Editing Maximum operating depth (section)

== Safe limit of partial pressure of oxygen==
Acute, or central nervous system oxygen toxicity is a time variable response to the partial pressure exposure history of the diver and is both complex and not fully understood.

[[Central nervous system]] oxygen toxicity manifests as symptoms such as visual changes (especially [[tunnel vision]]), ringing in the ears ([[tinnitus]]), [[nausea]], twitching (especially of the face), behavioural changes (irritability, [[anxiety]], confusion), and [[Vertigo (medical)|dizziness]]. This may be followed by a [[tonic–clonic seizure]] consisting of two phases: intense muscle contraction occurs for several seconds (tonic phase); followed by rapid spasms of alternate muscle relaxation and contraction producing convulsive jerking ([[clonus|clonic]] phase). The seizure ends with a period of unconsciousness (the [[postictal state]]).{{sfn|Clark|Thom|2003|p=376}}{{sfn|U.S. Navy Diving Manual|2011|p=44|loc=vol. 1, ch. 3}} The onset of seizure depends upon the partial pressure of oxygen in the [[breathing gas]] and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day.{{sfn|Clark|Thom|2003|p=376}}{{sfn|U.S. Navy Diving Manual|2011|p=22|loc=vol. 4, ch. 18}}<ref name="Bitterman" /> In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms.{{sfn|Donald, Part I|1947}} Decrease of tolerance is closely linked to retention of [[carbon dioxide]].{{sfn|Lang|2001|p=82}}<ref name="rebreather2.0" /><ref name="padi" /> Other factors, such as darkness and [[caffeine]], increase tolerance in test animals, but these effects have not been proven in humans.<ref name="pmid3705247" /><ref name="pmid8574677" />

The maximum single exposure limits recommended in the [[National Oceanic and Atmospheric Administration|NOAA]] Diving Manual are 45 minutes at 1.6 bar, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar.<ref name=dan/>