Oxygen Toxicity

 

Staying too long under high oxygen pressure can cause hyperoxie problems,
so there are depth limits (better O2-partialpressure-limits) and partial pressure-time limits.

Getting a spasm similar to epilepsy under water while breathing through a mouthpiece is much more dangerous than in the safe environment of a decompression chamber, which is why the limits are so wide spread. Workload, increases the risk of getting CNS problems when breathing gas with high ppO2. The risk of getting an oxygen-spasm increases with ppO2, exposition-time, workload, stress, fear and indisposition.

 ppO2-limits:

<0,12bar

- without adaptation: too low (fainting, death)

0,16bar

- minimum for full normal function

0,21bar

- God had build man for  

0,3 bar

- long time exposure (without lung toxicity problems)

0,5 bar

- maximum dose for long time diving (24h),
- first symptoms of "Lorraine Smith Effect"

1,3 bar

- usual solenoid controller setting on recreational CCR-dives

1,4 bar

- RAB: cold water / heavy work

1,6 bar

- RAB: warm water / easy dive

2,0 bar

- Marine (only in emergency cases)
- decompression chamber, max 3h

3,0 bar

- found in IDA71-Manual as short pass through exposition
- decompression chamber, max 2h

4,0 bar

- decompression chamber, max 40min.

 

Diving with Oxygen partial pressures above 0,5bar means that you have two clocks running that limit your dive by toxic effects.

 

CNS-Toxic, "Paul Bert Effect":

Oxygen partial pressures above 1,7bar within relatively short time will cause spasm similar to epilepsy. That is extremely dangerous when loosing your mouthpiece means inhaling water ! With higher partial pressure the time, until the risk of spasms becomes unacceptable, decreases rapidly.

The attacks mostly start with jerks around the mouth and the eyelids, before that nausea, dizzy feeling and tube-vision might be the first signals. Difficulties in breathing in the manner of quick shallow breathing attached with anxiety and suffocation feeling are leading very quickly to a tonic cramp of the entire body and unconsciousness.

U.S. Navy Diving Manual (1973):

 Normal Conditions

 Emergency

Time/min

ppO2-max/bar

Time/min

ppO2-max/bar

30

1,6

30

2,0

40

1,5

40

1,9

50

1,4

 

 

60

1,3

60

1,8

80

1,2

80

1,7

120

1,1

120

1,5

 

 

180

1,4

240

1,0

240

1,3

 

NOAA Limits under working conditions (1990):

ppO2/bar

Time/Day [min]

Time/Dive[min]

Time/Dive[min]
in Emergency

0,6

720

720

 

0,7

570

570

 

0,8

450

450

 

0,9

380

380

 

1,0

330

300

 

1,1

270

240

 

1,2

240

210

 

1,3

210

180

240

1,4

180

150

180

1,5

180

120

150

1,6

150

45

120

1,7

 

 

75

1,8

 

 

60

1,9

 

 

45

2,0

 

 

30

Between two Dives should be a surface time (ppO2=0,21bar) of at least 45 minutes.

 

ppO2
[bar]

CNS O2%
[%/min]

Duration
[min]

ppO2
[bar]

CNS O2%
[%/min]

Duration
[min]

0,50

0,00

1,22

0,48

208

0,60

0,14

714

1,24

0,51

196

0,64

0,15

667

1,26

0,52

192

0,68

0,17

588

1,28

0,54

185

0,70

0,18

556

1,30

0,56

179

0,74

0,19

526

1,32

0,57

175

0,76

0,20

500

1,34

0,60

167

0,78

0,21

476

1,36

0,62

161

0,80

0,22

455

1,38

0,63

159

0,82

0,23

435

1,40

0,65

154

0,84

0,24

417

1,42

0,68

147

0,86

0,25

400

1,44

0,71

141

0,88

0,26

385

1,46

0,74

135

0,90

0,28

357

1,48

0,78

128

0,92

0,29

345

1,50

0,83

120

0,94

0,30

333

1,52

0,93

108

0,96

0,31

323

1,54

1,04

96

0,98

0,32

313

1,56

1,19

84

1,00

0,33

303

1,58

1,47

68

1,02

0,35

286

1,60

2,22

45

1,04

0,36

278

1,62

5,00

20

1,06

0,38

263

1,65

6,25

16

1,08

0,40

250

1,67

7,69

13

1,10

0,42

238

1,70

10,0

10

1,12

0,43

233

1,72

12,5

8

1,14

0,43

233

1,74

20,0

5

1,16

0,44

227

1,77

25,0

4

1,18

0,46

217

1,79

31,25

3

1,20

0,47

213

1,80

50

2

 

