Bronchial Surfactant is an enzyme used by the UNSC for use inside Cryo-chambers. The cryo-inhalant is specifically designed to be accumulated in the throat during sleep, then regurgitated and swallowed upon reawakening. The liquid replaces nutrients lost during deep sleep. A person using Bronchial surfactant is theoretically able to sleep for extremely long periods of time, though the exact specifications are not known. People have been observed in cryo sleep for up to three weeks without using the fluid.
Some people do not have a problem using the drug, but Jacob Keyes had a profound disgust for it, believing that no matter how the formula is changed, it always tastes like lime-flavored mucus.