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When it comes to being couch potatoes, Americans aren’t alone. Physical inactivity has become a global pandemic, say researchers in a series of related papers published in the journal Lancet. According to one of the reports, lack of exercise causes as many as 1 in 10 premature deaths around the world each year — roughly as many as smoking.

About 5.3 million of the 57 million deaths worldwide in 2008 could be attributed to inactivity, the new report estimates, largely due to four major diseases: heart disease, Type 2 diabetes, breast cancer and colon cancer. The study finds that if physical inactivity could be reduced by just 10%, it could avert some 533,000 deaths a year; if reduced by 25%, 1.3 million deaths could be prevented. Say we got everyone off the couch and eliminated inactivity altogether: the life expectancy of the world’s population would rise by about 0.68 years (more, if you discount those who were already active), comparable to the effect of doing away with smoking or obesity.

For the study, led by I-Min Lee in the division of preventive medicine at Brigham and Women’s Hospital, scientists calculated something called a population attributable fraction (PAF), a measure of the contribution of risk factors like physical inactivity to diseases such as heart disease or diabetes, and even risk of death. The PAF told researchers how many cases of disease could theoretically be prevented if the risk factor were eliminated — that is, if all inactive people in a population were to start exercising sufficiently.

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Lee and his colleagues collected data on physical inactivity and outcomes of the four major diseases — heart disease, Type 2 diabetes, breast cancer and colon cancer — as well as rates for death from all causes. They then calculated PAFs for 123 countries. Overall, the estimates suggest that lack of exercise causes about 6% of heart disease, 7% of Type 2 diabetes, and 10% of breast and colon cancers worldwide.

Exercise has long been known to can lower risk factors like high blood pressure, high blood sugar and high cholesterol, which in turn reduces the risk of heart disease and diabetes. Physical activity also keeps heart vessels healthy and inhibits the formation of atherosclerotic plaques that can cause blood clots.

As for breast cancer, exercise may protect women by reducing fat — particularly dangerous belly fat, whose metabolic activity may trigger tumor growth in breast tissue. Colon cancer may work differently: researchers believe that exercise helps keep digestion regular and prevents potentially cancer-causing waste from encouraging abnormal growths in the colon.

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Current guidelines recommend that people get about 150 minutes of moderate exercise a week — a half-hour of brisk walking five times a week would do it. But in another Lancet paper published in the series, Pedro Hallal of the Federal University of Pelotas, in Brazil, and his colleagues found that 31% of adults worldwide (1.5 billion people) and 4 out of 5 teens aren’t exercising enough to meet that standard and therefore putting themselves at risk for chronic disease.

The researchers analyzed self-reports of exercise among adults in 122 countries, representing 89% of the world’s population, and among teens in 105 countries. Rates of physical inactivity were higher in high-income countries than in low-income nations. The Americas were overall the most sedentary region — with 43% of the population not exercising enough — while rates of inactivity were lowest in southeast Asia (17%).

One key reason is that we rely too much on modern conveniences like cars to get around. In the U.S., for example, fewer than 4% of people walk to work and fewer than 2% bike to commute; compare that to about 20% of people who walk to work in China, Germany and Sweden, and the more than 20% who bike their commutes in China, Denmark and the Netherlands, WebMD reports. Add to that the inordinate time most of us spend sitting — at the office, in front of the computer or watching TV.

Hallal estimates that sedentary people have a 20% to 30% greater risk of heart disease and diabetes than regular exercisers. But despite the deadly effects of lack of exercise, Hallal says physical activity doesn’t get the same attention or funding as other health risk factors. “It gets underfunded and undervalued,” Hallal told the Los Angeles Times. “But it’s huge everywhere in the world.”

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There was some encouraging news in the results as well: thanks to greater awareness about the importance of physical activity in improving health, about 31% of adults do report engaging in vigorous exercise three or more days a week.

Another paper in the Lancet series also examined what kinds of interventions might help people get active. Researchers analyzed 100 reviews of clinical and community-based efforts to encourage exercise and found some simple strategies that seemed to work: using signs to motivate people to use the stairs instead of the elevator, for instance, or offering free exercise classes in public places such as parks, especially geared toward women, lower-income folks and the elderly, groups who are less likely to get the recommended amount of exercise. Studies from the U.S., Australia, Belgium, Canada, England and Germany indicate that maintaining streets and improving lighting can boost activity levels by as much as 50%.

The authors of the study pointed to a particularly effective program called Ciclovía, which started in Bogotá, Columbia, and has spread to 100 other cities in the Americas. On Sunday mornings and public holidays, the program closes city streets to motorized vehicles, leaving roadways open for walkers, runners, skaters and bikers. Ciclovía attracts about a million people each week, the study notes, mostly people on lower incomes, and accounts for 14% of people’s weekly recommended exercise.

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Commenting on the Lancet series, many experts agreed that physical activity should be a global priority, though some took issue with the comparison with smoking. In an interview with WebMD, Timothy Armstrong, coordinator of the surveillance and population-based prevention program for the World Health Organization, noted that if the authors of the first paper had calculated the effects of smoking the same way they had for inactivity, the death statistics wouldn’t be quite so similar. Further, as Dr. Claire Knight of Cancer Research U.K. told the BBC, even if smoking and inactivity kill the same number of people, far fewer people smoke than are sedentary, making tobacco more risky to the individual.

Nevertheless, no one disagrees that the world population as a whole must start exercising more — and soon. “This is a super, super analysis,” Dr. James Levine, professor of medicine at the Mayo Clinic, told WebMD regarding Lee’s paper in the Lancet. “We know that as soon as somebody gets out of their chair, their blood sugar improves, their blood cholesterol and triglycerides improve, and that’s very consistent. Every time you get up it gets better. Every time you sit down it gets worse.”

The message, he says, is simple — get moving.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.