Insulation was by direct expansion, air, cork and slag wool.

Without the slag wool marker, the agency argued, geographic sampling would be impossible.

But late last year the agency dropped the broad sampling plan because the slag wool marker was rejected.

Under the plan, 150 buildings would be checked for traces of slag wool to determine the extent of any future cleanup.

Rock and slag wool, also known as mineral wool or mineral fiber.

Rock and slag wool.

The slag can also be used to create fibers used as an insulation material named slag wool.

Rigid panel insulation is made from fibrous materials (fiberglass, rock and slag wool) or from plastic foam.

After closure the slag heaps were reprocessed to make road stone, and slag wool; reprocessing continued until at least the early 1930s.

If slag wool and other traces of toxic soot are identified, the government has said, it will offer to clean the site and possibly the entire building.

E. Timothy Oppelt, interim chairman of the panel, said the rejection of the slag wool marker put severe limitations on the agency.

But the Environmental Protection Agency may be ready to drop the slag wool proposal completely and turn to an alternative plan that it hinted at earlier this year.

Mineral wools, including rock and slag wools, are inorganic strands of mineral fibre bonded together using organic binders.

In contrast, the more commonly used vitreous fibre wools, including insulation glass wool, stone wool and slag wool, are considered "not classifiable as to carcinogenicity in humans" (Group 3).

That proposal is far more modest than the agency's current plan to test for slag wool in 150 of the 7,000 residential and commercial buildings in sections of Lower Manhattan and Brooklyn.

If these elements are found at dangerous levels, and the slag wool indicator also is detected, the federal government will thoroughly clean the contaminated section and, in some cases, the entire buildings and their central ventilation systems.

He favored using slag wool as a marker, and he declined to comment on the rejection until the full technical panel on which he sits has had a chance to look over the peer reviewers' report.

An independent peer review panel rejected the agency's proposal to use slag wool, a type of insulating material found at the trade center, as a clue to determine how far the dust had spread from ground zero.

The plan was also criticized because it raises the possibility that if asbestos or lead is discovered without slag wool present, the owners or occupants will be left to clean up the hazards without help from the government.

There was slag wool from the towers' insulation, along with fiberglass and asbestos, wood and glass, plastic and colored specks of paper, bits of cotton fiber and organic compounds too infinitesimal to record.

But now they have determined that microscopic traces of slag wool, a type of insulating material, along with tiny particles of gypsum and concrete, can be taken as reliable evidence that trade center dust had passed that way.

Last Friday, a panel of independent scientists from around the country rejected a proposal by the federal Environmental Protection Agency to use a single element in the dust - slag wool insulation from the towers - in tests to prove its presence.

"In that context, we're also reanalyzing samples of dust from our inventory, and if we're able to successfully demonstrate that slag wool qualifies as a marker for trade center dust, we'll consider making adjustments to our plan," Mr. Steinberg said.

As they test for slag wool, inspectors will also look for more dangerous traces of the trade center collapse - asbestos, lead, glass fibers and polycyclic aromatic hydrocarbons, which are toxic soot and other substances from the fires that burned for weeks afterward.

But he said the agency was re-examining a proposal that was rejected by independent scientists last year to use a specific substance - a type of building insulation called slag wool that was used in the towers - as a marker to indicate the presence of trade center dust.