Introduction

The following article is part one of a two-part series article that was written by Harriet M. Ammann, Ph.D., D.A.B.T. She is a senior toxicologist for Washington State Department of Health, Office of Environmental Health Assessments.

Molds, a subset of the fungi, are ubiquitous on our planet. Fungi are found in every ecological niche, and are necessary for the recycling of organic building blocks that allow plants and animals to live. Included in the group "fungi" are 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 be able to grow.

Molds can grow on cloth, carpets, leather, wood, sheet rock, insulation (and on human foods) when moist conditions exist (Gravesen et al., 1999). Because molds grow in moist or wet indoor environments, it is possible for people to become exposed to molds and their products, either by direct contact on surfaces, or through the air, if mold spores, fragments, or mold products are aerosolized.

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. Molds have varying requirements for moisture, food, temperature and other environmental conditions for growth. Indoor spaces that are wet, and have organic materials that mold can use as a food source, can and do support mold growth. Mold 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 individual's exposure to mold parts or products, and the specific susceptibility of those exposed.

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

The most common response to mold exposure may be allergy. People who are atopic, that is, who are genetically capable of producing an allergic response, may develop symptoms of 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 with sufficient exposure.

Allergic reactions can range from mild, transitory responses, to severe, chronic illnesses. The Institute of Medicine (1993) estimates that one in five Americans suffers from allergic rhinitis, the single most common chronic disease experienced by humans. Additionally, about 14% of the population suffers from allergy-related sinusitis, while 10 to 12% of Americans have allergically-related asthma. About 9% experience allergic dermatitis. A very 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 whether this illness is solely allergic or has an infectious component. Molds are just one of several sources of indoor allergens, including house dust mites, cockroaches, effluvia from domestic pets (birds, rodents, dogs, cats) and microorganisms (including molds).

While there are thousands of different molds that 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 closely related mold species may cause allergy, yet that allergy may not be detected through challenge with the few purified mold allergens available for allergy 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 for atopic individuals.

Infection from molds that grow in indoor environments is not a common occurrence, except in certain susceptible populations, such as those with immune compromise from disease or drug treatment. A number of Aspergillus species that can grow indoors are known to be pathogens. Aspergillus fumigatus (A. fumigatus) is a weak pathogen that is thought to cause infections (called aspergilloses) 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. A. fumigatus is also fairly commonly implicated in ABPA and allergic fungal sinusitis. Aspergillus flavus has also been found as a source of nosocomial infections (Gravesen et al., 1994).

There are other fungi that cause systemic infections, such as Coccidioides, Histoplasma, and Blastomyces. These fungi grow in soil or may be carried by bats and birds, but do not generally grow in indoor environments. Their occurrence is linked to exposure to wind- borne or animal borne contamination.

A third group of possible health effects from fungal exposure derives from the volatile compounds (VOC) produced through fungal primary or secondary metabolism, and released into indoor air. Some of these volatile compounds are produced continually as the fungus consumes its energy source during primary metabolic processes. (Primary metabolic processes are those necessary to sustain an individual organism's life, including energy extraction from foods, and the syntheses of structural and functional molecules such as proteins, nucleic acids and lipids). Depending on available oxygen, fungi may engage in aerobic or anaerobic metabolism. They may produce alcohols or aldehydes and acidic molecules. Such compounds in low but sufficient aggregate concentration 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 released the highly toxic gas arsine from arsenic containing pigments (Gravesen, et al., 1994).

Fungi can also produce secondary metabolites as needed. These are not produced at all times since they require extra energy from the organism. Such secondary metabolites are the compounds that are frequently identified with typically "moldy" or "musty" smells associated with the presence of growing mold. However, compounds such as pinene and limonene that are used as solvents and cleaning agents can also have a fungal source. Depending on concentration, these compounds are considered to have a pleasant or "clean" odor by some people. Fungal volatile secondary metabolites 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 mucous membrane irritation, fungal volatile compounds may impact the "common chemical sense" which senses pungency and responds to it. This sense is primarily associated with the trigeminal nerve (and to a lesser extent the vagus nerve). This mixed (sensory and motor) nerve responds to pungency, not odor, by initiating avoidance reactions, including breath holding, discomfort, or paresthesias, or odd sensations, such as itching, burning, and skin crawling. 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 reactions in response to trigeminal nerve stimulation. Decreased attention, disorientation, diminished reflex time, dizziness and other effects can also result from such exposures (Otto et al., 1989)

It is difficult to determine whether the level of volatile compounds produced by fungi influence the total concentration of common VOCs found indoors to any great extent. A mold-contaminated building may have a significant contribution derived from its fungal contaminants that is added to those VOCs emitted by building materials, paints, plastics and cleaners. Miller and co-workers (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 or dizziness.

Odors produced by molds may also adversely affect some individuals. Ability to perceive odors and respond to them is highly variable among people. Some individuals can detect extremely low concentrations of volatile compounds, while others require high levels for perception. An analogy to music may give perspective to 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 certain odors including various perfumes, cigarette smoke, diesel exhaust or moldy odors. It is not know whether such responses are learned, or are time-dependent sensitization of portions of the brain, perhaps mediated through the olfactory sense (Bell, et al., 1993a; Bell et al., 1993b), or whether they serve a protective function. Asthmatics may respond to odors with symptoms.

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