Medical researchers are meeting this week in New Orleans to discuss the health effects of the ongoing oil spill in the Gulf of Mexico. But most of the discussion is about what isn't known.

The workshop was pulled together in a matter of days by the Institute of Medicine, a prestigious independent body chartered by Congress. Health and Human Services Secretary Kathleen Sebelius asked for the review.

Even though oil spills are fairly common, scientists at the two-day workshop say there's surprisingly little research on how they affect human health.

Since the 1960s, one researcher notes, there have been more than 30 major oil spills, nearly all of them involving shipwrecked tankers. But only about a quarter of them have been studied for toxic effects on humans. And the studies that have been done are often small and without comparison between groups of oil-exposed and unexposed people. In addition, none has so far looked at long-term consequences of exposure, such as cancer incidence.

At the end of a long day of questions and answers on what's known about the effects of oil spills on humans, activist John Hosey pretty much summed up the situation.

"The answers that people are getting are much like the oil coming out of the floor of the ocean," Hosey told the assembled scientists. "There's a lot of it coming up, but none of it's worth very much." Hosey is a clergyman with the Mississippi Interfaith Disaster Task Force.

Concern About Long-Term Problems

None of the distinguished researchers in the audience disagreed with his assessment.

That doesn't mean there's consensus on what evidence there is. Dr. Scott Barnhart of the University of Washington is on the side that doubts there's much reason to worry.

Photos: Oil Spill In The Gulf

It's true, Barnhart says, that cleanup workers exposed to crude oil often suffer acute effects — stinging eyes,