STS-46 MS Hoffman & MS Chang-Diaz wear masks during pre-breathe on OV-104

STS046-33-028 (4 Aug. 1992) --- With the possibility of an extravehicular activity (EVA) being added to the agenda, the two EVA-trained crew members begin their "pre-breathe" period on the space shuttle Atlantis' flight deck. Astronauts Jeffrey A. Hoffman (left), payload commander, and Franklin R. Chang-Diaz, mission specialist, reported to this station and began the "pre-breathe" process when problems developed during the extension of the Tethered Satellite System (TSS). When the human body is exposed to a sudden decrease in atmospheric pressure (for instance, from the 10.2 ppsi in the crew cabin to the 4.5 ppsi of the Extravehicular Mobility Unit (EMU) spacesuit), nitrogen traces in the bloodstream will expand. This expansion can create tiny bubbles and potential for the "bends". In order to lessen the effect, an astronaut must "pre-breathe" pure oxygen (the same pure oxygen that he will breathe in the suit) to help "purge" nitrogen from his/her bloodstream before exerting him/herself in the low-pressure environment of the suit. The "pre-breathe" exercise and the EVA turned out to be not needed as the TSS operations were resumed by remote operations.