Spring is in the air, when an outdoors enthusiast's thoughts turn to boat bottoms and how to protect them.

Picking an antifouling bottom paint never was fun, or even cheap, but at least it used to be easy -- just grab a gallon of Baltimore Red, a thick, coppery goo guaranteed to keep the slime and barnacles away for a summer or so.

Technology overtook the business recently with development of slick, self-polishing antifouling paints using organic tin as the toxin. Industry sources said these tributyltin-based paints held the promise of someday making bottom-sanding and bottom-painting an every-five-year nightmare instead of an annual or semiannual one, which was good news to mariners indeed.

The bad news is that Congress and the Environmental Protection Agency are worried TBTs are so poisonous they could imperil oysters, mussels and clams, and maybe other creatures as well.

TBTs are thought to be 40 to 1,000 times more toxic than copper, according to Dr. Robert Huggett of the Virginia Institute of Marine Sciences. He said the toxin has killed shellfish larvae at levels so low they weren't even measurable until recently.

Huggett said University of Maryland researchers found TBTs to be toxic to clam larvae at 16 parts per trillion. Meantime, researchers last summer were finding TBTs in hundreds of parts per trillion in water around marinas.

EPA reported TBTs in amounts "shown to adversely affect fish and shellfish" in several locations. Tests were conducted in Annapolis, Norfolk, San Diego, Lake Superior and Lake Ontario, the agency reported. The worst reading was at a San Diego marina -- 900 parts per trillion -- according to Pam Vlier of the EPA's Pesticide Program.

Much of the current research began after the Navy announced a plan last summer to paint all its 550 ships with organic tin-based bottom paints in an effort to cut down fuel use and dry dock time.

The Navy maintained in an environmental report the switch wouldn't create ecological hazards, but Virginia scientists, worried about huge oyster seed beds near the Norfolk Navy yard, took issue.

By December, Republican Sens. Paul Trible of Virginia and William Cohen of seafood-rich Maine were pushing legislation through Congress halting the Navy changeover until EPA could complete a special review of the dangers of TBTs, which may take years.

Just how much TBT already is in use is hard to pin down. Some reports indicate TBT-based bottom paint is on up to 90 percent of recreational boats, but industry sources said most bottom paints on the market have only a trace of TBT, about 1 percent, in a mostly copper base.

TBTs, however, are the main toxins in high-tech, racing sailboat bottom paints such as International's Micron and Woolsey's Miracol, though local boatyard operators say the call for these paints has not yet been widespread. TBTs also are the main toxin in paints made especially for aluminum boats, aluminum and copper being incompatible.

EPA reported that 250,000 to 300,000 pounds of TBTs are used annually in bottom paints, about a third of the total U.S. production of the pesticide. The rest goes in wood preservatives, disinfectants and industrial biocides.

But industry sources say that because of its superior efficiency and longer life, TBT is the bottom-paint toxin of the future if its continued use is approved by EPA. "I've been looking at antifouling paints for 30 years," said one industry scientist, "and I never saw anything come close in longevity."

The scientific reason is that tin-based compounds can be chemically bonded to the paint so they are released slowly and regularly. In copper bottom paints, by contrast, there is a big release of toxins when the boat goes in the water, then the remaining toxins slowly leach out with decreasing effectiveness.

In a press release, EPA reported no scientific evidence that TBTs can harm humans who eat seafood.

But Chuck Fox of the Environmental Policy Institute wondered, "What about the guy who applies the stuff?"

Since I'm that guy, I wonder, too.

"I hauled my boat two summers ago and wound up in dry dock next to a guy who worked at EPA," said Rob Mairs, a racing sailboat skipper from Edgewater, Md. "He wore one of these chemical-spill outfits to sand his boat, and he told me terrible things about tin-based paints.

"That was it for me. I took the stuff off with a paint remover. I didn't even want to sand it. I wouldn't go near it now with a 10-foot pole."