"I wouldn't want to live in a place where the separation (between the exhaust vent and supplemental air intake) is only 3 feet," Penney said in a recent interview. "Ten feet would be a minimal distance, as far as I'm concerned."

Penney, who has not reviewed the particulars of the HUD homes, is one of several professionals who have echoed Rodgers' and Bishop's general concerns, including former state Sen. Parley Hellewell of Orem, who owns a plumbing and heating company. But other experts are more cautious.

The odds of carbon monoxide poisoning because of the venting configuration are "incredibly low," notes Gordon Walker, director of the state division of Housing and Community Development. "But that doesn't mean it couldn't happen" under a "perfect storm" of weather conditions, he adds.

Rod Larson, industrial hygiene director of the Rocky Mountain Center for Occupational and Environmental Health in Salt Lake City, says that if he were building a house, he would want 10 feet between the exhaust vent and the air intake. But, he adds, the two men are making assumptions that aren't true.

Yes, Rodgers' Web site includes a video of exhaust being pushed, by a strong breeze, over to the air intake. But, Larson argues, "they're making assumptions that 100 percent of the exhaust is being picked up by the air intake. That's definitely never going to happen." Dilution with the ambient air would reduce the carbon monoxide in the exhaust to 1/100 of its original amount by the time it entered the house, he calculates.

You need to show some proof, Larson told the two crusaders; you need to provide carboxyhemoglobin tests of sick people, for example, and third-party evaluations showing how much carbon monoxide is actually being pulled into the homes. Otherwise, he says, "you're going to scare people," with no proof to back up the claims.

That's the response, too, from the Manufactured Housing Institute's vice president for regulatory affairs. "There has to be some documentation," Jeff Inks argues. "No evidence is being brought forward."

Rodgers and Bishop are frustrated by this Catch-22. They can't provide poisoning data because hospitals do not routinely test for carbon-monoxide poisoning, and nobody wants to spend money to test the homes. Larson guesses it would take at least $10,000 to do a proper third-party evaluation. "I'm operating on $750 retirement (income)," sighs Rodgers.