Exposure to cosmic radiation may also vary by the type of aircraft flown. Different aircraft tend to fly at different levels in the sky. Most major airline jetliners fly at close to 30-33,000 feet. Smaller planes fly at lower altitudes. Conversely, supersonic transport, including the Concorde, fly at higher altitudes. This illustrates that studies of the health effects of cosmic radiation should also consider exposure levels by identifying the types of aircraft flown.