The fields of medicine concerned with the maintenance of health, safety, and performance of those in aviation and space travel. Aerospace medicine is the sum of aviation medicine and space medicine -- health in flight both inside and outside the Earth's atmosphere. Aviation medicine is the study of the biological and psychological effects of aviation. Medically significant aspects of aviation travel include exposure to changing temperatures, large inertial forces, oxygen deprivation, and air sickness, as well as pilot fatigue. Aviation medicine is concerned, for instance, with the spread of disease by air travel and the adverse effects of noise and air pollution. Space medicine is the study of the biological and psychological effects of space travel. Medically significant aspects of space travel include weightlessness, strong inertial forces during liftoff and reentry, radiation exposure, absence of the day and night, and existence in a closed environment. Space medicine is concerned, for instance, with osteoporosis caused by weightlessness and the resultant increased risk of fractures.