In addition to ppO2, what is the second variable that contributes to total oxygen "dose"?

The second variable that contributes to the total oxygen "dose" is indeed the length of exposure. The concept of oxygen "dose" is integral in understanding how oxygen can affect divers during their activities.

When divers are exposed to elevated partial pressures of oxygen (ppO2), the total exposure is not determined by ppO2 alone but rather by both ppO2 and the duration of exposure to that pressure. This is because the longer the duration of time spent at a higher ppO2, the greater the amount of oxygen absorbed by the body, which can increase the risk of oxygen toxicity. Oxygen toxicity can manifest in various symptoms, especially when the ppO2 exceeds safe limits for extended periods, emphasizing the importance of monitoring both pressure and exposure time during dives using enriched air nitrox.

While other factors such as surface conditions, a diver's physical fitness, and the type of breathing gas used can influence diving safety and comfort, they are not primary contributors to the total oxygen dose like the combination of ppO2 and the length of exposure time. This understanding is vital for divers to manage their exposure effectively and mitigate risks associated with high levels of oxygen in their dives.