In the film Dune, when the Duke and Pual put on the desert suits, they are instructed to breathe in through the mouth and out through the nose when in the open desert. What does this accomplish?
Due to the planet Arrakis harsh desert environs the Fremen wore stillsuits, a technological device designed to hold, trap and recycle bodily fluids for weeks at a time when a Fremen was away from freestanding water supplies.
- Breathing in through the mouth and out through the nose ensure moisture-rich air being exhaled was captured by the stillsuit and processed ensuring as little moisture as possible was lost through exhalation.
A stillsuit is a full body suit worn in the open desert of the planet Arrakis that was designed to preserve the body's moisture.
It consisted of various layers that would absorb the body's moisture through sweating and urination, and then filter the impurities so that drinkable water would be circulated to catchpockets.
The individual could then drink the reclaimed water from a tube attached to the neck. A stillsuit, kept in working order and maintained properly, permitted the wearer to survive for weeks in the open desert.
The suit is described in Dune:
"It's basically a micro-sandwich — a high-efficiency filter and heat-exchange system. The skin-contact layer's porous. Perspiration passes through it, having cooled the body ... near-normal evaporation process. The next two layers . . . include heat exchange filaments and salt precipitators. Salt's reclaimed. Motions of the body, especially breathing and some osmotic action provide the pumping force. Reclaimed water circulates to catchpockets from which you draw it through this tube in the clip at your neck... Urine and feces are processed in the thigh pads. In the open desert, you wear this filter across your face, this tube in the nostrils with these plugs to ensure a tight fit. Breathe in through the mouth filter, out through the nose tube. With a Fremen suit in good working order, you won't lose more than a thimbleful of moisture a day..."