to the spacecraft, there will be the potential for the dust to contaminate surfaces and increase the risks of inhalation and skin exposure. Astronautical hygienists are testing materials with reduced dust retention times and the potential to control the dust exposure risks during planetary exploration. Novel ingress and egress approaches, such as suitports, are being explored as well. In NASA space suits, communications are provided via a cap worn over the head, which includes earphones and a microphone. Due to the coloration of the version used for Apollo and Skylab, which resembled the coloration of the comic strip character Snoopy, these caps became known as "Snoopy caps". Operating pressure Generally, to supply enough oxygen for respiration, a space suit using pure oxygen must have a pressure of about , equal to the partial pressure of oxygen in the Earth's atmosphere at sea level, plus and water vapor pressure, both of which must be subtracted from the alveolar pressure to get alveolar oxygen partial pressure in 100% oxygen atmospheres, by the alveolar gas equation. The latter two figures add to , which is why many modern space suits do not use , but (this is a slight overcorrection, as alveolar partial pressures at sea level are slightly less than the former). In space suits that use 20.7 kPa, the astronaut gets only 20.7 kPa − 11.6 kPa = of oxygen, which is about the alveolar oxygen partial pressure attained at an altitude of above sea level. This is about 42% of normal partial pressure of oxygen at sea level, about the same as pressure in a commercial passenger jet aircraft, and is the realistic lower limit for safe ordinary space suit pressurization which allows reasonable capacity for work. When space suits below a specific operating pressure are used from craft that are pressurized to normal atmospheric pressure (such as the Space Shuttle), this requires astronauts to "pre-breathe" (meaning pre-breathe pure oxygen for a period) before donning their suits and depressurizing in the air lock. This procedure purges the body of dissolved nitrogen, so as to avoid decompression sickness due to rapid depressurization from a nitrogen-containing atmosphere. Physical effects of unprotected space exposure The human body can briefly survive the hard vacuum of space unprotected, despite contrary depictions in some popular science fiction. Human flesh expands to about twice its size in such conditions, giving the visual effect of a body builder