The most common warning sign of a pulmonary embolism is sudden, unexplained shortness of breath. It often comes on without any exertion and can feel like you simply can’t get enough air no matter how deeply you breathe. Other hallmark signs include sharp chest pain that worsens when you inhale, a rapid heartbeat, and lightheadedness or fainting.
The Core Symptoms
A pulmonary embolism happens when a blood clot, usually one that formed in a deep leg vein, breaks loose and travels to the lungs, where it blocks blood flow. The symptoms tend to appear suddenly, sometimes within seconds, which is what distinguishes them from conditions that build gradually.
Shortness of breath is the single most frequent symptom. It happens because the clot prevents part of the lung from exchanging oxygen and carbon dioxide properly. Your body compensates by making you breathe faster and harder, but the blocked area of lung tissue can’t do its job regardless of effort. This is why the breathlessness feels disproportionate to whatever you’re doing, or strikes even at rest.
Chest pain is the second major warning sign. It’s often described as sharp or stabbing, and it typically gets worse when you take a deep breath, cough, or bend over. This type of pain, called pleuritic chest pain, differs from the squeezing or pressure sensation of a heart attack. However, a large clot can also strain the right side of the heart so severely that it mimics heart attack symptoms, with pressure behind the breastbone and pain radiating outward.
A rapid heart rate, generally above 100 beats per minute, is another key indicator. Your heart speeds up to try to push enough oxygenated blood through the body despite the blockage.
Less Obvious Warning Signs
Not every pulmonary embolism announces itself with dramatic breathlessness and chest pain. Some people experience subtler or more unusual symptoms that are easy to dismiss or attribute to something else.
- Fainting or near-fainting. A sudden drop in blood pressure or heart rate can cause you to pass out. This tends to happen with larger clots that significantly obstruct blood flow.
- Coughing up blood. A cough that produces blood-streaked or pink-tinged mucus can signal that lung tissue is being damaged by the clot.
- Unexplained anxiety or a sense of dread. Many people describe an overwhelming feeling that something is wrong before other symptoms fully develop.
- Wheezing or a new productive cough. These can be mistaken for asthma or a respiratory infection.
- Fever, abdominal pain, or flank pain. These atypical presentations are rarer but documented, and they can send both patients and clinicians looking in the wrong direction initially.
- Confusion or delirium. In older adults especially, a change in mental clarity can be the most prominent symptom.
About one-third of patients with a pulmonary embolism also show visible signs of a clot in the leg at the time of diagnosis. So the warning signs don’t always start in the chest.
Leg Clot Signs That Can Precede a PE
Most pulmonary embolisms originate as deep vein thrombosis (DVT), a clot in one of the deep veins of the leg. Recognizing a DVT before it travels to the lungs is one of the best opportunities to prevent a life-threatening event.
The classic DVT signs are swelling in one leg (not both), pain or cramping that usually starts in the calf, skin that turns red or purple in the affected area, and a feeling of unusual warmth over the swollen spot. The pain often feels like a deep soreness or a charley horse that doesn’t go away. If you notice these symptoms, particularly after a period of immobility like a long flight, a hospital stay, or recovery from surgery, they warrant urgent medical evaluation even before any chest or breathing symptoms appear.
Why Some Clots Cause No Symptoms at All
Not every pulmonary embolism produces noticeable symptoms. A meta-analysis covering more than 10,000 patients found that about 2.6% of people undergoing chest CT scans for unrelated reasons were found to have a pulmonary embolism they didn’t know about. These “silent” clots are more common in people who are hospitalized or who have cancer. Modern CT scanners can now detect smaller clots in the peripheral lung arteries that older technology would have missed, which partly explains the rising detection rate.
The existence of silent PEs doesn’t mean small clots are harmless. They can still cause long-term damage to lung tissue and increase the risk of future, larger clots. But it does mean that some people with risk factors may be living with clots that produce only vague symptoms like mild fatigue or slightly reduced exercise tolerance, symptoms too nonspecific to trigger alarm.
Who Is Most at Risk
Certain situations dramatically increase the odds of developing a clot that could reach the lungs. The biggest risk factors are immobility and anything that alters normal blood clotting. Specifically, watch for these circumstances:
- Recent surgery or hospitalization. Immobility during recovery is one of the strongest risk factors, particularly after orthopedic procedures on the hip or knee. Hospitalized patients are more than four times as likely to have an incidental PE compared to outpatients.
- Long periods of sitting. Flights, road trips, or desk work lasting several hours without movement can slow blood flow in the legs enough to allow clots to form.
- Cancer. Both the disease itself and its treatment increase clotting risk. Active cancer raises the likelihood of an incidental PE by about 80%.
- Hormonal factors. Oral contraceptives, hormone replacement therapy, and pregnancy all elevate clotting risk.
- Personal or family history. A previous DVT or pulmonary embolism significantly increases the chance of recurrence.
When Symptoms Become an Emergency
Certain combinations of symptoms signal a potentially massive clot and require an immediate call to emergency services. These red flags include severe difficulty breathing, chest or upper back pain, a heart rate that feels very fast or pounding, and loss of consciousness. A large PE can be fatal within hours if untreated, but with prompt treatment, mortality drops to under 2% even in intermediate-to-high-risk cases.
The challenge is that milder symptoms, like moderate breathlessness or calf pain, don’t always feel like an emergency. If you have any of the risk factors listed above and develop new, unexplained breathing difficulty or chest discomfort, treat it as urgent even if it seems manageable. Pulmonary embolisms are far more survivable when caught early, and the initial symptoms don’t always reflect the true size of the clot.