House and Senate conferees reached tentative agreement yesterday on a package of stringent controls intended to reduce industrial emissions of airborne toxins by 90 percent and force even tighter cutbacks of cancer-causing substances by early next century.

The agreement attempts to close a gaping hole in the Clean Air Act, which called for tough limits on such emissions in 1970. Such limits were enforced for relatively few pollutants, however, because of bureaucratic roadblocks and industry resistance. Many industries have been virtually unrestrained in their discharges of the most hazardous substances, exposing nearby communities to extraordinarily high risks of cancer.

Crafted by staff members and subject to ratification by conference members seeking to reconcile conflicting versions of a new clean air bill, the agreement would impose the first federal controls on coke oven emissions and municipal incinerators and sets forth the first guidelines to prevent accidents in chemical plants.

But in the face of heavy lobbying by the utility industry, conferees backed off controls of toxic emissions from power plants, which are primary sources of such dangerous metals as mercury and cadmium.

The agreement removes a major obstacle to completion of a clean air package by the time Congress adjourns next week. Considered less contentious than the acid rain proposals next up for negotiation, the air toxins pact still took a week to nail down and required the last-minute intervention of a dozen conference members at a session that broke up yesterday at 2:30 a.m.

"It's largely a guarantee that sometime in the beginning of the next century, people who are exposed to cancer from chemical pollutants will have the risk substantially reduced," said Sen. Dave Durenberger (R-Minn.), chief sponsor of the Senate provision.

The controls are aimed at a class of pollutants considered so hazardous that Congress in 1970 required that they be controlled at the source of emission. The agreement calls for controls on 189 pollutants known to cause cancer, birth defects, genetic damage, nervous system disorders and other serious diseases.

Although the worst polluters in the chemical, oil and steel industries have cut back voluntarily in recent years, the plants still expose downwind communities to risks of cancer hundreds of times higher than the government considers acceptable.

Under the conference plan, the industrial sources of 41 pollutants would have to install technological controls by 1995, with others to follow by the year 2003.

In cases where the risk of cancer remains high even after the best controls are implemented, the Environmental Protection Agency would be required to impose a tougher standard to protect the public health with an "ample margin of safety." Industry would have to limit its emissions or operations to meet that standard, which has roughly been interpreted to mean a lifetime risk of cancer of 1 in 10,000.

Most of the wrangling in the conference has not been over those abstract objectives but special regulations for certain powerful industries, such as steel, which uses giant coke ovens whose emissions expose people to risks of cancer as high as 1 in 55, according to the EPA.

Conferees agreed to give the industry until the year 2020 to meet the 1 in 10,000 "residual risk" standard, a generous extension that reflects the power of lawmakers from steel states. But to obtain the waiver, steel companies have to install increasingly stringent technology in 1998 and 2010 to reduce leaks from their oven doors.

Despite steel industry warnings that even weaker provisions jeopardized U.S. coking operations, John Stinson, a steel industry lobbyist, said that the industry "can live with this." He predicted costs of $6 billion to upgrade and replace coke ovens.