Halcyon Dive Systems
Rebreather Risks

Rebreather Risks

Understanding Rebreather Risks

Inherent Risks of Rebreathers

All rebreathers have an inherent risk greater than that found in open circuit scuba. Dive instruction should work to instill respect for the potentially fatal problems associated with rebreather diving, and active rebreather divers must take care not to become complacent. The following list outlines the most obvious problems associated with rebreather diving.


Because rebreathers recirculate a portion of each exhaled breath, carbon dioxide (CO2) generated by the body's use of oxygen must be eliminated before the gas can safely be returned to the diver. Failure to remove CO2 could cause hypercapnia - whose signs and symptoms include dyspnea, confusion, drowsiness, rigidity, spasms, loss of consciousness, and headache. CO2 is removed by directing exhaled gas through a CO2 "scrubber." The carbon dioxide in a diver's exhalation combines with water vapor in the loop, forming carbonic acid which is easily neutralized with a base material. Halcyon's rebreather scrubber is packed with a base material such as Sofnolime. As the carbonic acid passes through the scrubber, the CO2 molecules bind with the base material granules, neutralizing the CO2 from the exhaled gas. The byproducts of this chemical reaction are the formation of chalk, water vapor, and heat.

Hypercapnia's most troubling aspect is the difficulty of recognizing the symptoms in time to act. Divers should familiarize themselves with the technical problems that could cause CO2 build-up within the breathing loop:

Bypass of CO2 Scrubber

Since a rebreather is designed to allow gas to flow through the loop in only one direction through the scrubber, one-way valves are used on both sides of the mouthpiece. Should a one-way valve fail, the CO2-laden gas could simply flow back and forth in the hose without going through the scrubber.

Failure of CO2 Scrubber

Hypercapnia may also result from dives that exceed the scrubber material's duration. Rebreather divers must know how long their scrubber will last in the environment where they are diving. Certain environmental conditions, such as cold water, greatly reduce the scrubber material life. Water infiltration, either by a leak or condensation, into the scrubber can also reduce the efficiency of the scrubber medium. Divers who frequently reuse or are careless about replacing the scrubber medium are most likely to experience these problems.

Channeling due to a Poorly Packed Scrubber

In order for CO2 to be eliminated, it must come in contact with the scrubber medium. As the exhaled breathing mixture will flow through the scrubber along a path of least resistance, any pockets or voids in the scrubber media will allow some gas to pass through the scrubber with only a partial reaction to the medium. Care must be taken to ensure the scrubber media is thoroughly and correctly packed to avoid the gas forming channels, and thereby compromising the scrubber's effectiveness.


While water vapor is necessary to form the carbonic acid required to catalyze the CO2, large volumes of water in the scrubber will insulate the media from any gas flowing through the scrubber. It is imperative that all system integrity checks be done prior to every dive.


Breathing too high or too low a partial pressure of oxygen can also be life threatening. While most divers are familiar with the problem of too much oxygen (hyperoxia) thanks to Nitrox training, they may not be familiar with the problems of too little oxygen (hypoxia). Moreover, so long as a semi-closed rebreather diver does not exceed the MOD of the supply gas, the chances of hyperoxia are nonexistent barring obviously incorrect switches to the wrong breathing mixture. Hypoxia occurs when tissue cells are unable to receive sufficient oxygen through the arterial blood to maintain function.

How Hypoxia Can Occur

Hypoxia due to a problem with the rebreather:

  • Low fraction of oxygen in the mix (FO2) in conjunction with a corresponding low PPO2 due to shallow depth
  • Inadequate gas flow (especially in active rebreathers such as the Dolphin)
  • Inadequate purging or breathing bags
  • Gas addition failure of any type

Hypoxia due to a physiological problem:

  • Blockage of all or part of the pulmonary system air passages
  • Reduction of ventilation ability of lungs due to pneumothorax or lung failure
  • Reduction of oxygen exchange at the alveoli/capillary membrane or capillary tissue areas, by DCS, smoking, edema, etc…

A diver might experience Hypoxia if the total partial pressure of inspired oxygen dropped below certain values. We are used to breathing air at the surface, which contains roughly 21% oxygen (also known as a normoxic gas mixture), to give us a PPO2 of approximately .21 ATA. The table below reflects some commonly understood hypoxic levels.

Hypoxic Levels:

Normoxic .21 ATA
Mild Symptoms .16 ATA
Hypoxia Symptoms .14 ATA
Helplessness .11 ATA
Death .10 ATA

While hypoxia stops all normal cellular functions, brain cells are at the most risk. When hypoxia begins, the body reacts by raising its pulse rate and blood pressure to try and offset the effect by increasing circulation to deliver more blood. Unfortunately, since the brain is most affected, a diver could lose consciousness before recognizing the symptoms. The following symptoms often occur prior to hypoxia and may or may not be noticed by the diver:

  • Lack of concentration (which makes looking at CCR displays even more difficult)
  • Lack of muscle control
  • Inability to perform tasks requiring sharp skills (e.g. adding oxygen through solenoid)
  • Drowsiness, weakness, blackout
  • Failure of breathing control centers

Our minds are conditioned to inform us that we need to breathe when available air supplies start to deplete. Therefore, the best rebreather warning incorporates conditions that mimic an open circuit out-of-air situation. The Halcyon rebreather does this by ensuring that the loss of rebreather supply gas prevents the diver from continuing to breathe from an oxygen depleted supply. Without this convention, divers would rebreathe the same breath until they noticed the symptoms or lost consciousness.

What to do in the presence of symptoms:

Divers who feel as though they might be experiencing hypercapnia or hypoxia should immediately switch to an open circuit breathing supply to verify that the feeling is not related to gas in the breathing loop. These "reality checks" help insure that the diver is not surprised by problems in the breathing loop. Open circuit bailout regulators that are part of the breathing hoses facilitate the "reality check" and divers should always carry ample open circuit reserve gas supplies.