stingingly pungent. (Although rated as odorless, in such high percentages, the CO2 combines quickly with the moisture in your mouth to form carbonic acid.)
Is there a danger of lung infection? Stale water in an unclean BC can result in concentrations of bacteria or fungus that may cause a lung infection if inhaled. In a life-saving emergency you won’t be concerned about an infection that may develop days later. Practicing rebreathing with an unclean BC, however, is not wise. Frequent rinsing with well-chlorinated swimming pool water may be sufficient, but to be sure, use benzalkonium chloride, the antiseptic used to sterilize similar hospital materials. The concentrated form is sold in drug stores. Follow the directions for dilution and application. Rinse with fresh water before using the BC.
Can you avoid inhaling the water in the mouthpiece? Purging the BC mouthpiece is essential to avoid inhaling water. Most BC mouthpieces have holes in the end. That allows the water to be blown out with the first puff. If there are no holes, the mouthpiece can be purged by rolling so the water flows into the bottom of the BC.
If stray droplets owing to imperfect purging happen to cause a coughing spell, don’t remove the mouthpiece. Cough into the BC. The expelled air will be there, ready to inhale when the coughing spell is over.
Master purging in shallow water before trying BC breathing at depth.
Will rebreathing, increasing percentages of CO2 and decreasing percentages of O2, cause physiological problems? A study of buoyancy compensator rebreathing was conducted at Temple University. Twenty-five subjects, men and women, fat and thin, smokers and non-smokers, were tested rebreathing from a BC for one minute. Oxygen and carbon dioxide levels were monitored using a gas analyzer. There was no compromise of the physiology of any of these subjects.
In addition, 22 basic scuba students, after having used a regulator underwater only twice, rebreathed from their BCs for one minute in a pool ten feet deep. None had any problems.