Hurricane Katrina doubled the rate of serious mental illness in areas ravaged by the storm but the urge to commit suicide fell, partly because survivors bonded with each other, according to a study led by Harvard Medical School.

Billed as the biggest mental health study after Katrina killed about 1,500 people along the Gulf Coast, the survey showed that 15 per cent of 1,043 survivors were diagnosed with a serious mental illness five to eight months after the storm.

That figure suggests about 200,000 people from Alabama, Louisiana and Mississippi face serious mental illness because of Katrina, with about a third suffering from post-traumatic stress syndrome and the remainder depression, said Ronald Kessler, the study's lead researcher.

Almost 85 per cent of the survivors faced a major financial loss, including losing their home, and more than a third endured extreme physical adversity after Katrina struck a year ago and flooded 80 per cent of New Orleans, the survey showed.

Almost 23 per cent encountered extreme psychological adversity.

About 25 per cent reported having nightmares about their experiences - a figure that rises to almost 50 per cent for people who lived in New Orleans.

But Kessler, a professor of health care policy at Harvard Medical School, said the number of people reporting a desire to kill themselves declined, in part because many survivors had forged stronger ties with loved-ones and their community.

"We found an extraordinarily high proportion of our sample who said that despite the understandable sadness with all they lost and the understandable anxieties about the future ... that they felt closer to their loved-ones, they felt connected to the community in a way they didn't before," he said.

"They felt much more religious, they felt that they had a purpose in their life and a meaning," he said, noting that 88.5 per cent of the survivors in the survey said Katrina had helped them develop a deeper sense of meaning or purpose in life.

"Those are the people where these suicidal tendencies decreased," he said.

The study, led by the Harvard Medical School and funded by the National Institutes of Mental Health, was published in the World Health Organisation's "Bulletin" newsletter.

The researchers compared their survey with a snapshot of mental health taken in the same geographic area by the federal government in 2001-03.

The researchers plan to interview the same 1,043 survivors over seven years to track their recovery.