Every country has restrictions that it places on applicants to its spaceflight programs, and these occasionally include halitosis.
In the U.S., this is not a guideline. The National Aeronautics and Space Administration (NASA) has determined that candidates must have good vision, a resting blood pressure of 140/90 and be between 62 and 75 inches tall. For the Chinese administration, however, bad breath is a major taboo.
The China National Space Program (CNSP) specifically tests potential astronauts for halitosis, according to Mary Roach's popular science book, Packing for Mars. She notes that they do not necessarily look for bad breath because of its connection with gingivitis. Instead, they simply don't want their astronauts irritated with each other up there.
Roach points to an article in the Yangtse Evening Post, in which CNSP health official said that "the bad smell would affect their fellow colleagues in a narrow space." An employee at the Japanese aerospace program told her that snoring is another common deal-breaker.
Oral odor and snoring can be related to one another. Sleeping with the mouth open can dry out the tongue and palate. Without saliva to wash away bacteria, oral microorganisms begin to grow and multiply. As they consume matter on the tongue, they emit smelly sulfuric gases that give morning breath its distinctive smell.
Planning for extended space travel often highlights the importance of things that might otherwise be considered minutiae on planet Earth. Space food is notoriously bad, although astronauts have often declared having a favorite space meal. If meals get too repetitive on, say, a Mars mission, will astronauts go crazy? What if eating the same thing over and over causes bad breath?
These are problems that NASA is currently working on, according to the Los Angeles Times. While making freeze-dried, five-year-old food tasty may be a task for the agency's top scientists, eliminating bad breath can be simple. By brushing regularly and using a specialty breath freshening mouth rinse, both astronauts and the Earth-bound can keep their breath smelling stellar.