July 17, 2012 -- Not moving enough may be as hazardous to your health as smoking, a new study shows.

The study, which is published in The Lancet, estimates as many as 5.3 million deaths around the world were caused by physical inactivity in 2008.

By comparison, researchers point out that cigarette smoking is estimated to cause about 5 million deaths worldwide each year.

"Physical inactivity has a large impact on the health of the world. In fact, its impact is comparable to that of cigarette smoking," says researcher I-Min Lee, ScD, an epidemiologist at the Harvard School of Public Health, in a news conference.

Experts who were not involved in the research questioned that claim, however.

Timothy Armstrong, PhD, coordinator of the surveillance and population-based prevention program for the World Health Organization (WHO) in Geneva, Switzerland, points out that his organization estimates that inactivity causes fewer deaths -- about 3.2 million deaths around the world each year.

Armstrong says he thinks the Lancet researchers compared numbers that were arrived at in two different ways. "If the same methodology had been applied to smoking, I suspect we would not see the similar estimates for mortality," Armstrong says in an email to WebMD.

"That is not to say that physical activity or physical inactivity is not a major risk factor" for chronic diseases, he says. "It is. WHO currently ranks it fourth after high blood pressure, tobacco use, and high cholesterol."

The study further estimates that 6% of heart disease, 7% of type 2 diabetes, and about 10% of colon and breast cancers, are linked to lack of activity.