Indoor mold contamination--a threat to health?

The Fungus Among Us

Molds, a subset of the fungi, are ubiquitous on our planet. Fungi, which are found in every ecological niche and are necessary for the recycling of organic building blocks that allow plants and animals to live, include yeasts, molds, and mildews, as well as large mushrooms, puffballs, and bracket fungi that grow on dead trees. Fungi need external organic food sources and water to grow.

Subset: Molds

Molds can grow on cloth, carpets, leather, wood, Sheetrock, insulation, and human foods when moist conditions exist (Gravesen, Nielsen, Iversen, & Nielsen, 1999). To grow, they have varying requirements for moisture, food, temperature, and other environmental conditions. Many molds reproduce by making spores, which, if they land on a moist food source, can germinate and begin producing a branching network of cells called hyphae. Spores or fragments that become airborne can expose people indoors through inhalation or skin contact.

Molds can have an impact on human health, depending on the nature of the species involved, the metabolic products being produced by these species, the amount and duration of exposure, and the susceptibility of the individuals exposed.

Health effects generally fall into four categories: allergy, infection, irritation (mucous membrane and sensory), and toxicity.

Allergy

The most common response to mold exposure may be allergy. People who are atopic--that is, people who are genetically capable of producing an allergic response--may develop symptoms of an allergy when their respiratory system or skin is exposed to mold or mold products to which they have become sensitized. Sensitization can occur in atopic individuals who have had sufficient exposure.

Allergic reactions can range from mild, transitory responses to severe, chronic illnesses. The Institute of Medicine has estimated that one in five Americans suffer from allergic rhinitis, the single most common chronic disease experienced by humans. In addition, about 14 percent of Americans suffer from allergy-related sinusitis, while 10 to 12 percent have allergy-related asthma. About nine percent experience allergic dermatitis. A much smaller number, less than one percent, suffer serious chronic allergic diseases such as allergic bronchopulmonary aspergillosis (ABPA) and hypersensitivity pneumonitis (Institute of Medicine, 1993).

Allergic fungal sinusitis is a not-uncommon illness among atopic individuals residing or working in moldy environments. There is some question as to whether this illness is solely allergic or has an infectious component. Molds are just one of several sources of indoor allergens, including dust mites, cockroaches, effluvia from domestic pets (birds, rodents, dogs, cats), and other microorganisms.

While thousands of different molds can contaminate indoor air, purified allergens have been recovered from only a few of them. This means that atopic individuals may be exposed to molds found indoors and develop sensitization, yet not be identified as having mold allergy. Allergy tests performed by physicians involve challenge of an individual's immune system by specific mold allergens. Since the reaction is highly specific, it is possible that, even when some closely related mold species has caused the allergy, the allergy may not be detected through challenge with the few purified mold allergens available for tests. Thus, a positive mold-allergy test indicates sensitization to an antigen contained in the test allergen (and perhaps to other fungal allergens), while a negative test does not rule out mold allergy.

Infection

Infection from molds that grow in indoor environments is not a common occurrence, except in certain susceptible populations, such as those with immune systems compromised by disease or drug treatment. A number of Aspergillus species that can grow indoors are known to be pathogens. Aspergillus fumigatus is a weak pathogen that is thought to cause infections (called aspergilloses), but only in susceptible individuals. It is known to be a source of nosocomial infections, especially among immune-compromised patients. Such infections can affect the skin, the eyes, the lung, or other organs and systems. In addition, A. fumigatus is fairly commonly implicated in ABPA and allergic fungal sinusitis. Aspergillus flavus also has been found to be a source of nosocomial infections (Gravesen, Frisvad, & Samson, 1994).

Mucous-Membrane and Trigeminal-Nerve Irritation

A third group of possible health effects from fungal exposure derives from the volatile compounds (VOCs) produced through fungal primary or secondary metabolism and released into indoor air. Primary metabolic processes are those necessary to sustain an individual organism's life, including energy extraction from foods, and the synthesis of structural and functional molecules such as proteins, nucleic acids, and lipids. Some VOCs are produced continually during primary metabolic processes as a fungus consumes its energy source. Depending on available oxygen, fungi may engage in aerobic or anaerobic metabolism. They may produce alcohols or aldehydes and acidic molecules. In low but sufficient aggregate concentration, such compounds can irritate the mucous membranes of the eyes and respiratory system.

Just as occurs with human food consumption, the nature of the food source on which a fungus grows may result in particularly pungent or unpleasant primary metabolic products. Certain fungi can release highly toxic gases from the substrate on which they grow. For instance, one fungus growing on wallpaper has been found to release the highly toxic gas arsine from arsenic-containing pigments (Gravesen et al., 1994).