Recovery [h:min]

0:30

1:00

1:30

2:00

2:30

3:00

3:30

4:00

4:30

5:00

6:00

9:00

Multiplicator

0,8

0,63

0,5

0,4

0,31

0,25

0,2

0,16

0,13

0,1

0,06

0

Example:

1st Dive 2h at ppO2=1,3bar, 3h 15min Surface-Break at ppO2=0,21bar,
2nd Dive 1h bei ppO2=0,7bar

120Minutes*0,56CNS%/min=67,2CNS%

67,2CNS%*0,25=16,8CNS%

16,8CNS% + 60Minutes*0,18CNS%/min=27,6CNS%

 

 

Lung-Toxic, "Lorraine Smith Effect":

Symptoms may appear in any chronological order: cough without cough up, raising breathing resistance, complete inhaling becomes difficult, lower vital capacity, pain in the chest- and breastbone area, clumsiness and coordination problems.

In the early 60s a unit was defined to measure the lung toxic effects:
UPTD (unit pulmonary toxic dose) = OTU (oxygen tolerance unit) = CPTD (cumulative pulmonary toxic dose)

1 OTU = 1bar-ppO2 * 1minute 

 ppO2/bar

 0,5

 0,6

0,7 

 0,8

 0,9

 1

 1,1

 1,2

 1,3

 1,4

 1,5

 OTU/min

 0

0,285

0,49

 0,658

 0,881

 1

 1,18

 1,32

 1,47

 1,62

 1,77

 

 ppO2/bar

 1,6

 1,7

1,8 

 1,9

 2,0

 2,1

 2,2

2,3

2,4

 2,5 

 OTU/min

 1,92

 2,01

2,2

2,34

 2,48

2,61

2,74 

2,88

3,0

3,14

 

 Divedays

Average OTUs/day

Total OTU-Limit

1

850

850

2

700

1400

3

620

1860

4

525

2100

5

460

2300

6

420

2520

7

380

2660

>7  

300

 

 


 

Nitrogen Narcosis
("Raptures of the Deep")
 

What is it?
Nitrogen narcosis is an effect on the brain of gaseous nitrogen that occurs to divers who go below 100 FSW, due to the laws of partial pressures. Nitrogen is an inert gas existing in largest quantity in the atmosphere, 79% in air. It is inert, meaning that it does not take part in energy transformations. It is the gas that causes nitrogen narcosis through the effect of Dalton's law and it is the gas that causes decompression sickness on ascent from depth with reduction of pressure, (Boyle's Law). Nitrogen is the gas that determines decompression schedules.

What are some of the effects?

Complex reasoning decreases 33% and manual dexterity decreases 7.3%. The condition causes loss of motor function and decision making ability and can be more clearly defined as causing one to become "drunk", as with alcoholic beverages. The comparison to having had "three Martinis" is apt, and it has been stated that one should consider the narcotic effect of one Martini for every 50 feet of sea water.
What is Dalton's Law?
Dalton's Law states that the total pressure exerted by a mixture of gases is equal to the sum of the pressure of each of the different gases making up the mixture - each gas acting as if it alone was present and occupying the total volume. This same law causes oxygen toxicity and enhances the role of contaminant gases such as carbon monoxide and hydrocarbons.

The law is stated as:

p ATA=pO2 + pN2 + p other gases
thus: pN2= fN2 x ATA
How does nitrogen affect the nervous system?
There is a critical volume hypothesis that states there to be an increased volume of nitrogen in the membranes and this relates to solubility. This explains the pressure reversal of anesthetics. Nitrogen narcosis is potentiated by increased CO2 levels.

How can it be prevented?
Avoid deep diving below 100 feet sea water. Certain factors increase the possibility of nitrogen narcosis:

  • Cold
  • Stress
  • Heavy work and fatigue
  • CO2 retention
How is nitrogen narcosis treated?
Treatment of nitrogen narcosis is immediate controlled ascent to the surface, with the buddy or divemaster observing the diver for unusual behavior, administration of O2 and temporary cessation of diving. Prevention should be the best treatment, with no further diving below 100 feet.

