FACTBOX: What are tar balls and what is their impact?

(Reuters) - Tar balls found on beaches in the Florida Keys this week are not from the Gulf of Mexico oil spill leaking from a well owned by BP, the U.S. Coast Guard said on Wednesday, citing laboratory tests.

But tar balls linked to the spill have been found elsewhere on the Louisiana and Alabama coastlines, raising concerns about the disaster's ecological impact.

Following are some facts about tar balls:

- They are the remnants of crude oil dumped into the ocean by marine vessels or, in this case, by a blown-out undersea well. They are "little, dark-colored pieces of oil that stick to our feet when we go to the beach," according to the National Oceanic and Atmospheric Administration.

- During the initial stages of a spill, the oil will spread into a thin slick, leaving it susceptible to tearing by wind and wave action. The smaller patches that result often disperse over a wide area and some of the crude mixes with water to form an emulsion that looks like chocolate pudding.

- This mix is thicker and stickier than the original oil in the spill, but it can still be torn by wind and waves. The smaller pieces it breaks into are tar balls.

_ They can be as big as pancakes but are mostly coin-sized, according to NOAA.

THREATS TO HUMAN HEALTH/WILDLIFE

- Tar balls "are very persistent in the marine environment and can travel hundreds of miles," NOAA said

- "For most people, an occasional brief contact with a small amount of oil, while not recommended, will do no harm. However, some people are especially sensitive to chemicals, including the hydrocarbons found in crude oil," it said. "They may have an allergic reaction or develop rashes."

- Sea turtles are known to eat tar balls. Dr Gilly Llewellyn, the Oceans Program Manager for WWF-Australia, a conservation group, said tar balls can "attract a curious or hungry turtle" with often fatal results.

- Tar balls can also heat up and ooze into the sand, fouling crucial nesting habitat for turtles, said John Hocevar, the Oceans Campaign Director for Greenpeace USA.

TAR BALL CLEAN UP

- "There is no magic trick to making tar balls disappear. Once tar balls hit the beaches, they may be picked up by hand or by beach-cleaning machinery. If the impact is severe, the top layer of sand containing the tar balls may be removed and replaced with clean sand," NOAA said.

(Sources: Reuters, NOAA, WWF, Greenpeace)

(Compiled by Ed Stoddard)