Oxygen Toxicity Notes

Oxygen toxicity is a condition resulting from the harmful effects of breathing oxygen at elevated partial pressures.

Pulmonary Toxicity

Pulmonary toxicity is caused caused by long term exposure to oxygen with partial pressure in excess of 0.5 bar.

Symptoms of pulmonary oxygen poisoning begin slowly as a substernal irritation that becomes progressively more intense and widespread along with  with increased coughing.  Uncontrollable coughing occurs in severe cases, symptoms originating in the trachea and major bronchi associated with a constant burning sensation, which is worsened by inspiration.  The most severe symptoms are associated with shortness of breath on exertion or even at rest.  The onset of symptoms is variable among individuals but usually occurs after about 12 to 16 hours of exposure at 1.0 ata, 8 to 14 hours at 1.5 ata, and 3 to 6 hours at 2.0 ata. ( ata= 33 ft sea water)

Central Nervous System Toxicity

Central nervous system toxicity is caused by short term exposure to high concentrations of oxygen, typically involving partial pressures exceeding 1.6 bar. The exact mechanism isn't fully understood but it is thought that the high concentration of oxygen increases the formation of free radicals (atoms, molecules, or ions with unpaired electrons) to a point that damages cells and enzymes.

Signs and symptoms of central nervous toxicity:

• Convulsions - frequently without any warning.
• Visual symptoms including dazzle and constriction of visual field
• Ear related symptoms including ringing of the ears and auditory hallucinations.
• Nausea and spasmodic vomiting
• Twitching - this usually first appears in the lips and other facial muscles. This is the most common and well defined symptom.
• Irritability and behavioral changes including anxiety, confusion, and apprehension.
• Dizziness or vertigo.

PPO2	MAX	5	10	15	20	25	30	35	40	45	50	55	60
1.20	210	2%	5
1.25	195
1.30	180
1.35	165
1.40	150
1.45	135
1.50	120
1.55	82
1.60	45

NOAA CNS Oxygen Exposure limits: Table gives percentage of NOAA "allowable" limits for a single dive. At the 1.6 ATM exposure level the "CNS Clock" runs almost 4 times faster as at a PO2 exposure level of 1.4 atm. An exposure to a PO2 1.4 ATM is the maximum "recommended" limit for the working portion of any dive. Exposure levels above a PO2 of 1.4 ATM are shown for contingency planning and for calculations concerning the decompression (resting) portion of stage decompression dives.

Tracking Central Nervous System Toxicity

Multiple dives may be conducted providing that the final CNS exposure in a 24 hour period does not exceed 80%. If the 80% figure is reached, then a 12 hour surface break breathing normoxic air must be taken. Where repeat dives are conducted with a surface interval, a 50% surface reduction in the CNS load (breathing normoxic air) is assumed for every 90 minutes at the surface. This Residual CNS load is added to the next dives CNS load to give the two dive total at the end of the second dive.