Foxfish
Contributor
I've noticed browsing through forum threads on the recommended Nitrox limits, the question of how close you can sail to the MOD limits or time exposure limits based on PPO2 (partial pressure of oxygen) limits of 1.4 or 1.6 b often surfaces (pun intended). How conservative are the limits? How many people have run into problems when diving near or beyond the limits? Is it safe to exceed the limit for a short period of time.
Here are a few cases of illness or fatalities due to oxygen toxicity that have turned up in browsing the internet. Note that some of the cases occurred while diving within the prescribed limits.
Based on these reports I believe that the limits should be respected and exceeding them generally avoided.
While the risk of a 'hit' is low within the prescribed limits, there is still a chance that it can occur.
I'd be interested in seeing other reports of diver illness or fatality due to oxygen toxicity.
---------- Post added September 9th, 2013 at 11:07 PM ----------
The Dive:
The dive was planned to a ppO2 of 1.4 using a mix of 24/26. Because of the cave profile, a deco gas of 73% was used, since deco at 20ft was next to impossible. The dive was planned for a bottom run time of 30-35 minutes. This was Liz's and my second dive of the day; the first dive was at Little River, and there was a 5 hour surface interval.
We entered the water and ran the primary reel to the main line. I took a few pictures then clipped off the camera on the line due to its depth rating. We then we proceeded into the cave. As we approached the vertical fissure, Elizabeth moved in front to lead the dive. We descended to the crack and paused in the bedding plane while the third diver clipped our deco bottles to a line at 147 feet. At this time, Elizabeth signaled to me that she was not feeling 100%, but after a few moments she signaled she was fine. This was standard procedure, and occurred frequently on other dives after initial descent - such as at the bottom of the chimney in Peacock. Our buddy resumed the lead and we proceeded into the cave. As we approached the slightly deeper tunnel, the lead diver asked if it was ok to proceed a little further, and we all signaled yes. About 100ft into the smaller tunnel, Elizabeth signaled to turn the dive. This was at a max depth of 156ft, and at a runtime of 25 minutes.
We proceeded to turn and exit. Twice during the exit I turned and signaled "Are you ok?" and received a full "Ok" signal back. Immediately before retrieving my deco bottle I again signaled "Are you ok" and again received a complete "Ok" signal back. I retrieved my deco bottle and ascended 3 feet to allow Elizabeth to retrieve hers. She retrieved her bottle, and clipped off both clips properly. Immediately after clipping off her deco bottle at 147ft, at just short of 30 minutes runtime and less than 15 seconds after I had received an "Ok" from her, she reached up to her mask and air bubbles began venting from the top of her mask -- a sign I knew from prior dives that indicated she was dizzy. A few seconds later, the bubbles stopped and she began to fall sideways. I signaled our buddy and immediately went to her -- she was in a seizure. I ensured the regulator was secure in her mouth and righted her. At this point, the regulator fell out of her mouth. We tried pushing it back in numerous times but were unsuccessful every time -- her jaw would not budge. After remaining 3-4 minutes at depth (147-150ft) we began an ascent to our deco stop at 40 feet.
Upon reaching 40 feet, Elizabeth was still in a full seizure. We continued to try to push the regulator in her mouth, and frequently free-flowed the regulator in front of her mouth. I made my gas switch at 40 feet. At one point during the ascent we lost the line in the zero’ed vis, so I did a lost line drill while our buddy worked with Elizabeth. As we proceeded with our deco and struggled with her inflated drysut, Elizabeth was still seizing at 30ft, 12-15 minutes after she began. Because of my deco gases, my deco was shorter, so I left our buddy to ascend to my final stop, then I proceeded to the surface and called for help. We surfaced Elizabeth 45 minutes after the initial seizure, and was completely unresponsive.
Oxygen Toxicity fatality
---------- Post added September 9th, 2013 at 11:10 PM ----------
Arundo, New Jersey
1992July. A very experienced deep wreck diver knowingly dived beyond the NOAA oxygen limits while conducting an enriched air dive on the Arundo (135 fsw), suffered an oxygen seizure and drowned. The diver was breathing an EAN 40 (40% O2, balance nitrogen). This mix has a rated "Maximum Operating Depth" or MOD of 87 fsw (at a partial pressure of oxygen or PO2 of 1.45). However, the deck of the wreck is at 110 fsw with a maximum depth of 135 fsw, resulting in a PO2 of 1.7-2.0 atm which is well above the CNS toxicity threshold.
