For a time, the dust seemed to be everywhere, from the insides of downtown apartments to the very air that New Yorkers breathed. It is suspected of causing respiratory problems and may have long-term effects on health.

Most of the dust was swept up long ago, but small amounts of it doubtless remain, tucked in nooks and corners. Federal environmental officials had planned to test living and working places in Lower Manhattan and Brooklyn this year to make sure they are not still contaminated.

But the project has been stalled in part because even though the dust was seen, smelled or inhaled by millions of New Yorkers four years ago, there is no consensus about how to identify it today.