An experimental nontoxic spray that neutralizes the poison in poison ivy, oak and sumac is being recommended for widespread use among U.S. foresters this summer and may be available to the public by the beginning of next year's season, perhaps earlier.

The spray, discovered at least partly by accident, binds to -- or envelops -- the molecule in the poison ivy sap which, on contact with the skin, normally produces the characteristic itchy, blistering and weepy allergic reaction.

Fifty volunteers from the U.S. Forestry Service tried out the spray last summer in forests throughout the country. (In order to participate in the program, the volunteers had to have a moderate to severe sensitivity to the plants and work in jobs that required them to be around the plants.)

According to Art Jukkala, a forester with the agency's Missoula, Mont., Equipment Development Center, "the feedback has been so positive we are encouraging a lot of our field workers to use it this summer.

"Everything suggests it will be effective," he said, "but you never know when you get something like this out in the real world."

The spray offered apparently complete protection for some of the volunteers and seemed to reduce the severity of the reaction in those who did develop blisters.

If "Ivy Block," as the product is tentatively being called, is as effective as it now appears, it will mark the first genuine breakthrough in control of poison ivy, oak and sumac since Capt. John Smith described the ivy in his journal in the early 17th century. Poison oak is found mostly along the West Coast and sumac in uninhabited swampy areas. Poison ivy is ubiquitous in the eastern and midwestern states and Canada.

About half the American population will react to contact with urushiol, the chemical found in the sap of the poisonous plants, with only about 15 percent considered genuinely immune. Some 25 million Americans are so sensitive to it that a bout with the ivy-induced contact dermatitis "is one of the few outbreaks that can be considered a dermatologic emergency," said Dr. William L. Epstein, an authority on the problem.

Epstein leads a research project at the University of California at San Francisco School of Medicine on the prevention, treatment and cure of poison reactions. Solutions have remained stubbornly elusive.

Epstein's group evaluated Ivy Block at the request of the Forestry Service, and although they do not yet have enough evidence to publish results, they "do believe it is effective," said Epstein's assistant, Dr. Kimie Fukuyama.

About two years ago, Epstein and other urushiol researchers determined that there were a number of substances that would bind to the urushiol molecule in a test tube. 'Ivy Block' offered apparently complete protection for some volunteers and seemed to reduce the severity of the reaction in those who did develop blisters. These included aluminum silicates, which are found in spray-on antiperspirants, and egg white. Researchers found that spraying antiperspirants on clothing had some protective effect. If the poison touched the antiperspirant before it touched the skin, the reaction could be prevented or reduced in severity. It was during these tests that it was discovered that the most effective and long-lasting anti-ivy agent was not the active ingredient in the antiperspirant, but the inactive filler.

The active ingredient of "Ivy Block" is the inactive filler in antiperspirants. The filler is made by United Catalyst of Louisville, Ky.

"We are not a consumer company," said United Catalyst spokesman and chemist Tony Schwarz, "and we're still in a patent situation, so essentially we want to keep the components secret. We are, however, negotiating with an international consumer company to handle the over-the-counter business."

The company's products are used to regulate the flow of such other products as paints, inks, cosmetics and virtually anything that is used in an aerosol spray.

The components of Ivy Block are so neutral, says Schwarz, that they can be sprayed on the skin as well as clothing, and on pets who are likely to wander into poison ivy patches and bring home the heavy oil on their fur. The protective effect appears to last about 24 hours. (The antiperspirant lasted only about four hours.)

Native Americans, it is said, used to eat poison ivy leaves for protection against the toxic oils.

All that got them, say the experts, is poison ivy where it went in and poison ivy where it came out. (Inexplicably, though, not in between.)

In any case, eating poison ivy leaves is definitely not a good idea, and although there have been some oral agents purporting to protect against the ivy reaction only one was ever considered even partially effective. "Most dermatologists recommend against it," said Epstein, although it has been helpful in occasional special situations.

Epstein is still working on immunotherapy, possibly by modifying the urushiol molecule, and has developed treatment protocols -- very large doses of steroids -- which can abort attacks in extremely sensitive people if they get to the physician in time.

While it seems like everyone is complaining that they are having more poison ivy than ever, there's no evidence that this years crop is any worse than usual.

"I think the poison ivy crop is about the same as last year," says C. Benjamin Coffman, agronomist at the Weed Science Laboratory of the Beltsville Agricultural Research Center. "But poison ivy has a good rooting system and even in dry weather it will go find water when lots of other things won't."

Poison ivy has its good points, says Coffman. For instance, it is "a really good ground cover, better than kudzu."

During World War II, poison ivy was planted in Holland to protect the dikes. Although native to North America, it is now found all over Europe, at least in small quantities. "It is," says Coffman, "a good survivor, and may take the form of a vine, a small shrub, a plant or whatever is best in that particular environment."

Myths abound about how you get poison ivy, oak and sumac, and how it does or does not spread. Here are some facts:Pet fur is a splendid way to spread the sticky urushiol-carrying resin. The animals don't seem to be sensitive to the stuff themselves, so until a protective and harmless spray is actually available, be wary, and maybe a tad less affectionate.Just brushing against the leaves of the plant might not expose you. The sap is carried in the veins of the leaves, the stems and the roots. If the leaf is not damaged, you won't be affected. But, warns an article in a recent issue of the FDA Consumer, the magazine of the Food and Drug Administration, "what may look like an intact plant may not be, for insects chewing on the plants can cause breaks in the surface, releasing the sap."

Here are some of the myths:You can catch poison ivy from someone else's rash. No way. Both the plant and the sometimes weepy, blistery rash it can produce are popularly called poison ivy which causes part of the confusion. The poison ivy rash is an allergic reaction to the chemical in the plant. The liquid in the oozy blisters is a byproduct of the body's immune-system attempt to immobilize the toxic invader.You can spread the rash on yourself by bathing. Again, no way. Bathing immediately after exposure -- preferably within five minutes -- can wash the oil off and prevent the rash. Any soap will do -- not just the popularly touted yellow laundry soap.

The way poison ivy does spread on an individual is from the point of exposure -- the hands, say, while the oil is still present -- to the face, to brush a fly off your nose or for any reason to touch your face or neck or scratch your back.

Indeed, keeping the outbreak clean, weepy blisters or no, is important, because secondary infections are common and sometimes dangerous. Any outbreak on the face or genitals, or that covers 20 percent of the body, ought to be seen by a dermatologist. Practically nobody has ever written about poison ivy and oak without including the couplet, "Leaflets three; Let it be." This is not without cause. Clusters of three leaves, on vine, shrub, plant, bush are probably poison ivy or oak, although most plant experts now believe that what passes for poison oak in the East is merely a variety of ivy. It hardly matters.

What does matter is getting it out of your yard. Spray products must be used carefully, because they may damage other plants. Use them on days with no breeze. Do not try to dig out poison ivy unless you wear disposable gloves and the soil is soft and wet. If the roots break, they will sprout again, bigger and better.

And never, never burn poison ivy or oak. The heavy poisonous oil is carried in the smoke and can cause outbreaks not only on skin, but also in nasal passages and along the respiratory tract.