The heavy use of dispersants in the gulf oil spill is unprecedented and troublesome, two top Obama administration officials told a Senate subcommittee on Thursday. But use of the dispersants has been sharply reduced in recent weeks.

Lisa P. Jackson, administrator of the Environmental Protection Agency, said the daily use of dispersants is down 72 percent since May 26, when her agency and the Coast Guard told BP to use dispersant on the gulf’s surface only as a last resort.

“Every single thing we have done out at sea comes at some risk,’’ Ms. Jackson told the Commerce, Justice, Science and Related Agencies subcommittee of the Appropriations Committee.

The subcommittee chairwoman, Senator Barbara Mikulski, Democrat of Maryland, said she wanted assurances that dispersants would not become the “Agent Orange of this oil spill,” with “vile consequences.” Agent Orange was a herbicide dropped by American forces during the Vietnam War to kill trees and plants so soldiers could spot enemy guerrillas; studies have found an increased incidence of certain cancers and other diseases among Americans and Vietnamese who were exposed to it.

“Dispersants have been used in the Gulf of Mexico for 15 years,” Ms. Jackson said. “But it’s the volume that any average person, whether they have a chemistry degree or not,” would be concerned about, she said.

Dispersants are supposed to break up the oil into smaller droplets that naturally occurring microbes can digest, Ms. Jackson said, and “the microbes are thriving.” Should the microbes overproliferate, they could use up the dissolved oxygen in the water, leading to mass fish kills, she said, but so far, levels of dissolved oxygen are acceptable.

And if the well is successfully capped, that would mean not only an end to oil flowing into the gulf but also an end to dispersant use, she said.

Larry Robinson, the assistant secretary of commerce for oceans and atmosphere, testified that dispersants had not been found in fish or shellfish samples. The dispersants are intended to persist long enough to do their job and then to break down.