Could you give me the straight dope on “toxic” mold? I’m a home inspector and the science on this latest home hysteria seems a bit off. Nobody seems to know when/why/how the stuff turns up or turns “toxic.” I’d never heard of this stuff five years ago but now it seems like it’s everywhere.

–Kyle Corley, Palm Springs, California

I won’t say there’s nothing to this. For all we know toxic mold may turn out to be the worst threat to public health since yellow fever. But right now you’re not seeing toxic mold everywhere; you’re seeing publicity about toxic mold everywhere. That’s the inevitable consequence of big damage awards ($32 million to a family with a moldy mansion in Texas), big names (Erin Brockovich’s house in southern California is moldy too), and big-time media coverage (the New York Times Magazine ran a cover story on toxic mold last year). Just about every news outlet in the country has since chimed in. You can guess the reaction:

9 Insurance industry: Panic.

9 Real estate industry: More panic.

9 Trial lawyers and toxic remediation consultants: You know, this economy suddenly doesn’t look so bad.

Don’t get me wrong. Concern about toxic mold and other building-related health problems didn’t materialize overnight, and it’s not necessarily mass hysteria. On the contrary, it can be traced back to a real incident, the outbreak of Legionnaires’ disease at a Philadelphia hotel in 1976. Legionnaires’ disease is caused not by mold but by a specific bacterium, Legionella pneumophila, which had established itself in the hotel’s air-conditioning system; over 200 people contracted bacterial pneumonia, and 34 died. The outbreak planted the notion of pathogens lurking in walls in the public mind, and many instances of “sick building syndrome” have emerged since. Some of these have proven chimerical, but not all.

Two main factors are thought to contribute to building-related illness: widespread dependence on air-conditioning and the construction of tighter buildings in response to the energy crisis of the early 70s. Such closed environments can concentrate toxins or allergens and promote certain kinds of illness–asthma in children has risen sharply in recent decades, for example, and some say tight buildings are partly to blame.

Mold in particular got a closer look in 1994, when doctors noticed that a cluster of babies in Cleveland had developed bleeding in their lungs. Initial research suggested that the cause might be a toxin-producing mold called Stachybotrys chartarum. Like all molds it thrives in wet environments, and many of the lung-damaged babies lived in homes that had recently suffered major water damage.

Mold made headlines again a few years later. In 1998 the pipes in the Texas mansion belonging to Melinda Ballard and Ron Allison sprang a leak. Massive amounts of mold bloomed, but because most of the growth was inside the walls the couple didn’t realize what was going on for months. Meanwhile their son developed asthma, tremors, and learning problems, and Allison’s memory and ability to concentrate were so seriously impaired he lost his job. Ballard became convinced the problem was Stachybotrys and other molds. The couple sued, claiming their insurance company hadn’t moved fast enough, and in 2001 a jury awarded them $32 million. Since then mold-related insurance claims and lawsuits have multiplied, and builders and insurers are preparing for the worst.

So we’ve got a family with a frightening mold problem on the one hand and serious health problems on the other. It’s easy enough to believe Problem A caused Problem B. (According to the New York Times Magazine story, at one point a consultant walked into the Ballard home without a respirator; half an hour later he was puking his guts out, and he also suffered apparently permanent hearing loss. You’d need the soul of a tobacco lawyer to argue that such a thing was just coincidence.) But unlike civil juries or newspaper writers, scientists aren’t permitted to equate correlation with causation. They can’t be content merely to establish that A occurred, then B did; they have to find a precise mechanism of cause and effect. In the case of toxic mold this mechanism has yet to be determined. One recent review in a medical journal concludes, “The current public concern for adverse health effects from inhalation of Stachybotrys spores in water-damaged buildings is not supported by published reports in the medical literature.” Experts brought in to reexamine the baby cases in Cleveland say the original indictment of Stachybotrys was premature, and at the moment it’s fair to say that no scientific consensus on toxic mold has emerged. (Suggestion: Move a cage of guinea pigs into the Ballard house and see what happens.) With luck further research will clarify matters, but right now we’re not sure if we’re seeing the next asbestos or the next swine flu.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.