From UPTD to ESOT: Monitoring hyperoxic exposure in surface-oriented diving

A recent review suggested that the measure K = t² x pO₂ [4.57] (t is exposure time in h, pO in atm) should replace unit pulmonary toxic dose (UPTD) as an exposure index for pulmonary oxygen toxicity (POT) in surface-oriented diving. K would better predict reduction in vital capacity (VC) during exposure and allow prediction of recovery. Although K is more accurate estimating VC changes than UPTD, the calculation of K is more extensive, particularly when estimating hyperoxic exposure for dives with multiple pO₂ segments. Furthermore, and in contrast with UPTD, K is difficult to interpret on its own given its non- linear dimension of time. We suggest that a new metric: ESOT (equivalent surface oxygen time) should be used to replace UPTD. ESOT = t x pO₂ [2.285] (t is exposure time in minutes, pO in atm). ESOT=1 is thus the hyperoxic exposure reached after one minute of breathing 100% O₂ at surface pressure. Hyperoxic monitoring by ESOT is more practical than K to apply in an operational environment, with no loss of accuracy in POT prediction. In addition, it intuitively allows interpreting hyperoxic exposures on its own, analogous to UPTD. The daily hyperoxic threshold limits suggested by Risberg and van Ooij for two, five and an unlimited number of successive diving days would translate to ESOTs of 650, 500 and 420 respectively.