An analysis by the city Health Department of the World Trade Center Health Registry shows that 12 percent likely had PTSD when they were interviewed in 2003 and 2004 and that PTSD varied significantly by occupation.

The Health Department found that more than 21 percent of the unaffiliated volunteers not working with an organization such as the American Red Cross have PTSD -- or volunteer workers from construction, engineering and sanitation -- suffered from PTSD, compared to 12 percent of firefighters and 6 percent of police officers.

The study authors suggest that the non-emergency personnel had most likely had no disaster preparedness training or experience with previous emergencies and that firefighters, who lost many more colleagues as a result of the attacks on the WTC may have had to suffer from grief compounded by trauma.

The findings were published in the American Journal of Psychiatry.