There are more than 200,000 cases of pulmonary embolism, which usually begins as a blood clot in the leg that travels to the lung, in the U.S. each year, according to the National Library of Medicine. It can permanently damage lung tissue, other organs, or cause death, but many people who have it have no symptoms.
Pulmonary embolism is less common in Japan than in Western countries, said study coauthor Dr. Hiroyasu Iso, professor of public health at Osaka University Graduate School of Medicine, but Japanese people are becoming increasingly sedentary.
"We were surprised about the strength of the effect of television watching compared with the effects of advancing age, history of hypertension and diabetes mellitus, or body mass index in this study," Iso told Reuters Health by email. "We speculated that leg immobility during television watching had increased their risk of fatal pulmonary embolism."
In a number of studies of long haul travelers, the association between prolonged sitting and increased risk of pulmonary embolism did not vary by ethnicity, Iso said.
Other past studies have looked back at the lifestyle factors common to pulmonary embolism cases, but none had followed people over time to see if there was a link between their TV-watching time and their risk for embolisms, the study team writes in Circulation.
Between 1988 and 1990 Iso and colleagues asked more than 85,000 adults 40 to 79 years old in Japan how many hours they spent watching TV, then followed them for the next 19 years looking for deaths from pulmonary embolism. They also collected information on obesity, diabetes, cigarette smoking and high blood pressure, and tried to rule these factors out in the relationship between TV and blood clots.
Only 59 people in the sample died of pulmonary embolism, but compared to people who watched two and a half hours of TV or less per day, those who watched five or more hours were 2.5 times as likely to die of a clot.
Researchers calculated that among people who watched less than two and a half hours of TV, the rate of deaths from pulmonary embolism were 2.8 per 100,000 people per year, compared to a rate of 8.2 deaths per 100,000 per year for those who watched five or more hours daily.
Risk of pulmonary embolism death increased by 40 percent for each additional two hours of daily TV watching, they found.
"Time spent watching TV is a pretty reliable way to measure how much time people spend sedentary, or inactive," said Dr. Christopher Kabrhel, an Associate Professor of Emergency Medicine at Harvard Medical School in Boston who was not part of the new study. "If being sedentary puts you at risk for pulmonary embolism, and I believe it does, then it likely also puts you at risk of death from pulmonary embolism, as this study showed."
After TV watching time, obesity was the next most important factor predicting risk of death from pulmonary embolism, the authors found.
Since U.S. adults watch more TV than Japanese adults, the results may be even more important to Americans, the authors said in a statement accompanying the study.
"Nowadays, with online video streaming, the term 'binge-watching' to describe viewing multiple episodes of television programs in one sitting has become popular," lead author Dr. Toru Shirakawa, a research fellow in public health at Osaka University Graduate School of Medicine, wrote.
Travelers on long plane flights and people watching TV for long periods of time can stand up, stretch, walk around, or tense and relax their leg muscles for five minutes to reduce the risk of blood clots, they wrote.
"The results do not seem to be country-specific," Kabrhel told Reuters Health by email. "Being sedentary is bad for you wherever you live."