A new study published in the Journal of Occupational and Environmental Medicine finds that among U.S. employees who worked more than 12 hours per day or more than 60 hours per week, the rate of injury and illness was roughly 30–40 percent higher than among those working fewer hours.

Working overtime was associated with a still higher rate of injury, about 60–80 percent greater than among people who did not work overtime.

This research raises an interesting question regarding current employment trends and safety, according to an epidemiologist commenting on the study in JOEM.

In the United States, the average number of hours worked by all employed people and the average number of overtime hours for manufacturing workers have been on the increase since the 1970s. American workers—and many others around the world—have been working longer as global competition has intensified, explains Dr. Dana Loomis of the University of North Carolina.

But the overall rates of occupational injury and illness in the U.S. workforce have been declining with time.

One possible explanation for the apparent conflict between national trends and the findings is that longer hours may only result in greater risk for a subset of workers — perhaps those with greater potential exposure to the agent of injury, says Loomis.

Studies investigating the effects of extended work hours by occupation and industry might lead to insights about who is at risk when working hours increase, he says.