You’re in the middle of a punishing workout. Your heart races. Your muscles beg for mercy. Sweat rains down off your face. And your mouth is ripe with the unmistakable taste of blood. Wait, what?

It might sound odd to some, but many athletes know exactly what I’m talking about. A metallic, bloody taste in the mouth during intense exercise is not uncommon, but it can certainly be confusing. The taste often isn’t accompanied by any visible blood in your saliva, which makes it even more perplexing.

Truth be told, a couple of different thing can cause this sometimes scary sensation. Here’s why you might taste blood in your mouth during or after intense exercise.

Blood and Iron

Some people complain about a “bloody” taste in their mouth during or after exercise, while others say it’s more “metallic.” Odds are, they’re tasting something similar but classifying it a different way. The high iron content of blood gives it a metallic taste, so some perceive it as such.

But why do folks experience this taste in the first place?

Each case is different, but a few main culprits could be to blame. Working out in a cold, dry climate can lead to dryness and tiny cracks in the lining of the nose and throat, which can produce a “bloody taste” in the mouth. Or, if you have a blockage or infection in the salivary duct or severe tooth decay, heavy breathing could increase saliva production and give off a bad taste that might seem bloody or metallic.

But perhaps the most interesting potential cause is exercise-induced pulmonary edema.

What is an Exercise-Induced Pulmonary Edema?

Pulmonary edema is a condition in which excess fluid collects in the air sacs inside the lungs. In severe cases, it can cause serious breathing problems and have the potential to be life-threatening. However, researchers are now finding that it’s rather common to experience a mild pulmonary edema during intense exercise. This is known as an exercise-induced pulmonary edema, or EIPA.

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For example, a study, entitled “Small Changes in Lung Function in Runners With Marathon-Induced Interstitial Lung Edema,” looked for EIPA in participants in a 2012 marathon. Of the 36 participants, 17 percent were found to have suffered a mild pulmonary edema at some point during the race. Similar results have been found in rowers, cyclists and swimmers, with both short- and long-term bouts of exercise exhibiting the capability to produce EIPA. However, the symptoms are mild and can be expected to subside quickly.

Why might an EIPA cause you to taste blood in your mouth? The answer has to do with the permeability of the blood-gas barrier. The blood-gas barrier (also known as the blood-air barrier) is defined as “a thin tissue through which gases are exchanged between the alveolar air and the blood in the pulmonary capillaries.” When heart rate increases during intense exercise, the higher level of stress can compromise the integrity of the blood-gas barrier. This allows fluid (including red blood cells) to leak into the lung air sacs, which could potentially lead to a bloody or metallic taste in your mouth.

“Elevated pulmonary artery and left atrial pressures, coupled with a decreased intrathoracic pressure during inspiration, translate to increased capillary transmural pressures and the exudation of fluid from the capillary to the airspace,” wrote the authors of a study entitled “The Curious Question of Exercise-Induced Pulmonary Edema.”

If you find yourself actually coughing up blood during or after intense exercise, you should seek medical attention. However, if you simply experience the taste occasionally during or after exercise, and there are no other concerning symptoms, there’s probably nothing to worry about. According to the authors of the previously cited study, “the development of clinically important pulmonary edema with exercise remains a rare event.”