5.12 Developing acoustic criteria for a human environment is a complicated process which we had neither the time nor the skills to do in detail. However, there are some extreme conditions to be avoided. Within the hearing range mentioned above, sound intensities above 160 decibels cause rupture of the eardrum and deafness. Prolonged sounds between 90 and 120 decibels can damage the hearing (5.9). V.3: OPERATION AND COMFORT REQUIREMENTS V.3.1: Pseudogravity: The Skylab Life Sciences experiments show that a prolonged stay in a zero-g environment brings about bodily adaptations (loss of bone calcium, lessening of muscle tone, etc.) (5.10). Deductive reasoning would suggest that a prolonged stay in .5g or .75g environments would bring about similar adaptations but to a lesser degree. There is no data available on whether these adaptations would be permanent given a long stay (i.e., 10 years). If these effects are not permanent, they may require an intermediate readaptation period before the humans can return to Earth (for example, several months at .75g after 10 years at .Sq). If the effects~ permanent, the lower-than-one-g environment must be maintained for the natural lifetime of its inhabitants. Thus, based on the current available data, the choice of a pseudogravity level in a human space environment depends more on the expected gravitational future of that environment's inhabitants than on physiological criteria alone. The expected activities of the environment's population may require certain pseudoqravity levels as well. In the Skylab experiments, astronauts changing from a lg environment (Earth) to a zero-g environment suffered nausea and vestibular discomforts for 2-3 days (5.17). It is unclear whether similar effects would affect humans whose gravitational environment changed, for example, from .75g to .5g. Sudden changes in pseudogravity can cause dangerous situations (for example, increasing the weight someone is carrying). An excessive pseudogravity strains muscles, overloads the heart, and breaks bones.
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