Nine major companies have agreed to cut emissions of cancer-causing substances into the atmosphere at 40 chemical plants over the next 27 months, Environmental Protection Agency Administrator William K. Reilly announced yesterday.

The planned reductions, which would total 9.5 million pounds or 83 percent of emissions, are expected to significantly diminish the high cancer risks to populations downwind of the plants. But the voluntary reductions of chemicals that leak or are vented during the manufacturing process will have minimal impact on the flood of carcinogens dumped into the air nationwide -- 281 million pounds in 1988, the last year for which figures are available.

The EPA has authority to regulate emissions of so-called "hazardous" air pollutants, which can cause serious and irreparable harm in such tiny doses that Congress wanted them controlled at the industrial source of emission. But in the past 20 years, the agency has set standards for just eight of the hundreds of pollutants.

Seeking to cut through the red tape, Reilly set up a meeting in August 1989 with the nine corporate chiefs of 182 plants estimated to produce the highest cancer risks. He asked them to establish voluntary programs to cut emissions.

The agreement unveiled yesterday represents an "extraordinary voluntary effort, demonstrating that EPA and industry can work cooperatively," Reilly said.

But the steps pledged by the nine companies -- BASF Corp., Dow Chemical USA, Exxon Chemical Co., General Electric Plastics America, Goodyear Tire and Rubber Co., Occidental Chemical Corp., Reichhold Chemicals Inc., Texaco Chemical Co. and Texas Olefins Co. -- go no further than technological requirements now under final consideration by a House-Senate conference revising the Clean Air Act.

Both the House and Senate bills would require installation of the "maximum achievable control technology" in the next few years to curb plant emissions of 191 toxic air pollutants. The standard is expected to eliminate 99 percent of emissions of some chemicals.

Companies would be able to avoid the stringent standards if they voluntarily cut emissions 90 percent by 1992, a possible motive for yesterday's pact.

Reilly was unable to estimate to what extent the voluntary reductions would lower cancer risks, which, according to EPA theoretical calculations, run hundreds of times higher around the plants than is considered reasonable.

The Texaco plant at Port Neches, Tex., where odds of getting cancer from certain emissions over a lifetime are estimated to be 1 in 10, pledged to cut its carcinogenic emissions from 1.3 million pounds to 104,700 pounds by February.

Dow and General Electric took credit for pledges to reduce methylene chloride pollution by 6.3 million pounds. But the solvent has a limited future anyway because it depletes the stratospheric ozone layer and is targeted for phaseout.

Jeffrey Van, spokesman for the Chemical Manufacturers Association, said the agreements reflect an industry-wide "commitment to voluntarily reduce emissions."

But David Doniger, a toxics specialist with the Natural Resources Defense Council, said industry deserves little credit for volunteering to take steps soon to be required by law. "At most, it's a small down payment on the kinds of reductions we need to protect people," he said.