And that risk doubles when the weapons are guns, say researchers from the University of North Carolina at Chapel Hill.

The study compared 87 cases where employees were killed at work sites in North Carolina between 1994 and 1998 with 177 comparable work sites where there were no murders.

"We observed a small increase in the risk of homicide for workplaces that prohibited guns, but allowed other kinds of weapons," researcher Dana Loomis, a professor of epidemiology and member of the UNC Injury Prevention Research Center, said in a prepared statement.

"In contrast, there was a nearly sevenfold increase in the risk of a worker being killed in workplaces that allowed guns and other weapons," Loomis said.

"Our data suggest that, like residents of households with guns who are more likely to be victims of homicide, workers in places where the employer allows guns have a greater chance of being killed at work."

Loomis said the study results have a direct bearing on workplace safety policies. "In light of these findings, employers should question the risks and benefits of permitting firearms," he said.