LONDON (AFP) — Airline passengers suffer "significant" drops in oxygen levels in their bodies during flights, a study published in a British medical journal said on Monday.

Just over half of all the passengers studied had oxygen levels six percent lower than usual at an airliner's maximum altitude, a level at which many doctors would prescribe extra oxygen for hospital patients, according to researchers from Belfast, Northern Ireland.

Such drops in oxygen could help explain why some air travellers become sick during and after flights, according to the study published in Anaesthesia magazine.

Anaesthetists measured the oxygen levels of 84 passengers, aged between one year and 78, on the ground and at maximum altitude.

On average, oxygen levels fell by four percent when the passengers, none of whom had severe cardio-respiratory problems or required permission from their doctor to fly, reached cruising altitude.

The results could help understand illnesses connected to flights, said Dr Susan Humphreys, specialist anaesthetic registrar at the Royal Group of Hospitals in Belfast.

"We believe that these falling oxygen levels, together with factors such as dehydration, immobility and low humidity, could contribute to illness during and after flights," she said in the report.

"This has become a greater problem in recent years as modern aeroplanes are able to cruise at much higher altitudes."

Of the 84 passengers, 55 were on flights lasting more than two hours, while the others were on short-haul journeys. Similar results were obtained from both groups.