The Environmental Protection Agency yesterday ordered an emergency ban on major uses of two of the nation's most common weed killers.

The ban affects a heribicide known as 2, 4, 5-T, a significant component of "agent orange," the chemical used to defoliate jungles in Vietnam. The ban also includes silvex (know as 2, 4, 5-TP), a chemical used in 275 lawn products.

EPA officials said they ordered the ban after a new study, completed several days ago, showed a "singnificantly higher" rate of miscarriages in Alsea, Ore., soon after the national forests there had been sprayed with 2, 4, 5-T.

Scientists have known for years that the substance causes birth defects and tumors in animals. "But now we have the human evidence," said Steven Jellinek, assistant administrator of the EPA. "We have dead fetuses."

The ban covers the use of 2, 4, 5-T on U.S. forests, pastureland and powerline rights-of-way. But it does not include its application on cattle grazing land or rice fields, EPA officials said, because those uses do not present an imminent health hazard.

Both 2, 4, 5-T and silvex contain minute amounts of tetra-dioxin (TCDD), one of the world's mot deadly chemicals. A single drop, if it could be divided equally among 1,000 people, would kill them all.

Yesterday's action comes after eight years of legal battles and scientific debate involving the federal government, enviromental groups and Dow Chemical Co., a principal manufacturer of 2, 4, 5-T.

Court suits have been filed recently by Vietnam veterans who claim they are getting cancer from the "agent orange" spraying.

Dow, which sells the herbicides to dozens of other chemical companies for repackaging, said it had not reviewed the Oregon data. A Dow vice president, Etcyl H. Blair, however, said the company would fight the ban in court.

Calling the action "a political move," Blair said, "This is an example of government at its worst.... The bulk of scientific data gathered over three decades of use demonstrate there has never been a single documented incident of human injury resulting from normal agricultural use of these products....

"Government agencies in Australia and New Zealand recently completed extensive studies into allegations of human reproductive problems caused by 2, 4, 5-T and approved its continued use," Blair added.

However, EPA officials said yesterday that since the chemical was concocted at Fort Detrick, Md., during World War II, more than 40 studies have shown that 2, 4, 5-T and tetra-dioxin cause birth defects and tumors in animals.

But measuring exposure to humans proved more difficlut.Unlike other such chemicals, dioxin could not be found in human tissue, in the water or, for the most part, in food, because equipment was not sensitive enough to measure it.

While many doctors believe that it was responsible for the dramatic increase in birth defects among the Vietnamese, a rural Oregon school teacher brought to light the first definitive U.S. evidence.

Bonnie Hill of Alsea Ore. noticed last year that her miscarriage and those of several friends occurred shortly after the U.S. Forest Service sprayed nearby forests with weed killer. She wrote the EPA, which followed up with a study of local hospital records for the past six years. Statistically significant miscarriage increases were discovered in the months following the spray season.

The agency last April began the lengthy process that could lead to eliminating the herbicide. But agency officials considered the new evidence important enough to invoke, for the first time, EPA's emergency suspension powers. Hearings will continue on whether to make the ban permanent.

The EPA estimated that "four million people across the nation are at risk through its use in forestry, rights-of-way and pasture." An undetermined number may also be exposed through grazing land and rice-field applications, which prompted the Environmental Defense Fund to sharply criticize EPA for limiting the ban.

An emergency ban was ordered, the EPA said, because spraying season starts in March and 7 million pounds were about to be used on 1.2 million acres of forests, a million acres of pastures -- many surrounding farmhouses and drinking water streams -- and 600,000 acres of rights-of-way -- some straight through suburban towns.

The timber and paper industries, as well as western cattlemen, have been actively fighting a 2, 4, 5-T ban for years.

The Agriculture Department in 1970 prohibited its use in home gardens, recreation areas and on all food crops except rice. The agency tried to ban it on rice, but a court suit by Dow resulted in an injunction against any further action.