Fungi can also produce secondary metabolites as needed. These metabolites are not produced at all times since they require extra energy from the organism. They are the compounds that are frequently identified with typically moldy or musty smells associated with the presence of growing mold. Compounds such as pinene and limonene that are used as solvents and cleaning agents also can have a fungal source. Depending on concentration, these compounds are considered to have a pleasant or "clean" odor by some people. Volatile secondary metabolites may also impart flavors and odors to food. Some of these, as in certain cheeses, are deemed desirable, while others may be associated with food spoilage. There is little information about the advantage that the production of volatile secondary metabolites imparts to the fungal organism. The production of some compounds is closely related to sporulation of the organism. "Off" tastes may be of selective advantage to the survival of the fungus, if not to the consumer.

In addition to irritating mucous membranes, fungal VOCs may affect the "common chemical sense" that senses pungency and responds to it. This sense is primarily associated with the trigeminal nerve (and to a lesser extent the vagus nerve). It responds to pungency, not odor, by initiating avoidance reactions, including breath holding, discomfort, paresthesias, or odd sensations such as itching, burning, and crawling skin. Changes in sensation, swelling of mucous membranes, constriction of respiratory smooth muscle, or dilation of surface blood vessels may be part of fight-or-flight responses to trigeminal-nerve stimulation. Decreased attention, disorientation, diminished reflex time, dizziness, and other effects can also result from such exposures (Otto, Molhave, Rose, Hudnell, & House, 1989).

It is difficult to determine whether the level of VOCs produced by fungi influences the total concentration of common VOCs found indoors to any great extent. In a mold-contaminated building, fungal contaminants may make a significant contribution to VOCs emitted by building materials, paints, plastics, and cleaners. Miller and coworkers (1988) measured a total VOC concentration approaching the levels at which Otto et al., (1989) found trigeminal-nerve effects.

At higher exposure levels, VOCs from any source are mucous-membrane irritants and can have an effect on the central nervous system, producing such symptoms as headache, attention deficit, inability to concentrate, and dizziness.

Adverse Reactions to Odor

Odors produced by molds may adversely affect some people. The ability to perceive odors and respond to them is highly variable. Some people can detect extremely low concentrations of VOCs, while others require high levels for perception. An analogy to music may give perspective on odor response. What is beautiful music to one individual is unbearable noise to another. Some people derive enjoyment from odors of all kinds. Others may respond with headache, nasal stuffiness, nausea, or even vomiting to odors including various perfumes, cigarette smoke, diesel exhaust, and moldy smells. It is not known whether such responses are learned, are a time-dependent sensitization of portions of the brain, perhaps mediated through the olfactory sense (Bell, Schwartz, Petersen, & Amend 1993; Bell, Schwartz, Petersen, Amend, & Stini, 1993), or serve a protective function. Asthmatics may respond to odors with symptoms.

<http://www.doh.wa.gov/ehp/oehas/mold.html>.

References

Bell, I.R., Schwartz, G.E., Petersen, J.M., & Amend, D. (1993). Self-reported illness from chemical odors in young adults without clinical symptoms or occupational exposures. Archives of Environmental Health, 48, 6-13.

Bell, I.R., Schwartz, G.E., Petersen, J.M., Amend, D., & Stini, W.A. (1993). Possible time-dependent sensitization to xenobiotics: Self-reported illness from chemical odors, foods, and opiate drugs in an older adult population. Archives of Environmental Health, 48, 315-327.

Gravesen, S., Frisvad, J.C., & Samson, R.A. (1994). Descriptions of some common fungi. In S. Gravesen, J.C. Frisvad, & R.A. Samson (Eds.), Microfungi (p. 141). Copenhagen: Munksgaard.

Gravesen, S., Nielsen, P.A.; Iversen, R., Nielsen, K.F. (1999). Microfungal contamination of damp buildings--examples of constructions and risk materials. Environmental Health Perspectives, 107(Suppl. 3), 505-508

Institute of Medicine, Committee on Health Effects and Indoor Allergens, Division of Health Promotion and Disease Prevention (1993). Indoor Allergens: Assessing and Controlling Adverse Health Effects. (A.M. Pope, R. Patterson, & H.A. Burge, Eds.). Washington, D.C.: National Academy Press.

Miller, J.D., LaFlamme, A.M., Sobol, Y., LaFontaine, P., & Greenhalgh, R. (1988). Fungi and fungal products in some Canadian homes. International Biodeterioration, 24, 103-120.

Otto, D., Molhave, L., Rose, G., Hudnell, H.K., & House, D. (1989). Neurobehavioral and sensory effects of controlled exposure to a complex mixture of volatile organic compounds. Neurotoxicology and Teratology 12, 649-652.

Harriet Ammann, Ph.D., D.A.B.T.

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