When diving open circuit you are virtually always okay if you have gas to breathe. This stops being true when you move into technical diving where breathing your decompression gas too deep can kill you from an oxygen (O2) seizure and breathing your bottom gas for a deep trimix dive too shallow can kill you from hypoxia. When diving a rebreather the gas you are breathing can kill you if the partial pressure of oxygen in the gas is too high or too low, or if the partial pressure of carbon dioxide is too high. Experienced open circuit divers intuitively believe that as long as they have gas to breathe they are okay and as a result often forget to monitor the partial pressure of oxygen (pO2) in the breathing loop on a rebreather. This is frequently a factor in rebreather accidents.