MOST of the time, blood clots are a good thing. When you get injured, you need your blood to solidify and clump together at the site to help stop the bleeding. But sometimes clots crop up when they’re not needed, and that can spell trouble — especially if they form in the deep veins near your muscles.

“When blood clots form in this deeper system, they can be painful and very dangerous,” says Luis Navarro, founder of the Vein Treatment Center in New York City. This kind of clot is called a deep vein thrombosis, or DVT. They’re like roadblocks on your blood highway, causing traffic jams in your circulation and prevent the blood flow that keeps your system up and running.

Things can get even more serious if a DVT breaks away from its original spot and travels to your lungs. Then it becomes a pulmonary embolism (PE), a clot that prevents these vital organs from getting the oxygen and blood they need. That can damage your lungs and other organs and may even be fatal.

Here are the warning signs of a blood clot to watch for, so you can act quickly if one strikes.

Swelling in one limb

A swollen leg or arm is one of the most common signs of a DVT. “Blood clots can block the healthy flow of blood in the legs, and blood can pool behind the clot causing swelling,” says Dr. Navarro.

Leg or arm pain

Usually, DVT pain comes as a combo with other symptoms like swelling or redness, but sometimes it can stand alone. “Unfortunately, pain from a blood clot can easily be mistaken for a muscle cramp or strain, which is why the issue often goes undiagnosed and is specifically dangerous,” says Navarro.

DVT pain tends to strike when you’re walking or when you flex your foot upward.

Redness on your skin

While it’s true that a bruise is a type of blood clot, it’s not the kind you have to worry about. You can’t see a DVT. You might see some bruise-like discoloration, but you’re more likely to see red. A DVT causes redness in the affected limb and makes your arm or leg feel warm to the touch.

Shortness of breath

A blood clot in your lung slows your oxygen flow, and that can leave you feeling winded.

Chest pain

A pain in your chest may make you think heart attack, but it could be a pulmonary embolism. “Both a PE and a heart attack share similar symptoms,” says Navarro. However, PE pain tends to be sharp and stabbing, and feels worst when you take a deep breath.

Heart attack pain often radiates from upper areas of your body like your shoulders, jaw, or neck.

Unexplained cough

If you’re also having shortness of breath, fast heart rate, or chest pain, it could be a PE. “The cough will be dry, but sometimes people can cough up mucus and/or blood,” says Navarro.

A racing heart

When oxygen is low, your heart rate goes high to try to make up for the shortage. Feeling a flutter in your chest and having trouble with deep breaths could be your body sending out an SOS that you’ve got a PE lurking in your lungs.

Feeling faint

The lightheadedness is due in part to the chemical released from the lungs, changes in oxygenation of the blood, and in the blood pressure and heart rate. (SD-Agencies)