A long flight out of Australia can mean many hours in the same seat. Most people step off a little stiff and tired, then forget about it. For some travellers, though, sitting still for that long raises the chance of a blood clot in the leg. Knowing the signs and a few simple habits can make the trip safer and more comfortable.

This kind of clot is called deep vein thrombosis, or DVT. It is not just a flying problem. Long trips by car, bus or train can carry a similar risk. The good news is that most of the advice is easy to follow from the moment you board.

What deep vein thrombosis is

The Victorian Better Health Channel describes deep vein thrombosis as a blood clot that forms in the deep veins of the leg. The clot can block the flow of blood and oxygen. Anything that slows blood moving through those veins can play a part. Long periods of sitting are one of the common examples.

The bigger worry is what can happen next. Part of a clot can break loose and travel to the lungs. This is called a pulmonary embolism. Healthy WA describes it as life threatening and says it needs emergency hospital treatment.

Why long trips raise the risk

Better Health lists trips of more than four hours as a temporary risk factor. That covers air, road and rail travel. The risk mainly matters for people who are already more likely to get a clot. Dehydration is also listed, and the dry air of a long flight makes it easy to drink less than you should.

NHS travel health advice from Scotland says the absolute risk after a long haul flight is very small. It also says the risk rises the longer the trip goes on. So a short hop up the coast is a very different thing from a full day in the air. The longer the trip, the more these habits are worth the effort.

Who should take extra care

Some people carry a higher risk before they even pack a bag. Better Health names a long list of factors. The main ones are worth knowing before you book.

• Being over 40, or living with obesity
• Smoking
• A previous DVT, or a family history of blood clots
• Taking the contraceptive pill or hormone replacement therapy
• Cancer, heart failure or coronary heart disease
• Varicose veins or a blood clotting disorder
• Pregnancy, or the six weeks after giving birth
• Recent major surgery, a fracture or limited movement

Surgery is one that catches people out. Some travellers plan a procedure close to a holiday, such as dental implants. Others are recovering from LASIK or LASEK eye surgery when they fly. Healthy WA says people at high risk, including those with recent surgery or pregnancy, should talk to their doctor before long haul travel.

Simple habits on the plane

Movement is the main thing. Better Health suggests moving around before, during and after travel where you can. It also suggests simple leg and foot stretches every couple of hours to keep the calf muscles working. Sitting with your legs uncrossed helps too.

The NHS travel advice adds a few practical points. An aisle seat makes it easier to get up and walk. Flexing and pointing your ankles while seated helps blood flow in the calves. Try not to put your cabin bag where it stops you moving your legs.

If you already keep fit at home, the habit of regular movement will feel familiar. The ideas in fitness training carry over well to a long flight. Small, steady efforts count for more than one big stretch at the end. Even standing in the aisle for a minute or two helps.

Drinks, sleep and what you wear

Healthy WA suggests drinking plenty of water or juice on long trips. It also suggests avoiding too much coffee or alcohol. Better Health advises avoiding alcohol and smoking and wearing loose clothing. A water bottle you can refill after security is an easy way to keep up your fluids.

Light, sensible food can help you feel better on arrival as well. Packing a snack of kale chips or fresh fruit is a lighter choice than a pile of salty food. Drinking more water does mean more trips to the toilet. That is no bad thing, because each walk down the aisle gets your legs moving.

The NHS travel advice says to avoid excess alcohol, tranquillisers and sleeping tablets. These can make you less likely to get up and move during the flight. A light nap is fine, but try to wake and stretch at regular points. Setting a gentle alarm on your watch or phone can help.

Compression stockings and medicines

Compression stockings are often sold at airports and pharmacies. They are not the right choice for everyone. Better Health says some people may benefit from them and that you should check with your doctor whether they suit you. The NHS travel advice warns that stockings that fit badly can actually increase the risk of a clot.

That same advice says aspirin is not recommended to prevent travel related clots. Some people at higher risk may be prescribed an injected medicine by their doctor. If you already take blood thinners, follow your own doctor’s advice for the trip. Do not start or stop any medicine for travel without speaking to them first.

Warning signs during or after travel

A clot does not always show up during the flight. Signs can appear in the days after you land. Better Health says DVT usually affects one leg. It can cause throbbing, cramping, pain or tenderness in the calf or thigh. Swelling of the lower leg, ankle or foot is another sign.

The skin may feel warm or change colour, looking red, pale or blue. The NHS travel advice says to get urgent medical help for swollen, painful legs after travel, especially if one is worse than the other. It also says to seek urgent help for any trouble breathing. In Australia, call Triple Zero (000) in an emergency.

Frequently asked questions

How long does a trip need to be before DVT is a concern?

Better Health lists trips of more than four hours as a risk factor. This applies to planes, cars, buses and trains. The risk grows with the length of the trip. It matters most for people with other risk factors.

Should everyone wear compression stockings on a long flight?

No, they are not suited to everyone. Some people at higher risk may be advised to wear them. Stockings that fit poorly can raise the risk of a clot. Ask your doctor before you buy a pair.

Can I take aspirin before flying to prevent a clot?

The NHS travel health advice says aspirin is not recommended for preventing travel related clots. If you are worried about your risk, see your doctor before you go. They can tell you what suits your health. Never change your usual medicines without their advice.

What should I do if my leg swells after a flight?

Swelling and pain in one leg after travel needs prompt medical attention. Contact your doctor or go to an emergency department. If you have trouble breathing or chest pain, call Triple Zero (000). Do not wait to see if it passes.

This article is general information only and is not medical advice. Please speak with a doctor or qualified health professional about your own health before you travel.