Chemical dispersants, pumped into the water or sprayed on the surface of the Gulf slick, have been used to break up the oil. But late Wednesday BP said it would stop using dispersants underwater until tests can be done to make sure there are no major environmental consequences.
Dispersants have been used for years, but BP took the unprecedented step of pumping them directly down to the vicinity of the leaking wellhead, 5,000 feet below the surface of the water.
"When the dispersant and the oil mix, it just breaks the oil into smaller molecules, and the oil just breaks into smaller droplets," said Bob Fryar, a senior executive of BP. "At that time the ocean just takes over and it just degrades naturally."
Fryar, at BP in Houston, played down the risks, saying oil leaks into the Gulf of Mexico all the time.
"You know, actually, you typically have small oil seeps that come up from the ground already," he said. "There's probably one to two thousand barrels a day oil seep that's taking place in various places, and the earth just naturally takes care of that."
Biologists have been worried about other wildlife -- birds, fish and dolphins -- that could be in danger if the winds change and the oil spreads beyond the current slick area near the mouth of the Mississippi River.
Oil breaks down many animals' natural protection. A bird's feathers are naturally coated as insulation against the elements; a dolphin's eyes have an outer layer that shields them from toxins.
"An animal out there doesn't know that this is crude oil and it's toxic," said Moby Solangi, president of the Institute for Marine Mammal Studies in Gulfport, Miss. "All it knows is that this is something to play with."
Ayana Harry reported from Gulfport, Mississippi and Ned Potter wrote from New York. Additional reporting by Clayton Sandell in Houston.