How Dangerous Is Indoor Mold?

Outdoors, mold is everywhere. Filamentous fungi are essential to the breakdown of much of the detritus of the natural world. But mold does not restrict itself to living under the open sky. Give fungal spores--abundant in both indoor and outdoor air--a warm, wet space (no light required) and a carbon food source, and mold will grow inside four walls as easily as it does on leaves and fallen trees.

Indoor mold is, in fact, perceived by many to be a growing problem in the U.S., Canada, and Europe. Media coverage, litigation, and health concerns relating to indoor mold have dramatically risen over the past decade. Whether that upswing reflects an increase in the presence of indoor mold is difficult to say because nobody is tracking the incidence of indoor mold.

Nor is anyone tracking human exposure to mold in indoor environments on a national level, says Stephen Redd, chief of the Air Pollution & Respiratory Health Branch of the National Center for Environmental Health at the Centers for Disease Control & Prevention (CDC). Possible health effects are what drive his and CDC's interest in indoor mold. It is also primarily why other governmental agencies, such as the Environmental Protection Agency in the U.S. and those in other countries, are interested in this issue: They would like to know to what extent mold in indoor spaces is harmful to the health of people inhabiting those spaces.

At least to some degree, mold can be a health hazard. First, airborne mold spores or fragments can induce allergic symptoms in some people. Those symptoms range from an irritated nose and a cough to an exacerbated asthma attack, Redd says. Second, in rare instances, immunocompromised individuals, including cancer patients and those with HIV, can contract a mold infection, usually in a hospital. The mold invades the patient's lung tissue and proliferates in either the lung or the bloodstream. And third, Redd says, there is an open question as to whether the toxins that some mold spores produce have a negative health effect when inhaled indoors.

Molds themselves are not toxic. However, many molds do, under various circumstances, generate mycotoxins. "Mycotoxin is a big catch-all word," says Bruce B. Jarvis, professor of chemistry at the University of Maryland. "All mycotoxins are fungal metabolites that pose a health risk for humans and animals." Fungi use the toxins as a defense mechanism against other species while competing for space and food. Whether a fungus produces a mycotoxin depends on environmental triggers, including drying or loss of nutrients.

THE BEST KNOWN mycotoxins may be the carcinogenic aflatoxins, which are infamous in agriculture for causing liver failure and even death when humans and animals eat moldy corn or peanuts. No indoor mold has been clearly documented to induce such a drastic response from airborne exposure. Yet molds common to the indoor environment are capable of making hundreds of biologically potent compounds, some of which may play a larger or smaller role in causing human health effects than we now realize.

The most common fungal genera growing indoors include Penicillium, Aspergillus, and when very wet conditions prevail, Stachybotrys, Chaetomium, and some other genera. All of these fungi produce toxic metabolites of varying potencies. Fungi common in outdoor air are also found indoors, and these include Cladosporium, Alternaria, Aureobasidium, and Fusarium. These are associated with hay fever and asthma.

Establishing whether indoor mold growth causes significant adverse health effects is not an easy task. "The challenge," Redd says, "is separating what is really known from what is possible."

CDC has recruited the help of the Institute of Medicine--specifically, an IOM panel on damp indoor spaces and health--to prepare a comprehensive, objective review of what is known about the health effects of exposure to indoor mold. The panel's report is due this spring. Aino Nevalainen, who sits on the IOM panel and works in the Department of Environmental Health at the Public Health Institute in Finland, says: "People have experienced such great problems in characterizing the exposure that is possibly causing the health effects that many of them have given up. We know that mold growth is associated with harmful effects, but we don't know exactly what is so harmful in the mold growth. Even with asthma, there is a little uncertainty how well the documentation actually guarantees that there is a link."

Ronald E. Gots, chief executive officer of the International Center for Toxicology & Medicine, a firm that consults on public health litigation, emphasizes the uncertainty about whether mold causes severe adverse health effects. "There is no good scientific evidence today" that mycotoxic diseases arise from indoor environmental exposures to mold, he says. Furthermore, "it makes very little sense that one could actually get mycotoxicosis," or poisoned by mycotoxins from inhaling indoor levels of spores. "It's not enough of a dose."

True, but mycotoxins can still cause effects that are less severe than acute poisoning, says J. David Miller, Natural Sciences & Engineering Research Council Industrial Research Chair and professor of biochemistry at Carleton University in Ontario.

"If I take a drop of sulfuric acid and put it in a gallon of water, I could dump it on you, and it wouldn't make any difference. But if I were to put that drop of acid right on your skin, it would burn a hole. The way to think about the toxin issue is that when you inhale a particle that contains a toxic material--let's say the amount in one spore--the concentrations in that spore are often in the millimolar range, quite high in terms of concentration. So when that one spore lands on one alveolar macrophage, it is going to significantly damage that cell, kill it maybe. Can you afford to lose one cell? Yes." But once a number of cells are damaged, Miller says, "it would start to matter biologically."

"We have a lot of tolerance," he adds. "But it's always about how much. The fact is, we don't really know where the line gets crossed now. If it is true that mold and dampness is associated with exacerbating asthma and increasing upper respiratory disease, which I believe that it is, then there's the critical question: How much mold does that take?"

Many researchers agree with Miller that there is a growing body of evidence showing that indoor mold does cause illness. Miller believes chemists need to be involved in this work because their research on mycotoxin and mold spore metabolism will lead to establishing biomarkers of human exposure to mold. With a biomarker for mold exposure, health officials will be able to measure how much mold exposure is too much and establish standards that can be used in both legislation and litigation.