A pulmonary embolism can go undetected for days, weeks, or even longer, depending on the size and location of the clot. Up to 50% of people with a deep vein blood clot already have a clot in their lungs by the time the leg clot is diagnosed, yet only 33% to 40% of them ever had noticeable symptoms. Some people carry small clots in their lungs that are only discovered by accident during imaging for something else entirely.
The answer depends largely on how big the clot is and where it lodges. A massive clot blocking a major pulmonary artery can kill within hours. A tiny clot in a small branch vessel may sit there indefinitely, causing no symptoms at all or only vague ones you’d easily chalk up to stress, being out of shape, or a mild respiratory infection.
Why Many Pulmonary Embolisms Produce No Symptoms
Your lungs have a vast network of blood vessels. When a small clot lodges in one of the smaller branches (called subsegmental arteries), it may block so little blood flow that you never feel it. Studies using CT scans find that about 3.4% of hospitalized patients scanned for unrelated reasons turn out to have an unsuspected pulmonary embolism. Among outpatients getting routine chest scans, the rate is closer to 1%. These clots weren’t causing symptoms anyone noticed.
Cancer patients are especially likely to harbor silent clots. In one study, 70% of patients with incidentally discovered pulmonary embolisms had cancer. The combination of cancer-related clotting changes and frequent imaging means these clots get caught, but in people who aren’t being scanned regularly, the same clots could persist unnoticed.
Symptoms That Get Mistaken for Something Else
Pulmonary embolism is notoriously difficult to diagnose because its symptoms overlap with dozens of other conditions. The most common symptom is sudden shortness of breath, but it can also show up as chest pain that worsens with breathing, a racing heartbeat, dizziness, unusual anxiety, or sweating. Many people assume they’re having a panic attack, a pulled muscle, or a bout of bronchitis.
This overlap creates real diagnostic delays. A person might feel winded climbing stairs for a week or two and assume they’re just deconditioned. They might notice their heart racing at odd moments and blame caffeine or stress. Because there’s no single hallmark symptom that screams “blood clot in the lungs,” people often wait days or weeks before seeking medical attention, and even then, the diagnosis isn’t always the first thing a clinician considers.
What Happens to Untreated Clots Over Time
Your body has its own clot-dissolving system. Small clots can sometimes break down naturally over days to weeks without treatment. But this process is unreliable, and there’s no way to predict whether a given clot will dissolve on its own or grow worse.
The stakes of getting it wrong are serious. About one-third of people with an undiagnosed, untreated pulmonary embolism don’t survive. Among those who do get diagnosed, roughly 1 in 20 die within 30 days, and about 4 in 20 die within six months. A large clot can cause fatal heart strain within hours of lodging in the lungs, sometimes before there’s any chance to reach a hospital.
Even when an untreated clot doesn’t kill, it can cause lasting damage. Between 1% and 3.8% of people who survive a pulmonary embolism develop a condition called chronic thromboembolic pulmonary hypertension within two years. This happens when old clot material scars the inside of the lung’s blood vessels, permanently raising blood pressure in the lungs. The result is persistent shortness of breath, fatigue, and reduced exercise tolerance that doesn’t resolve on its own.
Small Clots: Do They Always Matter?
This is genuinely unsettled in medicine. Clots in the smallest lung vessels (subsegmental pulmonary embolisms) occupy a gray zone. Studies show that when these tiny clots are missed by imaging and patients go untreated, only 1% to 2% develop a new clot problem within three months. That’s a surprisingly low rate, considering that the imaging tests used in those studies were later shown to miss about 20% of small clots.
The implication is that many subsegmental clots probably resolve without treatment and without consequences. There’s no clear consensus on whether every small clot found incidentally on a CT scan needs blood-thinning therapy. Some clinicians treat them; others monitor. The clinical significance of these tiny clots remains genuinely uncertain, which is part of why a pulmonary embolism can exist in your lungs without you ever knowing it was there.
Who Is Most Likely to Have a Silent PE
Certain groups are more likely to develop clots that go unnoticed. People with active cancer top the list, both because cancer increases clotting risk and because their other symptoms (fatigue, shortness of breath from treatment) can mask the signs of a PE. People who’ve recently had surgery, been immobilized for long periods, or have a genetic clotting disorder also face higher risk.
Hospitalized patients are particularly vulnerable. The combination of bed rest, IV lines, and underlying illness creates ideal conditions for clot formation. The 4% incidental PE rate found in hospital inpatients reflects this reality. Many of those patients were being scanned for pneumonia, post-surgical complications, or cancer staging, and the lung clot was an unexpected finding.
Signs That Deserve Immediate Attention
Because silent PEs can exist for weeks without obvious symptoms, the warning signs worth acting on are the ones that appear suddenly or don’t have an obvious explanation. Shortness of breath that comes on without exertion, chest pain that sharpens when you breathe in, a heart rate that stays elevated at rest, or unexplained lightheadedness all warrant urgent evaluation, especially if you have any known risk factors for blood clots.
Coughing up blood, even a small amount, is a red flag. So is sudden swelling, warmth, or pain in one leg, which can signal the deep vein clot that often serves as the source of a PE. The combination of leg symptoms with any breathing change is particularly concerning and shouldn’t be attributed to something benign without investigation.