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.