The diver had told others in the past that he didn't follow the NOAA guidelines as he believed they were too conservative. An individual who knew the diver well believed he was probably diving the USN's "extreme exposure" limits for oxygen which are generally not considered conservative enough. In one case, the diver recommended that another follow his example (After all diving air at 250 fsw is a PO2 of 1.8 atm. No problem!) . The problem is that CNS toxicity is a function of both PO2, time and other factors, many of which are not well understood. His body was found approximately 45-50 minutes into the dive with regulator out of his mouth and 1500 psi in his doubles. Maximum depth on his computer was 132 fsw.
aquaCorps'
---------- Post added September 9th, 2013 at 11:12 PM ----------
Chester Polling, Massachusetts
1992July. An experienced 45 year old wreck diver suddenly lost consciousness during a 170 fsw air dive on the Chester Polling and drowned in the arms of his partner. The exact cause of his death is unknown. The team descended on the "near virgin" wreck at 140-170 fsw for what planned to be a short first dive of the day leaving their inflatable boat unattended but anchored into the wreck. Conditions were good and there was no current. About 10-15 minutes into the dive, the surviving partner called the dive and begin to ascend to the bow at 140 fsw to free their anchor.
The diver drifted back down to the bottom briefly for one more sweep of the area. When he returned to their ascent line he didn't look right to his partner who signaled, "OK?" The diver signaled, "NO- Not OK," but didn't indicate what was wrong. His partner grabbed him by the harness to maintain contact during their ascent. As they ascended the diver began moving his arms and legs and then his legs went limp at about 90 fsw. At 80 fsw his regulator fell out of his mouth and the diver lost consciousness. The surviving partner was "freaked" and tried to resuscitate the diver without success. At 15 fsw the surviving partner elected to complete a portion of his decompression before surfacing, removed the diver's weightbelt, inflated his BC and pushed him to the surface. There was no surface support person or anyone on their boat. The surviving partner completed about 5 minutes of air decompression, surfaced and went on oxygen. A nearby sailboat had picked up the drowned diver and had radioed the Coast Guard station which was only a few minutes away. CPR was applied to no avail. There were no life signs. The diver was evacuated to the hospital and pronounced dead. The autopsy stated the cause of death was drowning. It is highly unlikely that the event was an oxygen convulsion (a PO2 of 1.26 atm at low to moderate work levels). The diver had no previous history of cardiac problems and was reportedly in great shape.
aquaCorps'
---------- Post added September 9th, 2013 at 11:14 PM ----------
Devil's Eye, Florida
1992July. A trained cave diver lost consciousness and drowned while making an enriched air stage dive at Devil's Eye cave system. His partner survived. The multilevel dive was conducted using air as a travel mix and a bottom mix of EAN 40. The maximum depth of the dive was 104 fsw. The dive team staged into the system on an aluminum 80cf air stage which was breathed for approximately 15 minutes into the dive before the switch to EAN 40. After about 60 minutes into the dive the surviving partner turned to see the diver stop and begin shaking before losing consciousness and spitting the regulator out of his mouth. His partner tried unsuccessfully resuscitate the diver and then attempted to swim the unconscious diver out of the cave. Soon realizing that his efforts were futile, the surviving diver exited the cave to get help. The body was recovered a short time later by a recovery team.
Investigators believe that an oxygen seizure was the cause of death. Though PO2s for most of the multilevel dive were at or below 1.4 atm (83 fsw on EAN 40), due to the configuration of the cave there were multi-minute portions of the dive with PO2s as high as 1.5-1.7 atm (95-105 fsw) placing the profile outside of the NOAA Oxygen Limits (a maximum PO2 of 1.6 atm) which are based on "moderate" diver work levels. The team was reported to be swimming hard in the "upstream" system which would have resulted in CO2 build-up and possibly increased the diver's sensitivity to convulsion. The family refused an autopsy.
aquaCorps'
---------- Post added September 9th, 2013 at 11:16 PM ----------
Lake Jocasee, North Carolina
1992July. An experienced cave diver suffered an oxygen seizure during decompression following a special mix open water dive to 300 fsw in Lake Jocasee, N.C. was treated for fresh water drowning and luckily survived due to excellent top-side support.
