London, April 26: More than half of all air travellers suffer so much oxygen deprivation that they are at risk of deep vein thrombosis, according to a study.
The proportion of oxygen in the air breathed in on board an aircraft in flight falls from 21 per cent to as little as 15 per cent.
Researchers in Belfast found that in 54 per cent of those they examined during flight many hospital doctors would have prescribed extra oxygen because their blood was unable to carry oxygen to the body's organs easily.
As well as contributing to a dangerous thickening of the blood that can cause DVT, a drop in blood oxygen can impair mental performance, give passengers headaches and make them tired.
People with heart disease are more likely to suffer angina and those with unhealthy lungs have a greater risk of becoming ill.
Dr Susan Humphreys, an anaesthetic specialist at Belfast City Hospital and one of those involved in the research, to be published in Anaesthesia magazine next month, said: '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.
'This has become a greater problem in recent years as modern aeroplanes are able to cruise at much higher altitudes.'
Oxygen saturation is the proportion of haemoglobin in the blood able to bond with oxygen and carry it to the body's organs and tissues.
This was measured by Dr Humphreys and colleagues in 84 passengers aged from one year to 78 both at ground level and during flight. At ground level the saturation was 97 per cent while at cruising altitude it was 93 per cent.
In 54 per cent of passengers at cruising altitude it was 94 per cent or less ' the proportion below which many respiratory specialists said they would prescribe extra oxygen.
In one passenger, oxygen saturation dropped to 85 per cent during flight. The results were similar for passengers on short- and long-haul flights.
Previous fears over the risks of DVT associated with air travel have focused on immobility and dehydration.
Those behind the new research believe reduction of oxygen saturation could also be a significant factor.
Dr Rachel Deyermond, consultant anaesthetist at Ulster Hospital in Belfast, said: 'It has been shown that in low oxygen situations at altitude, the risk of blood clotting is greater.