A flurry of recent research has shown that excessive sitting increases the odds of obesity, heart disease, diabetes, some kinds of cancer, and even premature death. Now a new study is adding another health risk to that list: pulmonary embolism (PE), or a blood clot in the lungs.

The study, of nearly 70,000 female nurses who were followed for 18 years, found that those who spent hours sitting and sedentary in their leisure time were much more likely to have blood clots in their lungs compared to those who were more active.

And the risk remained even after researchers adjusted their data to take into account other factors including body weight, heart disease, smoking, and the use of medications like blood thinners and nonsteroidal anti-inflammatory drugs (NSAIDs).

“Women who sat the most had more than twice the risk of PE compared to women who sat the least,” says study researcher Christopher Kabrhel, MD, an attending physician and assistant professor of surgery in the department of emergency medicine at Massachusetts General Hospital, in Boston.

Most of the time, these blood clots, which can cause shortness of breath pain with breathing and increased heart rate, can be treated, but they can also be fatal. Researchers say that makes them a problem to take seriously.

“It’s the third most common cause of cardiovascular death in the country. It’s about as common as strokes,” says Kabrhel.

Though this study wasn’t able to definitively prove women in the study got blood clots because they sat so much, experts point out that sluggish blood flow, especially in the legs, is known to contribute to the problem. A piece of a clot in a deep vein, often in a leg, breaks off and travels to the lungs, causing a pulmonary embolism.

“Vein blood clots do occur in people who are not very mobile,” says James D. Douketis, MD, an associate professor in the school of medicine at McMaster University in Hamilton, Ontario, Canada.

“The extreme example is someone who’s had a stroke and is paralyzed. The risk of having a blood clot there is one in two,” says Douketis, who co-authored an editorial that accompanied the study but was not involved in the research.

In contrast, excessive sitting — in this case, for nearly six hours each day on average — increased the annual risk of having a pulmonary embolism from one or two out of every 1,000 adults to one or two in 500 adults.

Douketis points out that the risk from sitting appears to be slightly higher than the risk of blood clots associated with taking birth control pills and about half the risk of getting blood clots that a woman faces during pregnancy.

The study is published in BMJ.

“It’s a nice study,” says Jack E. Ansell, MD, chairman of medicine at Lenox Hill Hospital in New York City and an expert on thrombosis, or the formation of blood clots.

“There are always limitations to a retrospective review of this nature, but the findings look fairly definite,” Ansell tells WebMD. “It provides further, additional evidence that inactivity can be harmful.”

Sitting and the Risk of Pulmonary Embolism

For the study, researchers combed through data collected by the Nurses’ Health Study, a long-running investigation that’s collecting information about the health of thousands of registered female nurses in the U.S.

The study collects information about lifestyle habits as well as any newly diagnosed diseases every two years by questionnaire.

Nearly 70,000 women were included in the current research. Their average age in 1990 was 56.

Researchers split them into groups based on how much time they said they spent sitting down each day.

About 43% of women reported sitting less than 10 hours each week in their leisure time, 52% reported sitting between 11 and 40 hours each week, and 5% reported sitting for at least 41 hours each week.

Over 18 years, there were 268 cases of pulmonary embolism that didn’t have a known cause.

Even after controlling for variables known to be associated with the risk of lung blood clots, researchers found that women who were the most physically inactive had a rate of pulmonary embolism that was 2.34 times higher than women who reported sitting the least.

In a separate analysis that looked at how much exercise the women reported, researchers found no association between physical activity and the risk for pulmonary embolism, suggesting that it was the hours they spent sitting, rather than the hours they spent exercising that counted more — a phenomenon that’s been noted in other studies.

The study also showed that women who were more physically inactive had more heart disease and hypertension than those who reported being more active at home.

The study has some important limitations.

Because the Nurses’ Health Study only asked about sitting in a couple of the questionnaires, the researchers had to assume that the women in the study kept the same habits they reported in 1990 over the 18-year period of observation.

Douketis points out that personal habits like sitting aren’t steady exposures. People can decide to be more active, or they may become more inactive over time in response to worsening health.

And even though the study took into account a number of important variables, epidemiologists know that it’s impossible to take into account all the factors that may be at work.

For example, women who reported being most inactive in the study showed associations with heart disease even in the early years of the study, raising the question of whether physical inactivity was a trigger or a consequence of an already present disease.

Even with all those caveats, experts say it would be wise for people who find themselves sitting on the sofa, or really sitting in any setting for hours at a time — a car, a plane, a desk — to get moving.

“It’s the same thing that I would recommend for people who fly on airplanes where you’re in somewhat cramped conditions for long periods of time,” Ansell says. “Get up and move about, if you can. At least do some leg and foot exercises to keep the blood flowing.”