The long term dosage of oxygen is measured in units called OTU's . Hypoxic and Hypoxia - In general, both of these terms relate to a less than normal amount of Oxygen. Hypoxia is the physiological condition associated with breathing too low of a partial pressure of Oxygen. HPNS is a condition which results from breathing Helium under high pressures. Other symptoms can include euphoria, nausea, vomiting, lack of appetite and drowsiness.
Symptoms sometimes moderate or entirely dissapear with continued exposure.
This limit is to protect from Oxygen toxicity . The mushroom valve on a rebreather is the one way valve on either side of the mouthpiece which keeps the breathing gases traveling the loop in one direction. The valves are very important to ensure that the gases are scrubed of CO2 and that there is enough O2 in the breathing loop. The vavle is called a mushroom valve because the diaphram of the valve resumbles a mushroom. The mushroom valve is one of the places in the breathing loop that can have a very direct effect on the work of breathing .