Oxygen Toxicity Concerns

In DAN Oxygen Provider classes, there is often a question about oxygen toxicity and whether or not this is an issue at the first responder level. This article addresses that concern and provides a reason for believing the oxygen toxicity risk to a diver receiving treatment from a diving oxygen provider is minimal.

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Oxygen is a primary component in the biochemical reactions that sustain life. We cannot survive without it. There is, however, a downside to oxygen. Oxygen is a chemically extraordinarily promiscuous element. It is always seeking other chemical components with which to react. One view of atmospheric science maintains that if it were not for the much more chemically inert and abundant nitrogen present in our atmosphere, that our planet long ago would have burned to cinders in an oxygen-driven torch. This concern was first expressed by Lavosier (who gave oxygen its name in the late 1770's). He observed the increase in flammability of a candle exposed to an oxygen-rich atmosphere and stated, 'the animal powers be too soon exhausted in this kind of pure air." Consider the biochemical reaction:

Oxygen + Anything It can find ==> "Bad Stuff" (Often called Reactive Oxygen Species)

Oxygen, in biological systems, continually forms "bad stuff," (extremely reactive materials like free radicals, super-oxides, radical anions, etc. Think of this "bad stuff" as "body rust.") that interferes with normal physiological processes. Because these undesirable constituents are always formed in an oxygen atmosphere, our body has evolved over geological time a set of natural biochemical defenses (like super-oxide dismutase, reducing enzymes systems like glutiathione, etc) to cope with these undesirable "bad stuff" species as they are formed.

Problems, like CNS oxygen toxicity, occur when the amount of this "bad stuff" formed exceeds the body's capacity to deactivate this "bad stuff." Le Chatelier's principle (an increase in one of the components on the left side of a chemical reaction will force the reaction process to the right). suggests that an increase in oxygen concentration will generate more "bad stuff." The higher the oxygen concentration, the more "bad stuff:" formed. So, increases in oxygen partial pressure will lead to enhanced formation of so-called highly reactive oxygen species. It is the presence of too much and/or too many of these "bad stuff" moieties that leads to physiological problems.

Chemical reactions (like Oxygen + Anything It can find ==> "Bad Stuff" ) require that the reacting molecules collide with each other in order to react. So, in general, the more concentrated the reactants (chemists call this "mass action"), the faster the reaction proceeds. So, an increase in the partial pressure of a gaseous reactant will increase the reaction rate.

One way of understanding oxygen toxicity is to assume that body removal of "bad stuff: proceeds at a fixed rate. A homeostasis rate developed over time to adequately dispose of "bad stuff" while allowing the necessary oxygen reactions that drive our energy metabolism to proceed. (If all oxygen reactions were stopped, we would cease to function.) As oxygen molecules per unit volume of tissue increase (either by increase in concentration of oxygen in the breathing mix or by increases in pressure), more "bad stuff" forms. Since waste disposal of "bad stuff" works at a fixed rate, the "bad stuff" begins to accumulate. This increase in "bad stuff" eventually overwhelms the body defenses and accumulated highly reactive oxygen species do their damage.

There are two types of oxygen toxicity: Central Nervous System (CNS) toxicity can lead to a highly excitatory state (like an epileptic seizure). The rapid depth-related onset strongly suggests a direct chemical reaction with oxygen involving the formation of a central nervous system excitant (like NO^. ). Since empirical evidence (diver seizure and death) has indicated the approximate threshold (varies with individual diver's body chemistry on day of dive) is 1.4 to 1.6 ata, divers limit their risk to this malady by keeping the partial pressure of oxygen in their breathing mix below 1.4 ata. Breathing pure oxygen in a first aid scenario at sea level (1.0 ata oxygen) is well below the established threshold. NOAA has developed a table for approximating the depth/time dependent exposure. Use of this table is typically discussed in oxygen-enriched dive training classes. CNS toxicity is not considered a significant risk factor in air diving at recreational depths. It is a major risk factor in non-air diving to depths beyond typical recreational levels.

The second type of oxygen toxicity is termed pulmonary or whole-body oxygen toxicity. This situation arises from changes in the respiratory and circulatory system's cellular structure when exposed to high concentrations of oxygen. Typically, this takes many hours and is characterized by gradually increasing difficulty in breathing. The respiratory people have developed a system of measuring oxygen exposures to monitor amount of oxygen dose received (and therefore, relative risk). They have defined an Oxygen Toxicity Unit (OTU) as 1 OTU = 1 minute of breathing 100% oxygen at sea level. Since it has been observed that most people can tolerate 24 hours of breathing pure oxygen without trouble, the accepted allowable dose is 1440 OTU's (1 OTU per min x 60 min/hr x 24 hr/ day) per day

I believe the relative concerns are about oxygen toxicity can be best expressed by looking at some computer-generated numbers, with the understanding that different computer programs may give slightly different absolute values for the numbers quoted. The exact numerical value is NOT as important as the relative magnitude of the number and its comparison to the accepted tolerance limits. Besides, in the clinical situation, even though numbers are important tracking devices and guidelines, therapeutic protocols (time/depth/breathing gas) are evaluated and determined by the medical staff based on individual patient behavior and needs. Lastly, while 1440 OTU's is the allowed daily dose of oxygen, under physician's care in a hyperbaric procedure, doses in excess of 1700 OTU's may be acceptable.

