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When is Nitrox Dangerous

  • Using enriched air nitrox mixtures of 40% oxygen or less can use their everyday regulators

Oxygen Toxicity and Scuba Diving

  • This most commonly happens when a diver breathes partial pressures of oxygen greater than 1.6 ata, such as breathing EANx32 beyond 130 feet. Most training organizations recommend a maximum oxygen partial pressure of 1.4 ata for this reason.

  • There is frequently no warning of the impending convulsions - a diver is perfectly fine one moment and convulsing the next.

  • risks losing his regulator and drowning or pulmonary barotrauma if the convulsions begin while his airway is closed.

  • may sometimes notice other warning signs and symptoms. These include visual disturbances such as tunnel vision; auditory anomalies such as ringing ears; nausea and dizziness; twitching - especially of facial muscles; and mood changes such as irritability or euphoria.

  • must immediately ascend to a shallower depth to reduce the partial pressure of oxygen.

  • Pulmonary oxygen toxicity occurs when the cells in a diver's lungs are damaged or experience cell death. It is primarily a risk for technical divers, as the condition occurs when divers breathe elevated partial pressures of oxygen for extended periods of time, such as breathing pure oxygen on a series of decompression stops. Most divers can breathe a partial pressure of oxygen of 1.4 - 1.5 ata for 8 - 14 hours before feeling the effects of pulmonary oxygen toxicity.

  • a dive with an oxygen partial pressure of 1.4 ata for 60 minutes uses about 33% of a diver's allowable oxygen exposure for the day

  • If decompressing with high partial pressures of oxygen for long periods of time, be sure to take appropriate air breaks to reduce the risk of pulmonary oxygen toxicity.

  • Strenuous exercise and poorly functioning regulators can elevate a diver's carbon dioxide levels, which causes him to retain oxygen and increase his risk of oxygen toxicity.

  • Some medications, such as decongestants containing Psuedoephedrine HCl, act as oxygen exciters, accelerating the onset of oxygen toxicity at unusually low partial pressures or shortened exposure times.

Practical guide to nitrox diving

  • allows divers to lengthen no-decompression limits, shorten surface intervals and get an added safety buffer for decompression sickness in certain diving circumstances.

  •  One is the amount of exposure, or the pressure of oxygen in the lungs, and the second is the length of the exposure. Combined, these two factors are called the oxygen limit.

  •  a diver breathing air (21 percent O2) at 132 feet (5 atm) is breathing a partial pressure of oxygen (PPO2) of 1.05 or the equivalent of breathing 105 percent oxygen at the surface. (5 atm x 0.21 = 1.05, 100 x 1.05 = 105 percent)

  • a diver breathing EAN 35 will have a maximum operating depth of 99 feet, or 4 atm, when using the recreational limit of 1.4 PPO2: 1.4 / 0.35 = 4 atm, 33 feet x (4 atm - 1 atm--the one surrounding the earth) = 99 feet.

  • at a PPO2 of 1.4, divers are allowed 150 minutes of bottom time--even if it is all completed on a single dive.

  • The best application of nitrox is in the 50- to 100-foot range

  • Custom mixes like this really do give you the best bang for the buck. For example, using EAN 32 at 70 feet only adds 10 minutes to your 50-minute bottom time. But using EAN 40 at 70 feet will double your bottom time to 100 minutes, definitely giving you more dive for your dollar without compromising your risk of DCS. Technical divers call this selecting the "best mix" for the dive and it is done by completing a simple formula: PPO2 limit/ATA (pressure at the maximum depth of the dive).

    • Normoxic nitrox: Regular air, or cleaned air with 21 percent oxygen

    • EAN: Any mix with more than 21 percent oxygen

    • EAD: Equivalent Air Depth. The relationship between nitrogen absorbed at a particular depth breathing nitrox and the depth at which an equivalent absorption rate would occur breathing air.

    • PPO2: Oxygen partial pressure, pressure in atmospheres x O2 percentage = PPO2

    • MOD: Maximum operating depth, PPO2 / O2 percentage = MOD

    • Best Mix: PPO2 limit/ATA (pressure at the maximum depth of the dive) = O2 percentage for the dive

Daily limits for CNS oxygen toxicity

  • The oxygen exposure time for a single dive is compared to the Single Dive Exposure Limits on the NOAA table (1.6 for 45 mins, 1.5 for 120 mins 1.4 for 150 mins and so on). The suggested working limit for this type of exposure is 80 percent of the limits. (i.e. 1.4 for 120 mins, 1.5 for 96 mins or 1.6 for 36 mins). 

  • If a diver reaches the limits of the Single Exposure Time on a single dive then he must take at least a two-hour interval on the surface, breathing normal air. This surface interval is thought to reduce the CNS loading by about half. Current thinking is that CNS loading is subject to a 90-minute half-time.

  • If two dives are conducted with less than a two-hour surface interval, treat them as a single dive for the purposes of CNS tracking. In other words, the in water times are added together and compared against the Single Exposure Time. If one dive is at a greater oxygen partial pressure than the other, that pressure is the one used with the combined in-water times of the two dives, to calculate total CNS loading.

  • If two or more dives are conducted within a single 24 hour period with more than two hours at the surface between each dive, then the total in water times are added and compared against the Daily Limit to arrive at the diver’s CNS loading.

  • If a series of dives in a 24 hour period reaches the Daily Limits, then a 24 hour surface interval breathing air is the safest option to be taken before diving again.

  • Remember, the NOAA daily limit is for a 24-hour period NOT a calendar day.

What you need to know to dive safe with nitrox

  • When filling tanks with a Nitrox blend, the tank is exposed to pure oxygen that's diluted later using regular air. If cylinders aren’t treated to withstand this high level of oxygen, they can explode. Tanks used for Nitrox and those used for regular air are NOT interchangeable. 

  • ​There's an even greater reduction in the risk of getting ill from decompression sickness, so it's highly valuable to those who are more susceptible to it, including individuals who are older, overweight, injured, fatigued, or have dealt with DCI in the past.

Analyzing enriched air tank
Maxtec MaxO2 nitrox analyzer
Round up or Round down for oxygen level in decimal places

  • I suggest round up especially if you are doing multiple nitrox dives for several days. It is because it helps to reduce the risk of CNS. Risk of decompression sickness on the otherhand will not be significantly increased if each dive is limited to within 1 hr and not more than 5 dives per day. Why? Because as long as you are diving within your computer limits, above dive times with air will not get decompression sickness neither, not mention nitrox.

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