Low back pain is one of the most common complaints among adults around the world. Researcher Sangwoo Tak from the University of Massachusetts at Lowell set out to look at how often it affects workers.

Tak analyzed surveys of more than 130,000 workers in 70 countries, conducted by the World Health Organization in 2002 and 2003. He says overall, almost a third of workers reported having low back pain in the month prior to the survey. He says many people complaining of pain do physical labor.

"We found, of course, farmers and fishery workers and also elementary workers, who are basically street cleaners, shoe shiners and garbage pickers," Tak said. "? In low-income countries, we see these sorts of unstable occupational groups."

Tak found some variation of rates of low back pain from one country to another. But he says what was interesting is that the numbers were lowest in low-income countries. Tak says there are many reasons.

For one, people in different cultures may react differently to pain and measure it differently. "You know, some people talk about pain when they really have severe pain, but some people don't really care," he said. "I mean, compatibility of pain scale is really a difficult issue in this kind of survey."

Differences in how countries collect data can also affect how frequently low back pain gets reported. Tak says he worries that the problem is actually under-reported in many countries and occupations.

"Military personnel could be high-risk group but we didn't find any elevated risk," he said, "because this is one time snapshot survey, we can't see the long-term effect of their occupational exposure. So it's probably military people if they have pain or injuries they wouldn't stay in the military".

Tak says when low back pain affects a worker's ability to stay employed, it becomes a serious public health problem. He says further research could help countries develop policies to better prevent workplace injuries. He presented his study at this month's American Public Health Association annual meeting in Boston, Massachusetts.