NEW YORK (AFP) - The City of New York has launched a far-reaching study to monitor the physical and mental health of people who were close to the World Trade Center (WTC) when it was destroyed by terrorist attacks two years ago.

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The WTC Health Registry, Mayor Michael Bloomberg's office said, will be "a giant database, a comprehensive health survey on those most directly exposed to the events of September 11."

It said the study would focus primarily on the more than 200,000 people in the surrounding area when the twin towers collapsed in flames, dust and ashes after being hit by two hijacked passenger jets.

"Even if you have been healthy since 9/11, it's important for you to sign up," said a statement urging those concerned to participate in the 20-year project.

"Beyond the loss of life and physical destruction experienced..., the disasters full impact may not be known for years to come.

"Thats because it is unclear what kind of effect, if any, the smoke, dust, and debris from the WTC collapse may have on public health over the long term."

The statement said input from the study would be compiled and made available to the public via the Internet every four months.