A little knowledge can be a dangerous thing, but in Hilary Ellison’s case it saved her life. A science teacher at a Hampshire comprehensive, Hilary had spent an entire weekend at her dining table filling out forms and writing reports in readiness for her school’s Ofsted inspection. She ploughed through the paperwork, sitting for hours on end, while her husband and three sons supplied her with food and coffee. By the Sunday evening she noticed one of her legs had become swollen. Comparing the measurements around her calf confirmed her suspicions. From the short time she had spent training as a nurse before becoming a teacher, Hilary recalled that this was the first sign of something serious - deep vein thrombosis (DVT).

At first, her case puzzled the doctors. None of the factors that would usually put someone at high risk of a thrombosis applied to her. She didn’t smoke or take the Pill. She hadn’t undergone hormone replacement therapy. She had no family history of blood disorders, nor had she been on any recent long journeys.

“All along, I told the doctors that I had sat as still as I would on a plane for long enough to get half way round the world, only moving for calls of nature,” she recalls. “I think that they thought I was exaggerating.”

After a course of treatment she was soon back at work. “Sadly I was out of hospital, and back at work, just in time for the Ofsted inspection,” she jokes.

The recent controversy over “economy class syndrome”, linking deaths from DVT with long-haul flights, prompted Hilary to write to The TES (her story appeared on February 2) and recall her lucky escape of a few years ago. “I just wanted to make other teachers more aware of it,” she says.

If anything, teachers’ workloads have since increased, raising the likelihood - though still rare - of a case like hers happening again. A study conducted last year by the Office of Manpower Economics showed that the average secondary teacher spends just over 16 hours a week marking and preparing work - two hours more than the last such study, four years earlier. Over a school year, that adds up to a lot of time spent sitting still.

John Scurr, a consultant surgeon at University College Hospital, London, was not entirely surprised to read of Hilary’s case. “We have seen people who have sat at desks for very long periods of time with similar sorts of problems,” he says. “It is clearly very unusual for the majority of teachers - as it is for most airline passengers. But a teacher who is relatively elderly or who has had other illnesses, such as lung or heart disease or some sort of cancer, is more likely to get a thrombosis.”

Research conducted by Dr Scurr, soon to be published in the medical journal The Lancet, is sure to ruffle a few feathers in the airline industry. He scanne the blood of 200 passengers before and after long-haul flights and found that 10 per cent had developed clots in transit. Although these were small and not life threatening, the research will confirm the link between air travel and DVT risk.

Too late for 55-year-old teacher Susan Fuller, from Essex, who died of a pulmonary embolism caused by DVT on a flight home from Bangkok in August 1999, one of several such deaths which have brought calls for better seating and health and safety information on aircraft. There are no records of annual flight-related DVT deaths, but estimates vary from dozens to hundreds.

Part of the reason is the difficulty in identifying the event that caused the thrombosis in the first place - clots can form and remain in the bloodstream for some time before causing a blockage. But Dr Scurr says the general health message must be to avoid situations - whether travelling or working - which present a risk. “Flying is one aspect of it,” he says. “What I suspect with this teacher is that she sat down for a long period of time, quite still, in much the same way as people sit on a plane, a bus or train.

“You are likely to get blood clots if you sit still for long periods. Lots of us have a slight tendency to form them - although most of the time they will dissolve and go away. It is only when they grow and get bigger that they cause a problem.”

Thirty years ago deep vein thrombosis resulting from air travel was virtually unheard of. But the increase in intercontinental flights, combined with a reduction in leg room, has brought a spate of cases. In the course of her job, Hilary Ellison unwittingly recreated those conditions at her dining room table. She’ll be happy if her story serves as a cautionary tale to others. Because even a glowing Ofsted report isn’t worth the risk.

Sitting targets

Deep vein thrombosis (DVT) has been known about for more than half a century. In 1940, it killed 23 people as they slept in their deckchairs while sheltering from bombing raids in London Underground stations. The deaths were due to the wooden frames of the chairs cutting off circulation in their legs.

DVT is so called because a blood clot usually occurs in the deep veins of the leg. If it travels through the circulatory system to the thinner veins of the lung, it can cause a pulmonary embolism, blocking the flow of blood and often proving fatal.

Between 5 and 10 per cent of us have “sticky” blood which is prone to clotting.Smoking, drinking, old age or illness can increase the threat of thrombosis.

On long-haul flights, dehydration and reduced cabin pressure can add to the effect of cramped seating. The Thrombosis Research Institute recommends that you flex your legs, ankles and toes while sitting to maintain circulation, walk along the aircraft aisle every hour or so, and drink plenty of fluids. Taking an aspirin a few hours before flying will thin your blood and reduce the chance of clots forming.