A diver breathing 100% Oxygen at sea level (1 ata pressure) accumulates 1 OTU per minute. An injured diver breathing from a demand inhalator mask typically consumes a DAN jumbo D cylinder in approximately 50 minutes. So, each DAN cylinder consumed represents about 50 OTU 's delivered to the patient. Thus, a diver would need to consume (1440 0TU allowed per day / 50 OTU per cylinder) 28.8 DAN jumbo D cylinders in a continuous 24-hour session to reach the allowed whole body daily dose. Most divers do NOT carry this quantity of oxygen with them AND, more importantly, transfer of the patient to the emergency medical chain of response occurs long before oxygen toxicity becomes a critical factor.

Now, let's address accumulation of OTU's during recreational diving on typical recreational diving breathing gases.

Table One examines the % CNS toxicity (as a measure of CNS toxicity risk) and accumulated OTU's (as a measure of whole body or pulmonary toxicity risk) for dives on air to the no decompression limits of the US Navy tables. These numbers are well below the daily-allowed dose of 1440 OTU.

Depth (fsw)	Time (min)	% CNS	OTU's
40	200	0	0.00
50	100	12	9.10
60	60	8	14.67
70	50	8	19.04
80	40	7	20.40
90	30	7	19.11
100	25	6	19.07
110	20	6	17.85
120	15	5	15.51
130	10	4	12.10
140	10	5	13.50

There has been some discussion in the recreational diving literature, especially in the early 1990's, about withholding oxygen to divers who have made dives using a breathing mix of oxygen-enriched air. So, let's look at "the numbers" for oxygen-enriched air. Table Two is a compilation of oxygen toxicity values for dives to the no decompression limits while breathing NOAA Mix 1 (32 % oxygen). Again, these values are far below thresholds of concern.

Depth (fsw)	Time (min)	% CNS	OTU's
40	310	55	149.19
50	200	45	133.17
60	100	29	84.16
70	60	20	60.80
80	50	22	59.42
90	40	20	54.38
100	30	18	46.32
110	25	18	43.55
120	25	22	47.71
130	20	36	42.15

In addition, Table Three is a compilation of oxygen toxicity values for dives to the no decompression limits while breathing NOAA Mix 2 (36 % oxygen). As with Mix 1, these values are below thresholds of concern.

Depth (fsw)	Time (min)	% CNS	OTU's
40	310	68	200.48
50	200	56	166.46
60	100	35	103.36
70	60	26	73.35
80	60	31	85.01
90	50	31	80.43
100	40	31	72.39
110	30	42	60.36

To help put this is a bit more of a proper perspective, consider (only for the purposes of examining the "numbers" for oxygen toxicity: this is certainly not a suggested mission profile. The "numbers" here are taken to absurdity only for purposes of illustration) making three consecutive dives, with no surface interval, to 60 fsw for 60 minutes. The accumulated % CNS and OTU's for this series of decompression dives is shown below in Table Four.

Table 4: Oxygen Toxicity Values For Three Consecutive Dives of 60 fsw for 60 minutes

Mix	% CNS	OTU's
Air	25	43.90
Mix1 (32 % O2)	54	152.40
Mix 2 (36 % O2)	64	188.49

It should be obvious that even after three consecutive 60 fsw dives for 60 minutes, a diver would be well below the allowed OTU accumulation of 1440. I also suggest that, especially in Michigan waters, thermal considerations have more control of the dive durations than whole body oxygen toxicity while diving conventional mixes to traditional recreational diving depths.

Hyperbaric treatment of DCI will involve added exposure to high concentrations of oxygen and significantly add to the OTU accumulation. Table Five lists approximate OTU values for the standard US Navy Treatment Tables.

So, even with a DCI treatment for an air embolism (Table 6A) for a diver injury at the conclusion of three consecutive dives to 60 fsw on NOAA Mix 2, the total whole body oxygen exposure (820 + 188 = 1008 OTU's) is less than the acceptable standard daily dose of 1440 OTU's.

As a first responder on the scene of a diving accident, the administration of the highest concentration oxygen available (see Why100 %) is the definitive treatment for a diagnosed diving malady. There is no justification to the belief that concerns over oxygen toxicity should mandate with holding this critical "denitrogenation" agent from a recreational diver suffering from DCI who has been diving on air or oxygen enriched air It is, however, important that first responders note the time and type of administration device (an estimate of O₂ concentration delivered) utilized in treatment so that medical professionals can track OTU's should treatment in a chamber be required. It is also important to monitor patient ease of breathing, especially in exposures of more than a couple of hours. If the patient begins to show signs of discomfort or reports respiratory distress, then a few minutes of air breathing should remove the discomfort. Following this air break of a couple of minutes, the patient would be returned to the highest concentration oxygen available until either the gas is exhausted or the patient is transferred to a higher medical authority.

Bottom Line: Under most recreational diving conditions, first responders treating a diving incident with oxygen will transfer medical care of an injured diver to the emergency medical community long before oxygen toxicity becomes an issue.

Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 200 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

Use of these articles for personal or organizational profit is specifically denied.

Treatment Table	Approximate OTU's
5	297
6	607
6A	820