The agency classifies it as a possible carcinogen based on rat exposure studies. It also has listed bifenthrin among a group of pesticides to be tested for their potential to act as "endocrine disruptors," which may affect humans or wildlife, even at low doses.

Health officials point out that the pesticide is currently registered by the EPA for use in controlling ticks. Participants are advised not to walk in the sprayed area for 24 hours and to keep pets away. Since the chemical is highly toxic to aquatic life, they've ruled out testing it on any yard that's within 100 feet of water.

And they say the study, as well as the information given to volunteers, was approved by both state and federal agency institutional review boards, which are set up to safeguard people participating in research.

"I won't say they're not putting themselves at any risk, because I never say that about anything," Mitchell said. But he added, "I believe the risks have been adequately conveyed."

Dr. Robert S. Lawrence, director of the Johns Hopkins University's Center for a Livable Future, disagreed after reviewing the packet of information supplied to prospective volunteers. Lawrence said the fact-sheet should have spelled out more clearly that, while nothing's been proven, questions have been raised about the long-term safety and potential impact on humans of bifenthrin.

Preliminary results for all three states from the first year of the study indicate that the yards treated with pesticide had 62 percent fewer ticks overall than the "control" yards sprayed with water, according to officials.

But the people in the treated households reported finding just as many ticks on their bodies as the residents of untreated properties, and there were basically the same number of Lyme disease cases reported in both groups.

Federal and state health officials say that if after a second year and more analysis there's no difference in tick bites or infection among the two groups, then they'll advise the public that spraying yards with pesticide really doesn't help prevent Lyme disease. Mitchell said he, for one, sees a certain irony in that, given the criticism the study has received from pesticide opponents.

But critics say such an outcome — discouraging more pesticide use — would not justify the risks to which they believe the study has exposed human subjects.

"We have no idea if we've caused more harm than good," Carella said.

An earlier version misstated the goals of the Maryland Pesticide Network. The Sun regrets the error.

Preventing Lyme disease:

•Apply tick repellants to skin or clothing

•After being outdoors, check your body for ticks

•Take a shower promptly to wash off any insects that haven't yet attached themselves

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