Blood clots in the lungs, known as pulmonary embolisms, almost always start somewhere else in your body. A clot forms in a deep vein, usually in the legs, then breaks free and travels through your bloodstream until it lodges in the lung’s arteries. Preventing a pulmonary embolism means stopping that clot from forming in the first place or catching it before it migrates. The strategies range from simple daily habits to medical interventions your care team may recommend after surgery or during a hospital stay.
How Clots Form and Travel to the Lungs
The process starts with deep vein thrombosis (DVT), a clot that develops in the large veins of your lower legs, thighs, or pelvis. Three conditions raise your risk: slow blood flow (from sitting still too long or being bedridden), damage to a blood vessel wall (from surgery, injury, or inflammation), and blood that clots more easily than normal (from genetics, hormones, or certain medications). When any combination of these is present, a clot can form silently in a deep vein.
The danger comes when part or all of that clot detaches. It rides the bloodstream through increasingly larger veins, passes through the right side of the heart, and gets pumped into the pulmonary arteries. Once it reaches a vessel too narrow to pass through, it blocks blood flow to a section of the lung. Small clots may cause mild symptoms. A large one can be life-threatening within minutes. That’s why prevention focuses heavily on the legs, where most of these clots originate.
Keep Blood Moving Throughout the Day
Prolonged stillness is the single most controllable risk factor. Whether you’re at a desk, in a recliner recovering from illness, or on a long flight, hours of sitting allow blood to pool in your calves and thighs. The fix is straightforward: interrupt that stillness regularly.
If you work at a desk, stand and walk for a few minutes every hour. Even small movements help. Flexing your ankles, pressing the balls of your feet into the floor, and contracting your calf muscles act as a pump that pushes blood back toward your heart. Regular exercise, even moderate walking for 30 minutes most days, keeps your circulatory system functioning well and reduces clot risk over time. If you’re recovering from surgery or illness and can’t walk yet, your care team will likely have you doing ankle pumps and leg exercises in bed for this exact reason.
Travel Precautions for Long Trips
Flights, car rides, and train journeys lasting more than four hours carry a measurable increase in clot risk. The combination of cramped seating, low cabin humidity, and reduced movement creates near-ideal conditions for a DVT to form. The CDC recommends choosing an aisle seat when possible so you can get up and walk around every two to three hours. On road trips, schedule stops at the same intervals.
While seated, exercise your calf muscles by pulling your toes toward your shins, then pointing them away. Rotate your ankles in circles. These movements aren’t a substitute for walking, but they keep blood circulating between breaks. Wear loose, comfortable clothing that doesn’t constrict your waist or legs. Avoid crossing your legs for extended periods.
Graduated compression stockings offer real protection during travel. In a study of airline passengers over age 50 on flights longer than eight hours, none of those wearing compression stockings developed DVT, while about 10% of those without stockings did. Stockings in the 20 to 30 mmHg range (often labeled “class 1”) are widely available without a prescription and are the type most commonly studied for travel-related clot prevention.
How Compression Stockings Work
Graduated compression stockings apply the most pressure at the ankle and gradually decrease pressure moving up the leg. This gradient helps push blood upward against gravity, preventing it from pooling in the lower veins where clots tend to form. In hospitalized patients, DVT developed in 13% of those wearing compression stockings compared to 26% of those without, cutting the risk roughly in half.
Stockings come in three general categories: low compression (under 20 mmHg), medium compression (20 to 30 mmHg), and high compression (above 30 mmHg). For travel and general prevention, low to medium compression is typical. Higher compression levels are usually reserved for people with existing vein problems or on medical advice. The stockings need to fit properly to work. Too loose and they won’t compress enough; too tight and they can actually restrict blood flow. Most pharmacies carry sizing guides, or your doctor can help you find the right fit.
Staying Hydrated
Dehydration concentrates your blood, raising its viscosity and theoretically making clot formation easier. This connection, based on a well-established principle of clot science called Virchow’s triad, is why you’ll hear “drink plenty of water” as standard advice before flights and during hospital stays. Research confirms that dehydration during physical stress alters how your body breaks down clots, though the direct link between mild everyday dehydration and clot risk is harder to quantify precisely.
The practical takeaway: drink water consistently throughout the day, especially during travel, after exercise, in hot weather, and during illness that causes fluid loss. Limit alcohol and caffeine before and during long flights, as both promote fluid loss. You don’t need to overhydrate, just avoid running dry.
Medical Prevention After Surgery
Surgery is one of the highest-risk scenarios for clot formation. The combination of tissue injury, anesthesia, and bed rest creates all three conditions that promote clotting. Hospitals use a layered approach to counter this.
The first layer is early movement. Current protocols aim to have patients standing under medical supervision within 24 hours of surgery, with activity gradually increasing from there. For orthopedic procedures involving the legs, rehabilitation starts even sooner with specific exercises: ankle pumps, quadriceps contractions, knee bending, and calf massage, all designed to keep blood flowing through the operated limb.
The second layer is mechanical compression. Intermittent pneumatic compression (IPC) devices are inflatable sleeves wrapped around your calves that rhythmically squeeze and release, mimicking the pumping action of walking. Hospitals prefer these for patients who have a high bleeding risk because, unlike blood thinners, they carry no risk of hemorrhage. They can be applied during surgery on the opposite leg and used immediately afterward regardless of drains or wound status.
The third layer is blood-thinning medication. For patients at moderate to high clot risk who aren’t at elevated risk of bleeding, preventive doses of anticoagulants are standard. These are given at much lower doses than treatment for an existing clot, just enough to reduce the blood’s tendency to form dangerous clots. Your surgical team decides which combination of these three layers you need based on the type of surgery, your medical history, and your individual risk profile.
Who Faces Higher Risk
Some people need to be more vigilant than others. Your risk of forming a clot in the legs (and by extension the lungs) increases with age, obesity, smoking, pregnancy, use of hormonal birth control or hormone replacement therapy, cancer and cancer treatment, a personal or family history of blood clots, and inherited clotting disorders. Having multiple risk factors compounds the danger.
If you fall into any of these categories and face a situation that adds further risk, like upcoming surgery, a long international flight, or extended bed rest from illness, talk to your doctor beforehand. They may recommend compression stockings, a short course of preventive blood thinners, or both. People with known clotting disorders sometimes take anticoagulants long-term specifically to prevent pulmonary embolism.
Recognizing the Warning Signs
Prevention also means catching a clot early, before it reaches the lungs. DVT in the legs typically causes pain or tenderness (often in the calf), swelling, warmth, and reddish or bluish skin discoloration. These symptoms usually affect one leg, not both. Not all DVTs cause noticeable symptoms, but when these signs appear, especially after surgery, travel, or prolonged immobility, they warrant urgent medical evaluation.
If a clot has already reached the lungs, the symptoms shift. Unexplained shortness of breath is the most common sign. You may also experience sharp chest pain that worsens when you breathe in, a cough (sometimes with blood), rapid heartbeat, or lightheadedness and fainting. These symptoms can come on suddenly and escalate quickly. A pulmonary embolism is a medical emergency, and recognizing these signs early can be the difference between a treatable event and a fatal one.