Ms. Schonbrun, 52, has multiple chemical sensitivity syndrome, which she says leaves her vulnerable to whiffs of substances that her system can no longer tolerate. She said that she was sensitive to ingredients in herbicides as well as pesticides, and that exposure to the chemicals could put her body into anaphylactic shock.

“It’s an illness that you have to depend on the kindness of neighbors and friends and hope for the best,” she said.

But Ms. Schonbrun said she had found that was not enough and turned to lobbying the City Council, the mayor, the neighborhood mediator and anyone else she thought might be able to help.

Her goal was to get neighbors either to refrain from using such chemicals or to notify her before applying them.

Her condition and her campaign have left local officials and neighbors grappling with just how much responsibility they all have in coping with one woman’s ailment.

“It’s a pretty complicated situation,” said Diggs Brown, a Fort Collins councilman who has met with Ms. Schonbrun. “How do you balance the rights of one neighbor who is using legal chemicals on their lawn on private property and somebody who apparently has a chemical sensitivity?”

Just the mention of chemical sensitivity can be controversial, as there remains widespread doubt in the general medical community about whether it is a real diagnosis. Even the term “multiple chemical sensitivity syndrome” is polarizing in the medical and environmental health communities. Ms. Schonbrun said that she had been classified as “disabled with chemical injury” and that she received disability checks.

“The bottom line is that the condition is very much in dispute,” said Dr. Edward B. Holmes, director of the Occupational and Environmental Health Clinic at the University of Utah. “There seems to be, in my experience, a significant number of people that have this kind of conglomeration of symptoms that fit into a pattern strongly with psychiatric conditions.”

Dr. Holmes said that in rare cases someone might have a true allergy to a specific chemical.

On the other side of the debate are specialists like Dr. Claudia S. Miller, professor of environmental medicine at the University of Texas Health Science Center at San Antonio, who said practitioners were hearing about more cases of chemical sensitivities.

“There is no reliable prevalence data because the illness presents in such varied ways,” Dr. Miller said in an e-mail message. “But estimates of the number of persons who report having multiple intolerances that they recognize and that cause them to rearrange their lives ranges from 3 to 6 percent of the population.”

Dr. Miller said chemical sensitivities or intolerances could be caused by a specific exposure to a toxic substance or by a number of exposures. There is no known cure.

Often, people with severe chemical sensitivities remove themselves from urban areas completely, but Ms. Schonbrun said she could not face such isolation and did not want to be too far from her family or health care facilities.

After her diagnosis six years ago, Ms. Schonbrun left her job as a nurse in San Diego and moved with her husband, Bob, to Tucson, where they stayed for a year. When she became even sicker, the couple moved to Fort Collins, a college town of about 130,000 with strong agrarian roots, to build a “safe” house with features that include an elaborate venting system and a tar-free roof to allow her to live as free of offending chemicals as possible.

Ms. Schonbrun has added her name to the Colorado Department of Agriculture’s registry of pesticide-sensitive people so she will be notified and can stay indoors when there is any commercial spraying.

Some people question the Schonbruns’ choice to relocate to their tidy and rapidly expanding subdivision with farms and ranches close by.

“With a condition like that, they choose to move into a residential area,” said Curt Richards, who lives across the street from the Schonbruns and said that the dispute had escalated to the point that he had obtained a restraining order against Mr. Schonbrun. “The bottom line is, we’re not breaking any laws. We have modified how we take care of our property that requires more of my time and money, but that’s not good enough.”

Ms. Schonbrun said she had twice had to call for an ambulance when her neighbors used herbicides she could not tolerate.

The Schonbruns bought the first batch of nontoxic herbicides for some neighbors, including Mike Cada, who lives next door.

“I’m willing to do it as long as it’s effective,” Mr. Cada said, noting that the nontoxic mixture cost at least 40 percent more than traditional herbicides.

No local ordinances require private citizens to notify neighbors when they apply pesticides. According to the National Conference of State Legislatures, six states have laws that address overspraying and improper use of pesticides around humans(especially children); two others are considering such laws.