In the first three years after the terrorist attacks, researchers found, reports of doctor-diagnosed illness climbed by 18 percent in a nationally representative sample of adults. The jump was highest in those with pre-existing health conditions, but people who were healthy before 9/11 also experienced an increase in physician-diagnosed ailments. Not surprisingly, use of healthcare services rose as well.

The study sheds light on the lingering effect of "collective traumas," such as natural disasters, presidential assassinations and terrorist attacks. In the case of 9/11, researchers said, watching events unfold on live TV appears to have contributed to the development of stress-related illness.

"We cannot underestimate the impact of collective stress on health," said E. Alison Holman, UCI assistant professor of nursing science and a health psychologist. "People who work in health professions need to recognize symptoms related to stress and need to consider the potential effect of indirect exposure to extreme stress."

The study, co-authored by Holman and Roxane Cohen Silver, UCI professor of psychology & social behavior, appears online in the journal Social Science & Medicine. It involved almost 2,000 adults who completed Internet surveys in the days, months and years after 9/11.

Participants disclosed whether a physician had diagnosed them with any of 35 illnesses, such as heart disease or diabetes, and the number of times they had seen a doctor in the past year for each disorder. Sixty-three percent had viewed the 9/11 attacks live on TV, and 4.5 percent had been directly exposed to them.

"Those who watched the attacks live on TV -- as opposed to those who learned about them only after they happened -- experienced a 28 percent rise in physical ailments over the following three years," Holman said.

The percentage of the sample with at least one physician-diagnosed disorder over that period climbed from 79.2 percent to 89.5 percent, according to the study.

Respondents also visited their medical providers more often, and this was associated with higher rates of cardiovascular, endocrine, gastrointestinal and hematology-oncology illnesses in the three years after 9/11. Further analyses suggest that the increased incidence of disorders and service use was above and beyond that expected from an aging sample.

"Large-scale collective traumas such as 9/11 often set in motion a series of events, such as personal loss, economic hardship and fears about the future," Holman said. "Under these circumstances, stress can take its toll in the form of illness, even among people who were nowhere near the World Trade Center towers on Sept. 11."

The study, funded by the National Science Foundation, builds upon previous research by Silver and Holman into 9/11 stress response. In a paper published in 2008, they linked it to a 53 percent jump in cardiovascular ailments over the next three years.

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