Hyperoxic exposure monitoring in diving: A farewell to the UPTD.

Depending on pO2 and exposure time hyperoxic breathing gas may cause injury in many organs including the lungs. Pulmonary oxygen toxicity (POT) may be asymptomatic, but will initially present as a tracheobronchitis in symptomatic subjects. A number of objective measurements of POT have been investigated, but the decrement in vital capacity (VC) has remained the most accepted outcome measure. The unit pulmonary toxic dose (UPTD) has been established as the most common exposure index for POT in diving. UPTD is calculated based on the pO2 and exposure time. A literature search identified five models predicting POT, but no model would accurately predict VC change for the full range of pO2 variation and exposure time relevant for surface-oriented diving. Nevertheless, compared to UPTD, the K-index (K = t2*pO24.57, where t = time (hours) and pO2 = inspired pO2 (atm)) suggested by Arieli performed better for pO2 > 150 kPa and allowed estimation of recovery. We recommend that the Arieli K-index should replace UPTD as the POT exposure index for all surface-oriented diving. Based on the limited data available we suggest a daily threshold of K = 120 for a maximum of two diving days followed by two days of recovery. For five consecutive days of diving, we recommend that the threshold should not exceed K=70 and two recovery days should be allowed. For multiday diving without days of recovery, the daily exposure should probably be limited to K = 40-50.