As many as 50 million Americans are living sedentary lives, putting them at increased risk of health problems and even early death, a leading expert in exercise science told the American Psychological Association (APA).

Speaking at APA's 117th Annual Convention, Steven Blair, PED, called Americans' physical inactivity "the biggest public health problem of the 21st century."

Blair is a professor of exercise science and epidemiology at the University of South Carolina's Arnold School of Public Health. He is one of the world's premier experts on exercise and its health benefits and was the senior scientific editor of the 1996 U.S. Surgeon General's Report on Physical Activity and Health.

Research has shown approximately 25 percent to 35 percent of American adults are inactive, Blair said, meaning that they have sedentary jobs, no regular physical activity program and are generally inactive around the house or yard. "This amounts to 40 million to 50 million people exposed to the hazard of inactivity," Blair said in an interview. "Given that these individuals are doubling their risk of developing numerous health conditions compared with those who are even moderately active and fit, we're looking at a major public health problem."

Blair's extensive research comes primarily from the Aerobics Center Longitudinal Study (ACLS), in which he found that fitness level was a significant predictor of mortality. The ongoing study began in 1970 and includes more than 80,000 patients. The researchers periodically measured the participants' body composition and body mass index, and each patient underwent a stress test. Researchers also looked at numerous other factors including the participants' medical histories.

One follow-up study of 40,842 longitudinal study participants showed poor fitness level accounted for about 16 percent of all deaths in both men and women. The percentage was calculated by estimating the number of deaths that would have been avoided if people had spent 30 minutes a day walking. This percentage was significantly higher than when other risk factors were considered, including obesity, smoking, high cholesterol and diabetes. The ACLS also found that moderately fit men lived six years longer than unfit men.

More examination of 14,811 female patients in the ACLS showed that women who were very fit were 55 percent less likely to die from breast cancer than women who were not in good shape. This was after the researchers had controlled for BMI, smoking, family history of breast cancer and other possible risk factors.

Blair also highlighted the benefits of exercise on the mind, referring to recent emerging evidence that activity delays the mind's decline and is good for brain health overall. Blair said he thinks psychologists can be integral in helping patients understand the health hazards of being inactive and encouraging people to look for more ways to get moving. "Over the past few decades, we have largely engineered the need for physical activity out of the daily lives of most people inindustrialized societies," said Blair.

The message should be simple, he said: Doing something is better than doing nothing, and doing more is better than doing less, at least up to a point. "We need numerous changes to promote more physical activity for all, including public policies, changes in the health care system, promoting activity in educational settings and worksites, and social and physical environmental changes. We need more communities where people feel comfortable walking. I believe psychologists can help develop better lifestyle change interventions to help people be more active via the Internet and other technological methods."

For more information on the American Psychological Association, visit www.apa.org.