Harvard health experts answer your questions

Q. Within a three-month period, both my sister and her husband (both in their 40s) were hospitalized with pulmonary embolisms. Any thoughts on the cause? Could it be something in the air?

A. A pulmonary embolism is a blood clot (or a piece of one) that forms somewhere in the body, usually the veins of the legs, gets loose, travels through the bloodstream, and lodges in an artery in the lungs. The consequences are extremely variable, ranging from no symptoms at all if a small clot blocks a small artery to sudden death if a large one blocks a larger artery. In between those two extremes, symptoms may include sudden breathlessness, pain in the chest when taking in a deep breath, and a new pain or swelling in the calf or thigh, caused by the clots in the legs where the pulmonary embolism originated.

It's important that blood have the ability to clot. Without clotting, we'd bleed to death whenever a blood vessel wall was breached. But there's no good purpose served by a clot forming inside a vein that's not bleeding.

Pulmonary embolism: traveling trouble

So why does it happen? Many reasons. Something, such as a surgical procedure, can directly injure the veins of the legs or pelvis. Pressure -- from a baby in the uterus, for example -- can narrow vessels so blood moves through them more slowly than usual, which can make clots more likely to form. Any circumstance that keeps the legs cramped and not moving -- like a long airplane flight or sitting at your desk for a long time -- also slows down blood flow and makes clot formation more likely. Certain medicines, including oral contraceptives and hormone therapy, can make the blood itself "stickier." Some diseases -- different types of cancer, for example -- and the trauma of surgery may have the same clotting effect. This is all pretty old news.

But since the early 1990s, researchers have discovered a group of inherited conditions that cause thrombophilia, the medical term for an increased tendency to form blood clots. Factor V Leiden and protein C deficiency are two examples. One or more of these inherited conditions affect between 25% and 40% of people who develop blood clots in the legs and pelvis. As you might expect, having more than one increases the risk. Likewise, having one of them and another risk factor -- being pregnant, taking an oral contraceptive, having hip replacement surgery -- also ratchets up the risk.

You've asked if "something in the air" could have caused your sister and her husband to both have pulmonary embolisms. I'm not aware of anything contagious that would, or any chemical (unless they're both heavy smokers).

I am wondering if they both travel a lot and whether they might have taken a long airplane trip together.

Because they are both in their 40s, which is younger than most people who suffer a pulmonary embolism, I'd recommend that they see a specialist -- a hematologist -- with special expertise in blood clotting.

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