Utilizing a pair of large inflatables for surface support , safety divers and an continuous ascent/decompression line system, the eight minute planned jump to 300 fsw was conducted on trimix 14/33 (14% oxygen, 33% HE, balance N2. Max. working PO2 = 1.41 atm) with two intermediate mixes, an EAN 32 (@130 fsw) and an EAN 60 (@ 60 fsw) to be followed by surface supplied oxygen at 20 fsw. Back up oxygen bottles were carried by team members. Total planned decompression time was 61 minutes. Prior to reaching the 20 fsw oxygen stop, PO2's on the dive were at or below about 1.4 atm with the exception of two minutes at 120-130 fsw (PO2= 1.5-1.6 atm), and 6 minutes at 50-60 fsw (PO2=1.5-1.7) during the intermediate gas switches. The dive team discussed and dismissed the need for "air breaks" (the practice of breathing air for 5 minutes every 20-25 minutes during oxygen decompression which greatly reduces sensitivity to convulsions) as unnecessary during the oxygen decompression phase of the dive due to the short time (36 minutes) involved .
The dive proceeded as planned without incident until about 20 minutes into the oxygen decompression. The diver unclipped from the decompression line switching to his oxygen stage in order to swim over and check on a second team on a nearby decompression line on the second support boat. He did not communicate what he was doing to his partner who lost visual contact with the diver as soon as he swam off. Swimming slowly the diver lost some buoyancy, drifted down to about 35 fsw (PO2=2.06) and he believes he dozed off for several moments due to his excessive fatigue. He startled awake when his breathing became abnormal and quickly checked his depth as the onslaught of oxygen toxicity began. Fortunately experience took over. Holding his regulator in his mouth with one hand he hit his power inflator with the other as the seizure began. His actions saved his life. As he ascended uncontrolled, he was aware of losing his regulator at about 10 feet and hit the surface convulsing, face down and helpless before losing consciousness. The diver was rescued within moments of surfacing by the team's support personnel. His breathing had stopped. CPR was applied and the diver was resuscitated. He was soon evacuated to a nearby hospital, treated for fresh water drowning and recovered.
Though the diver's profile would normally be considered "light" from an oxygen tolerance perspective the short spike to 35 fsw coupled with the lack of an "air break" apparently led to trouble. Extenuating circumstances appear to be his condition before making the dive. A paramedic by profession, the diver had just come off of a 24 hour shift and had less than 2 hours of sleep the night before the dive. Fluid intake had been minimal and little food had been consumed over the previous 14 hours. Diver fatigue was believed to be the main cause of the accident.
aquaCorps'
---------- Post added September 9th, 2013 at 11:29 PM ----------
There seems to be variances in understanding of some of the limits. I was taught that MOD refers to the maximum operating depth you can dive for the PPO2 that the diver selects. The maximum allowable partial pressure is 1.6 b, but it is recommended that divers base their limits such as the MOD on 1.4 b. I believe this is the typical way to define these limits in recreational diving. A person could dive more conservatively if susceptible to other risk factors.
Here are a few cases of illness or fatalities due to oxygen toxicity that have turned up in browsing the internet. Note that some of the cases occurred while diving within the prescribed limits.
Based on these reports I believe that the limits should be respected and exceeding them generally avoided.
While the risk of a 'hit' is low within the prescribed limits, there is still a chance that it can occur.
I'd be interested in seeing other reports of diver illness or fatality due to oxygen toxicity.
---------- Post added September 9th, 2013 at 11:07 PM ----------
The Dive:
The dive was planned to a ppO2 of 1.4 using a mix of 24/26. Because of the cave profile, a deco gas of 73% was used, since deco at 20ft was next to impossible. The dive was planned for a bottom run time of 30-35 minutes. This was Liz's and my second dive of the day; the first dive was at Little River, and there was a 5 hour surface interval.
We entered the water and ran the primary reel to the main line. I took a few pictures then clipped off the camera on the line due to its depth rating. We then we proceeded into the cave. As we approached the vertical fissure, Elizabeth moved in front to lead the dive. We descended to the crack and paused in the bedding plane while the third diver clipped our deco bottles to a line at 147 feet. At this time, Elizabeth signaled to me that she was not feeling 100%, but after a few moments she signaled she was fine. This was standard procedure, and occurred frequently on other dives after initial descent - such as at the bottom of the chimney in Peacock. Our buddy resumed the lead and we proceeded into the cave. As we approached the slightly deeper tunnel, the lead diver asked if it was ok to proceed a little further, and we all signaled yes. About 100ft into the smaller tunnel, Elizabeth signaled to turn the dive. This was at a max depth of 156ft, and at a runtime of 25 minutes.
