According to this document,
Quote:
Along with the tolerance curves, Dr. Christian J. Lambertsen and colleagues at the Institute for Environmental Medicine, University of Pennsylvania, developed the current tracking method for pulmonary oxygen toxicity in the form of the unit pulmonary toxic dose (UPTD) and the cumulative pulmonary toxic dose (CPTD). The UPTD is more commonly referred to in the diving community as the oxygen toxicity unit (OTU). One UPTD (or OTU) is the degree of pulmonary oxygen toxicity produced by breathing 100% O2 continuously at a pressure of 1 atmosphere absolute (ATA) for 1 minute. The CPTD calculation (see Equation 1 below) converts any continuous oxygen exposure (PO2 above 0.5) and time combination to be expressed as UPTDs (or OTUs).
The CPTD calculation is carried out for each segment of the dive profile and the results (expressed as OTUs) are summed up to produce the total number of OTUs for the dive. This number can then be compared against the daily and multi-day limits established by Dr. Bill Hamilton and colleagues in the NOAA Repetitive Excursions (REPEX) Procedures Report. These published limits have been widely adopted by the technical diving community.
Bolded emphasis is mine. Perhaps the referenced "NOAA Repetitive Excursions (REPEX) Procedures Report" is the source?