Sitting for long periods of time has become increasingly common in modern society, both at work and at home. With desk jobs and screen time dominating our days, some health experts have warned that remaining sedentary for too long could negatively impact circulation and increase the risk of dangerous blood clots.
What causes blood clots?
Blood clots, also known as deep vein thrombosis (DVT), occur when blood thickens and clumps together. This usually happens in the deep veins of the leg, but clots can also form in other parts of the body like the arms or pelvis. Clots form as a result of sluggish blood flow, often when someone is immobile or sedentary for an extended period of time. Sitting still causes the blood to pool in the veins of the legs and pelvis, where it can start to clot if this stagnation persists for too long. Other risk factors like obesity, smoking, pregnancy, and certain medications can also increase someone’s risk of developing clots.
How sitting promotes blood clotting
When you sit for a long time, a few things happen that can set the stage for blood clots:
- Reduced muscle activity – Moving your leg muscles helps pump blood back to your heart. When your legs are immobile for hours, your calf muscles don’t contract, allowing blood to pool.
- Bent leg posture – Sitting with your knees bent decreases blood return from your lower legs compared to standing or lying down straight.
- Increased coagulation – Stagnant blood is more likely to clot. Long periods of sedentary time increase certain proteins that cause blood to coagulate.
All of these changes to normal blood flow and circulation when sitting create conditions that encourage abnormal clotting. The longer someone sits still, the greater the risk.
What does the research say?
A number of studies have found connections between greater sitting time and increased risk of blood clots:
- A meta-analysis of observational studies found that those who sat for more than 8 hours a day had a 2.8 times greater risk of DVT compared to those who sat less than 4 hours per day.
- Another study found women who sat 10+ hours a day and did not exercise had a 4 times greater risk of thromboembolism (blood clots) than active women with less sitting time.
- Research among office workers observed that sitting in a chair continuously for 4 hours without getting up increased blood clotting factors by over 40%.
- A study that had participants sit motionless for periods of 2-8 hours saw progressive increases in blood clotting factors the longer they sat.
While more research is still needed, these findings suggest a strong connection between long sedentary time and increased risk of clots in the veins.
At what point does sitting become dangerous?
There is no definite threshold after which sitting suddenly becomes dangerous for clots. Research has found heightened risks with different cutoffs like 4, 8, and 10 hours of sitting per day. The risk rises gradually the more sitting occurs, especially if sitting time is accrued in prolonged, uninterrupted bouts.
Sitting for a full 8-hour workday would certainly be considered high risk, but detrimental effects appear to start accumulating after even just 1-2 hours of uninterrupted sitting. For optimal health, it’s recommended to take movement breaks at least every 30-60 minutes.
Who’s most at risk?
Some groups of people may be more vulnerable to the risks of prolonged sitting:
- Desk workers – Office jobs require long hours of uninterrupted sitting.
- Frequent fliers – Sitting immobile on flights for 4+ hours increases DVT risk.
- Elderly – Less active lifestyles contribue to poorer circulation.
- Overweight/obese – Excess weight places more pressure on veins.
- Smokers – Smoking damages blood vessels and increases clotting.
- Pregnant women – Hormonal changes increase DVT risk.
- Cancer patients – Tumors can release pro-clotting substances.
- On birth control – Estrogen in pills promotes clotting.
- Post-surgery – Immobility during recovery raises risk.
Those with a history of blood clots, vascular conditions, or genetic clotting disorders should be especially mindful of prolonged sitting.
Signs and symptoms
The most common signs of a blood clot in the leg are:
- Swelling in one leg or one area of a leg
- Pain in the leg, often in the calf. The pain may feel “warm” or sore to the touch.
- Redness or blueish skin discoloration
- Visible surface veins
Shortness of breath, rapid heart rate, chest pain, or coughing up blood can be signs of a pulmonary embolism if a clot travels to the lungs. Seek immediate medical attention if you experience any of these symptoms.
| Risk Level | Sitting Duration |
|---|---|
| Low | 0-3 hours |
| Moderate | 4-7 hours |
| High | 8+ hours |
Prevention
If you have a desk job or lifestyle that involves a lot of sitting, there are things you can do to lower your risk of blood clots:
Take regular movement breaks
Avoid sitting for more than 1-2 hours at a time. Set a reminder to get up and walk around, march in place, stretch, or do knee raises. This boosts blood flow and prevents stagnation.
Exercise regularly
Make time for exercise even with a sitting job. This could be a daily lunchtime walk or gym session after work to get your leg muscles pumping.
Wear compression socks
Compression socks apply gentle pressure to improve circulation in the legs and veins.
Lose weight if overweight
Excess weight strains leg veins and lowers blood flow. Losing even a few pounds can improve circulation.
Drink plenty of water
Hydration prevents blood from thickening and clotting.
Avoid crossing legs
Crossing your legs can pinch veins and restrict blood flow in the calves.
Elevate your feet when possible
Place your legs above heart level while sitting or resting to utilize gravity to promote blood return.
Go for walks during long flights or car rides
Move around the cabin and stretch your calves when flying. Take breaks to walk around at least once per hour during long road trips.
Talk to your doctor about medications
Blood thinners like aspirin may be recommended if you have DVT risk factors.
Treatment
If a clot is detected, treatment usually involves blood thinner medication to prevent further clot growth and allow the body to slowly break it down and dissolve it. Rest, compression stockings, and elevation of the affected leg are also recommended. If clots keep recurring, an inferior vena cava filter may be surgically implanted to prevent clots from reaching the lungs and causing potentially fatal complications.
The bottom line
Prolonged sitting substantially raises the risk of developing a blood clot, especially when sitting for 8 hours or more in a single day. The immobility leads to sluggish blood flow that allows blood to pool and start clotting in the veins of the legs or pelvis. Taking regular movement breaks, staying active, wearing compression socks, and staying hydrated can all be helpful precautions. Listen to your body and seek medical help promptly if you experience any symptoms of a possible blood clot.
