Air travel is becoming increasingly more accessible to people both through the availability of cheap flights and because the airlines are now able to cater for individuals of all ages and disabilities. The wide bodies of many new aircraft permit the airlines to have very flexible seating options. Airline operators currently have an important role in determining die comfort and spaciousness of the seating in their aircraft. Passengers who remain seated for the bulk of a flight may risk oedema. or deep vein thrombosis. This could be particularly important for larger people in certain economy class seats. The absence of smoking on planes has encouraged designers to cut back on the rate of cabin ventilation and hence introduce filtered recirculated air to the aircraft cabin. In new planes the ventilation rate is under pilot control and savings (economies) can be achieved by using decreased ventilation. A lower ventilation rate may lead to 'less comfortable air quality' in some parts of the plane and an increased risk of possible cross-infection from other passengers on the flight. Technological advances in jet engine design has permitted larger passenger planes to fly longer distances and at greater altitudes than ever before. The higher flying altitude is associated with a lower cabin pressure, which has an important physiological effect on oxygen saturation in the blood of both crew and passengers, particularly for die very young, the elderly and those who are less fit.