More than half of air passengers are risking their health flying by being starved of oxygen, a study says.
Oxygen levels dropped by 4% on average during flying
A Belfast hospitals and university team found oxygen levels in the blood dropped so low in 54% of passengers that they would need extra supplies.
Oxygen levels dropped by 4% on average the study of 84 passengers found.
But the UK air regulator said people with heart and lung problems should consult their doctor before flying but healthy people need not be concerned.
Oxygen levels on the ground averaged 97%, but once their plane was in the air they fell to 93% on average, the study published in the Anaesthesia journal said.
Some physicians put hospital patients with blood levels below 94% on extra oxygen.
The researchers said the drop could increase the chance of breathing difficulties, headaches and angina attacks for those who suffered from them.
Anaesthetists took oxygen levels from 55 passengers on long-haul flights lasting more than two hours, with the remaining on short-haul flights.
The measurements were similar for both groups, which were made up of people aged between one and 78 years old.
Lead researcher Dr Susan Humphreys, an anaesthetic specialist registrar at the hospital, said: "We believe these falling oxygen levels, together with factors such as dehydration, immobility and low humidity, could contribute to illness during and after flights.
"This has become a greater problem in recent years as modern aeroplanes are able to cruise at much higher altitudes."
The study comes after House of Lords Science and Technology Select Committee and Department of Transport reports called for more research to be carried out on the health impact of flying.
But a spokesman for the Civil Aviation Authority, the UK air regulator, said healthy passengers need not worry.
"The oxygen levels would not be harmful to healthy passengers, we are only talking about people with health problems, such as lung and heart.
"The advice to those people is consult with your doctor before flying, and that still stands.
"We have always known oxygen levels fall when a plane is flying."