Risk Assessment for Divers

  • Severity of Harm possible - Drowning would be the worst case scenario.
  • Likelihood of Harmful Outcome. The likelihood of a serious outcome is dependent upon numerous factors we are unable to predict. Given the usual controlled recreational diving situation a harmful outcome is very unlikely.
  • Risk factors, Avoidable? Yes [see modifiers above]
  • Is it Worth It? Yes. The risk of nitrogen narcosis is far overweighed by the personal advantages of recreational scuba diving. This is a personal viewpoint.


 


 

High Pressure Nervous Syndrome


 

Helium

During the 1930s the U.S. Navy tested other gases as a substitute for nitrogen. Their scientists conducted experiments using rare gases such as helium, neon, and argon. After numerous trials, helium was selected as the most suitable gas to dilute oxygen for deep diving.

Helium is the second lightest element known to man; in fact, only hydrogen is lighter. Helium is one seventh as light as air and our atmosphere only contains 5 part per million of helium. Certain natural sources in the U.S. and Canada contain as much as 2% helium and this is where much of this gas is
collected.

Helium is chemically inert; it has no color, taste, or odor. These characteristics make it an almost perfect gas for diving. However, helium has two disadvantages. First, helium is extremely expensive due to its rarity. Secondly, helium has high heat conductivity and will rob body heat from a diver at a rapid rate. There are also other major disadvantages to diving with helium mixes; it is more difficult to decompress, i.e., deeper and longer stops are required as compared to air; and there is a greater risk of a serious case of decompression sickness if stops are not done per plan.

In deep saturation diving, under rapid compression rates, divers sometimes suffer from a phenomenon known as the High Pressure Nervous Syndrome (HPNS). 

HPNS

NOAA has the following to say about HPNS:

"At diving depths greater than 600 fsw (183 msw), signs and symptoms of a condition known as the high pressure nervous syndrome (HPNS) appear and become worse the faster the rate of compression used and the greater the depth or pressure attained. HPNS is characterized in humans by dizziness, nausea, vomiting, postural and intention tremors, fatigue and somnolence, myoclonic jerking, stomach cramps, decrements in intellectual and psychomotor performance, poor sleep with nightmares, and increased slow wave and decreased fast wave activity of the brain as measured by an electroencephalogram (Bennett et al. 1986).

First noted in the 1960's, HPNS was referred to initially as helium tremors. Since that time, numerous studies have been conducted that were designed to determine the causes of HPNS and to develop means of preventing it (Bennett 1982).

Methods of preventing or ameliorating HPNS include using a slow and steady rate of compression to depth, using a stage compression with long pauses at selected intervals, employing exponential compression rates, adding other inert gases such as nitrogen to helium/oxygen mixtures, and selecting personnel carefully. At present, the data suggest that adding 10 percent nitrogen to a helium/oxygen mixture, combined with the use of a proper compression rate, ameliorates many of the serious symptoms of HPNS (Bennett 1982)."

HPNS can result from diving mixtures that contain helium. Here are three actions that you can take to avoid HPNS. They are:
-Don't dive Heliox (O2/He) deeper than 400 FSW.
-Don't dive Trimix (O2/He/N2) deeper than 600 FSW.
-Note: Adding as little as 10% Nitrogen to He/O2 mixes buffers the mix to the point that it can be used to 600 FSW without experiencing HPNS.
-Use (very) slow descent rates. Descending slower than one FSW per minute beyond 400 FSW on Heliox and 600 FSW on Trimix keeps HPNS at bay. Unfortunately, this slow rate of decent is only practical in commercial diving and is of no use in tech diving.


References

Bennett, P.B. 1982b. The high pressure nervous syndrome in man. In: The Physiology and Medicine of Diving and Compressed Air Work. (P.B. Bennett and D.H. Elliot, eds), Balliere-Tindall, London. pp. 262-296.

 Bennett, P.B. 1990. Inert gas narcosis and HPNS. In: Diving Medicine, Second Edition (A.A. Bove and J.C. Davis, eds.).W.B. Saunders Company, Philadelphia. pp. 69-81.

 Bennett, P.B, R. Coggin, and J. Roby. 1981. Control of HPNS in humans during rapid compression with trimix to 650 m (2132 ft). Undersea Biomed. Res., 8(2): 85-100.