A Case of Caustic Cocktail


  • An assisted diver on a closed-circuit rebreather (CCR)  
  • Diving with friends on what should have been a regular recreational tech dive 
  • Miscommunication with crew caused an issue with the unit 

Reported Incident

A diver with ample experience on his CCR unit, is a disabled veteran, and dives with assistance. While diving with friends, the vessel had a new crew that were not yet trained on the proper handling of their equipment. The diver explained to the crew member their need of assistance – completing their pre-dive checklist and pre-breathe obligation while making way, then when in the water a crew member would pass the rebreather to the diver. When the new crew member was asked to hand them the rebreather, the diver noticed the crew member was letting the hoses drag on the boat while bringing it over. After making the final checks, the group proceeded with the dive. During descent, the affected diver took in a breath at 33 fsw (10 msw) and immediately noticed something was wrong and called the dive. The diver reports that they believe they inhaled a caustic cocktail.  

The diver explained that they couldn’t talk for about 30 minutes once back onboard, and that their throat was burning for a few days. The diver did not seek any medical attention or physician follow up but did refrain from diving for six consecutive days and had their equipment fully serviced before diving again.  

The diver thinks the hose might have come unattached at some point but that it did not fill with water until they were significantly submerged.   


Rebreathers are packed with a scrubber material to remove the carbon dioxide from exhaled gas. When water mixes with this material a chemical reaction produces a highly concentrated caustic soda, better known to rebreather divers as “caustic cocktail”. This can be dangerous if inhaled or ingested. Pre-dive checklists for rebreathers are essential in making sure the unit is functioning properly before the dive, as there are a number of considerations to diving with a rebreather. Water entering the canister which houses the scrubbing material can occur in many different ways, from something as simple as not closing the breathing loop to something more severe like a leak or disconnection. 

The reporting diver reports completing their checklist and did not mention any other abnormalities or other factors. The decision to call the dive and refrain from diving was one made from experience, as any delay in treatment or further exertion could have led to a much more serious incident. 


The first aid treatment for ingestion of caustic soda is to immediately flush repeatedly with water, preferably freshwater, but seawater is still effective if freshwater is unavailable. Doing so helps to reduce contact time of the caustic soda and dilute any residual solution, minimizing harm. Crew members working dive charters should be properly trained in the handling of diver’s equipment should they need to intervene for emergencies or assist divers as needed while onboard the vessel. Communication is important when working with divers that need assistance and buddy checks and proper briefings should be completed with those providing assistance as well as the buddies going diving.