Researchers found that among the contractors studied, 25 percent met criteria for PTSD, 18 percent screened positive for depression and half reported alcohol misuse. Despite their troubles, relatively few get help either before or after deployment.

"Given the extensive use of contractors in conflict areas in recent years, these findings highlight a significant but often overlooked group of people struggling with the after-effects of working in a war zone," said Molly Dunigan, co-author of the study and a political scientist with RAND, a nonprofit research organization.

The results are from an anonymous online survey of 660 people who had deployed on contract to a theater of conflict at least once between early 2011 and early 2013. The study attracted participants through several methods, including contacting individual companies and trade associations and posting links to the survey on websites and blogs. It is the first survey to examine a broad range of deployed contractors, not just those who provide security services.

While the contractors surveyed suffered problems at higher rates than military members, researchers caution that such rates cannot easily be compared because of variations in measures and methodology. Among U.S. troops deployed to Iraq or Afghanistan, PTSD rates are estimated at 4 percent to 20 percent, depression rates are estimated at 5 percent to 37 percent and alcohol misuse is estimated at 5 percent to 39 percent.

"Because many of the contractors participating in the study -- 84 percent -- had previously served in the armed forces, it is difficult to gauge whether their work on contract is the source of these problems, or whether they stem from prior military experiences," Dunigan said. "However, the critical thing to take away from this study is that a significant number of people working on the ground in conflict environments are suffering from these problems, and many of their deployment-related health needs are not being addressed."

Researchers say more resources are needed to help contractors at all stages of their deployments.

"The military has programs before, during and after deployment to help service members address deployment-related mental health problems," said Carrie Farmer, co-author of the study and a RAND health policy researcher. "The majority of contractors we surveyed reported that they did not have access to similar resources."

Companies also may want to put more stringent pre-employment screening methods in place, but this might also inadvertently eliminate some potential hires with highly specialized skill sets.

"The critical thing to keep in mind is that mental health problems can be addressed. There are evidence-based treatments for these conditions," Farmer said.

Sixty-one percent of those responding to the RAND survey were U.S. citizens, 24 percent were from the United Kingdom and the rest were citizens of Australia, South Africa, New Zealand and other nations.

Contractors have deployed extensively around the world in the past decade, supporting U.S. and allied forces in Iraq and Afghanistan, as well as assisting foreign governments, nongovernmental organizations and private businesses. Duties of contractors have included base support and maintenance, logistical supports, transportation, intelligence, communications, construction and security services.

During the height of the conflicts in Iraq and Afghanistan, the number of private military and security contractors employed by the U.S. Department of Defense outnumbered U.S. troops in both theaters. There were 155,826 contractors alongside 152,275 U.S. troops in Iraq in 2008, and 94,413 contractors alongside 91,600 U.S. troops in Afghanistan in 2010.

Although contractors are not supposed to engage in offensive combat, researchers say they may be exposed to such stressors as gunfire, improvised explosive devices, serious injury, kidnapping, the deaths of fellow personnel and the psychological aftermath of killing.

The RAND study found differences between the U.S. and U.K. contractors surveyed, with the former reporting nearly twice the rate of PTSD and depression as U.K. contractors. Contractors from the U.K. also reported better preparation, lower levels of combat exposure and better living conditions than U.S. citizens, on average. However, respondents who were citizens of countries other than the U.S. and U.K. reported even better experiences in these categories.

"While we tried to control for combat experience, there may be unmeasured differences in what the two groups were exposed to, or there may be social or cultural differences in how they respond to combat," Farmer said.

Transportation contractors in particular reported the lowest levels of preparation, the highest combat exposure and the worst living conditions of all the job specialties surveyed. Logistics and maintenance contractors fared best in all three categories, while maritime security contractors fared well in terms of combat exposure and living conditions. Contractors who carried a weapon felt better prepared for deployment than those who did not.

A number of contractors also reported physical health problems as a result of deployment, including traumatic brain injuries, respiratory issues, back pain and hearing problems. Although most had health insurance, only 28 percent of those with probable PTSD and 34 percent of those screening positive for depression reported receiving mental health treatment in the previous 12 months.