We proceeded to turn and exit. Twice during the exit I turned and signaled "Are you ok?" and received a full "Ok" signal back. Immediately before retrieving my deco bottle I again signaled "Are you ok" and again received a complete "Ok" signal back. I retrieved my deco bottle and ascended 3 feet to allow Elizabeth to retrieve hers. She retrieved her bottle, and clipped off both clips properly. Immediately after clipping off her deco bottle at 147ft, at just short of 30 minutes runtime and less than 15 seconds after I had received an "Ok" from her, she reached up to her mask and air bubbles began venting from the top of her mask -- a sign I knew from prior dives that indicated she was dizzy. A few seconds later, the bubbles stopped and she began to fall sideways. I signaled our buddy and immediately went to her -- she was in a seizure. I ensured the regulator was secure in her mouth and righted her. At this point, the regulator fell out of her mouth. We tried pushing it back in numerous times but were unsuccessful every time -- her jaw would not budge. After remaining 3-4 minutes at depth (147-150ft) we began an ascent to our deco stop at 40 feet.
Upon reaching 40 feet, Elizabeth was still in a full seizure. We continued to try to push the regulator in her mouth, and frequently free-flowed the regulator in front of her mouth. I made my gas switch at 40 feet. At one point during the ascent we lost the line in the zero’ed vis, so I did a lost line drill while our buddy worked with Elizabeth. As we proceeded with our deco and struggled with her inflated drysut, Elizabeth was still seizing at 30ft, 12-15 minutes after she began. Because of my deco gases, my deco was shorter, so I left our buddy to ascend to my final stop, then I proceeded to the surface and called for help. We surfaced Elizabeth 45 minutes after the initial seizure, and was completely unresponsive.
Oxygen Toxicity fatality
---------- Post added September 9th, 2013 at 11:10 PM ----------
Arundo, New Jersey
1992July. A very experienced deep wreck diver knowingly dived beyond the NOAA oxygen limits while conducting an enriched air dive on the Arundo (135 fsw), suffered an oxygen seizure and drowned. The diver was breathing an EAN 40 (40% O2, balance nitrogen). This mix has a rated "Maximum Operating Depth" or MOD of 87 fsw (at a partial pressure of oxygen or PO2 of 1.45). However, the deck of the wreck is at 110 fsw with a maximum depth of 135 fsw, resulting in a PO2 of 1.7-2.0 atm which is well above the CNS toxicity threshold.
The diver had told others in the past that he didn't follow the NOAA guidelines as he believed they were too conservative. An individual who knew the diver well believed he was probably diving the USN's "extreme exposure" limits for oxygen which are generally not considered conservative enough. In one case, the diver recommended that another follow his example (After all diving air at 250 fsw is a PO2 of 1.8 atm. No problem!) . The problem is that CNS toxicity is a function of both PO2, time and other factors, many of which are not well understood. His body was found approximately 45-50 minutes into the dive with regulator out of his mouth and 1500 psi in his doubles. Maximum depth on his computer was 132 fsw.
aquaCorps'
---------- Post added September 9th, 2013 at 11:12 PM ----------
Chester Polling, Massachusetts
1992July. An experienced 45 year old wreck diver suddenly lost consciousness during a 170 fsw air dive on the Chester Polling and drowned in the arms of his partner. The exact cause of his death is unknown. The team descended on the "near virgin" wreck at 140-170 fsw for what planned to be a short first dive of the day leaving their inflatable boat unattended but anchored into the wreck. Conditions were good and there was no current. About 10-15 minutes into the dive, the surviving partner called the dive and begin to ascend to the bow at 140 fsw to free their anchor.
The diver drifted back down to the bottom briefly for one more sweep of the area. When he returned to their ascent line he didn't look right to his partner who signaled, "OK?" The diver signaled, "NO- Not OK," but didn't indicate what was wrong. His partner grabbed him by the harness to maintain contact during their ascent. As they ascended the diver began moving his arms and legs and then his legs went limp at about 90 fsw. At 80 fsw his regulator fell out of his mouth and the diver lost consciousness. The surviving partner was "freaked" and tried to resuscitate the diver without success. At 15 fsw the surviving partner elected to complete a portion of his decompression before surfacing, removed the diver's weightbelt, inflated his BC and pushed him to the surface. There was no surface support person or anyone on their boat. The surviving partner completed about 5 minutes of air decompression, surfaced and went on oxygen. A nearby sailboat had picked up the drowned diver and had radioed the Coast Guard station which was only a few minutes away. CPR was applied to no avail. There were no life signs. The diver was evacuated to the hospital and pronounced dead. The autopsy stated the cause of death was drowning. It is highly unlikely that the event was an oxygen convulsion (a PO2 of 1.26 atm at low to moderate work levels). The diver had no previous history of cardiac problems and was reportedly in great shape.
aquaCorps'
---------- Post added September 9th, 2013 at 11:14 PM ----------
Devil's Eye, Florida
1992July. A trained cave diver lost consciousness and drowned while making an enriched air stage dive at Devil's Eye cave system. His partner survived. The multilevel dive was conducted using air as a travel mix and a bottom mix of EAN 40. The maximum depth of the dive was 104 fsw. The dive team staged into the system on an aluminum 80cf air stage which was breathed for approximately 15 minutes into the dive before the switch to EAN 40. After about 60 minutes into the dive the surviving partner turned to see the diver stop and begin shaking before losing consciousness and spitting the regulator out of his mouth. His partner tried unsuccessfully resuscitate the diver and then attempted to swim the unconscious diver out of the cave. Soon realizing that his efforts were futile, the surviving diver exited the cave to get help. The body was recovered a short time later by a recovery team.
