Blood clots become dangerous when they start to move, this is because if they block blood vessels in the lungs (called a pulmonary embolism) they can prove fatal.
Inactivity and being immobile for long periods, including lying in a hospital bed or sitting on a long journey, can cause blood flow to slow down and increase the likelihood of a clot.
During a long flight, not only is your ability to move around limited but the dry environment can lead to dehydration, especially if you have a few alcoholic drinks. Being dehydrated can cause blood to become thicker and more prone to clotting, therefore increasing your risk of DVT.
Long-haul flights (over six hours) can be associated with a risk of DVT, but the good news is that if you're in generally good health there's very little chance of this happening.
In hospitals you will be encouraged to get up and move around as much as possible and given compression stockings to wear to reduce the risk of clots.
DVT often occurs in the leg and presents with pain and swelling, as well as warm skin around the area of the clot.However, you don't always experience symptoms, so if you're in a high-risk group make sure you take preventative measures before long periods of being immobile.
An assessment by a doctor, along with blood and imaging tests, are required for a DVT diagnosis. An ultrasound or CT scan is usually enough for a diagnosis, but it can also be confirmed by a D-dimer blood test, which detects pieces of broken down clot, and another test where a dye is injected into the blood and an X-ray used to discover any potential clots.
Who gets it?
You're most likely to develop DVT if you're over the age of 40, have a previous history or family history of DVT, have existing health problems, or are going through extreme hormonal changes (only in women).
While men tend to have a higher risk in general, pregnancy, the oral contraceptive pill and taking hormone replacement therapy (HRT) for the menopause are all factors that could increase a woman's chance of experiencing DVT.
Living with other serious health conditions can also increase your chances of developing DVT, including (but aren't limited to) rheumatoid arthritis, heart and lung disease, and even cancer.
Cancer cells have the potential to make blood clot more and radiotherapy and chemotherapy can damage and weaken your veins, making clots more likely.Having an illness that reduces your mobility can also increase your chances of developing DVT, especially if you're confined to a hospital bed.
You're considered at high risk of DVT if you've had surgery under a general anaesthetic lasting more than 30 minutes in the previous 4 weeks.
Without treatment DVT can lead to a more serious and potentially fatal condition called a pulmonary embolism, which occurs in about 10 per cent of DVT patients. This is when the blood clot reaches your lungs and blocks a blood vessel.Symptoms of a pulmonary embolism include breathlessness and chest pain, and it needs urgent investigation and treatment.
If you experience DVT, doctors are most likely to prescribe drugs called anticoagulants. These are used to thin the blood which can stop clots from getting bigger, as well as prevent others forming.
A number of sources recommend aspirin to prevent blood clots in air travellers, but experts say there isn't enough evidence to suggest it has any effect on developing DVT.