Boyle’s Law, which relates to the expansion of gases, is the principal gas law to remember in the context of flight physiology. Essentially, gas will expand as altitude increases and atmospheric pressure decreases. As altitude decreases, the opposite occurs. Therefore, during an aeromedical evacuation, any gas in an enclosed space will try to expand. A change in altitude from sea level to 8000 feet (2500 metres) will expand an enclosed volume of gas by 35%. Relevant gas-filled structures that may be affected are listed in the box over the page. Slow changes in altitude can minimise the effects of gas expansion.