A study by the University of California, Irvine linked psychological stress responses to the attacks to a 53 per cent increase in heart problems, such as high blood pressure and stroke in a three-years period after Sept 11, 2001.

The study, published in the January edition of the 'Archives of General Psychiatry', is the first of its kind to demonstrate the long-term effect of the 9/11 attacks on cardiac health.

Most participants in the survey watched the attacks on live television, while one-third had no personal connection to them.

Most subjects had no pre-existing heart problems and the results persisted even when risk factors such as smoking and obesity were taken into account.

"It seems that the 9/11 attacks were so potent that media exposure helped to convey enough stress that people responded in a way that contributed to their cardiovascular problems," Alison Holman, an assistant professor of nursing science at UCI and the study's lead researcher said.

The three-year study involved 1,500 adults surveyed at random whose health information had been recorded before the attacks.

Researchers quizzed participants about their stress responses in the weeks after the attacks and issued annual follow-up questions ending in late 2004.