NEW YORK (AP) — Congress closed out 2010 by passing a law that will provide nearly $2.8 billion in cash payments to people sickened by toxic dust at the World Trade Center, but lawmakers left a big question unanswered in the legislation.

They didn’t say which illnesses would be covered.

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Only a few diseases were singled out by name in the act, including asthma and a handful of other respiratory ailments. Federal administrators still have to decide whether to cover other conditions, like cancer.

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Doctors said deciding which diseases are linked to ground zero and which occurred naturally could be tough.

Tens of thousands of people were involved in the trade center cleanup. Many are now middle aged, when illnesses of all types become more common.

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