Investigators believe that an oxygen seizure was the cause of death. Though PO2s for most of the multilevel dive were at or below 1.4 atm (83 fsw on EAN 40), due to the configuration of the cave there were multi-minute portions of the dive with PO2s as high as 1.5-1.7 atm (95-105 fsw) placing the profile outside of the NOAA Oxygen Limits (a maximum PO2 of 1.6 atm) which are based on "moderate" diver work levels. The team was reported to be swimming hard in the "upstream" system which would have resulted in CO2 build-up and possibly increased the diver's sensitivity to convulsion. The family refused an autopsy.
aquaCorps'
---------- Post added September 9th, 2013 at 11:16 PM ----------
Lake Jocasee, North Carolina
1992July. An experienced cave diver suffered an oxygen seizure during decompression following a special mix open water dive to 300 fsw in Lake Jocasee, N.C. was treated for fresh water drowning and luckily survived due to excellent top-side support.
Utilizing a pair of large inflatables for surface support , safety divers and an continuous ascent/decompression line system, the eight minute planned jump to 300 fsw was conducted on trimix 14/33 (14% oxygen, 33% HE, balance N2. Max. working PO2 = 1.41 atm) with two intermediate mixes, an EAN 32 (@130 fsw) and an EAN 60 (@ 60 fsw) to be followed by surface supplied oxygen at 20 fsw. Back up oxygen bottles were carried by team members. Total planned decompression time was 61 minutes. Prior to reaching the 20 fsw oxygen stop, PO2's on the dive were at or below about 1.4 atm with the exception of two minutes at 120-130 fsw (PO2= 1.5-1.6 atm), and 6 minutes at 50-60 fsw (PO2=1.5-1.7) during the intermediate gas switches. The dive team discussed and dismissed the need for "air breaks" (the practice of breathing air for 5 minutes every 20-25 minutes during oxygen decompression which greatly reduces sensitivity to convulsions) as unnecessary during the oxygen decompression phase of the dive due to the short time (36 minutes) involved .
The dive proceeded as planned without incident until about 20 minutes into the oxygen decompression. The diver unclipped from the decompression line switching to his oxygen stage in order to swim over and check on a second team on a nearby decompression line on the second support boat. He did not communicate what he was doing to his partner who lost visual contact with the diver as soon as he swam off. Swimming slowly the diver lost some buoyancy, drifted down to about 35 fsw (PO2=2.06) and he believes he dozed off for several moments due to his excessive fatigue. He startled awake when his breathing became abnormal and quickly checked his depth as the onslaught of oxygen toxicity began. Fortunately experience took over. Holding his regulator in his mouth with one hand he hit his power inflator with the other as the seizure began. His actions saved his life. As he ascended uncontrolled, he was aware of losing his regulator at about 10 feet and hit the surface convulsing, face down and helpless before losing consciousness. The diver was rescued within moments of surfacing by the team's support personnel. His breathing had stopped. CPR was applied and the diver was resuscitated. He was soon evacuated to a nearby hospital, treated for fresh water drowning and recovered.
Though the diver's profile would normally be considered "light" from an oxygen tolerance perspective the short spike to 35 fsw coupled with the lack of an "air break" apparently led to trouble. Extenuating circumstances appear to be his condition before making the dive. A paramedic by profession, the diver had just come off of a 24 hour shift and had less than 2 hours of sleep the night before the dive. Fluid intake had been minimal and little food had been consumed over the previous 14 hours. Diver fatigue was believed to be the main cause of the accident.
aquaCorps'
---------- Post added September 9th, 2013 at 11:29 PM ----------
There seems to be variances in understanding of some of the limits. I was taught that MOD refers to the maximum operating depth you can dive for the PPO2 that the diver selects. The maximum allowable partial pressure is 1.6 b, but it is recommended that divers base their limits such as the MOD on 1.4 b. I believe this is the typical way to define these limits in recreational diving. A person could dive more conservatively if susceptible to other risk factors.
