Types of Mold

There are approximately 400,000 types of mold, of which less than 100,000 have been named. Approximately 1,000
types of mold are found indoors across North America. Less than 80 molds are suspected of causing some form of
illness, and only a few of them are considered toxic.


Molds are organized into three groups according to human responses: Allergenic, Pathogenic and Toxigenic.

Allergenic Molds

Allergenic molds do not usually produce life-threatening health effects and are most likely to affect those who are
already allergic or asthmatic. The human system responses to allergenic molds tend to be relatively mild, depending on
individual sensitivities, typically producing scratchy throats, eye and nose irritations and rashes.

Pathogenic Molds

Pathogenic molds usually produce some type of infection. They can cause serious health effects in persons with
suppressed immune systems. Healthy people can usually resist infection by these organisms regardless of dose. In some
cases, high exposure may cause hypersensitivity pneumonitis (an acute response to exposure to an organism).

Toxigenic Molds

Mycotoxins can cause serious health effects in almost anybody. These agents have toxic effects ranging from short-term
irritation to immunosuppression and possibly cancer. Therefore, when toxigenic molds are found further evaluation is


The most common types of mold found indoors include:
• Aspergillus and its subspecies (A. flavus, A. versicolor)
• Cladosporium
• Penicillium
• Alternaria
• Stachybotrys atra (S. atra), also known as “Black Mold.”

Often, mold spores, whether dead or alive, cause adverse health effects, primarily of a respiratory nature, including hay
fever-like allergic symptoms.

Many of these molds, primarily S. atra, also produce chemical toxins known as “mycotoxins,” which are generated and
released into the air within the mold spores, leading to the “toxic mold” designation. Exposure to these toxins can occur
through inhalation, ingestion, or skin contact, and can result in symptoms including dermatitis, cough, rhinitis, nose
bleeds, cold and flu symptoms, headache, general malaise and fever.


Alternaria sp – Extremely widespread and ubiquitous. Outdoors it may be isolated from samples of soil, seeds, and
plants. It is commonly found in outdoor samples. It is often found in carpets, textiles, and on horizontal surfaces in
building interiors. Often found on window frames. The species Alternaria alternata is capable of producing tenuazonic
acid and other toxic metabolites which may be associated with disease in humans or animals. Alternaria produces large
spores having sizes between 20 – 200 microns in length and 7 – 18 microns in width, suggesting that the spores from this
fungi are deposited in the nose, mouth, and upper respiratory tract. It may be related to bakers asthma. It has been
associated with hypersensitivity pneumoniti, sinusitis, deratomycosis, onychomycosis, subcutaneous
phaeohyphomycosis, and invasive infection. Common cause of extrinsic asthma (immediate-type hypersensitivity: type
I). Acute symptoms include edema and bronchiospasms, chronic cases may develop pulmonary emphysema.

Aspergillus sp – A genus of fungi containing approximately 150 recognized species. Members of this genus have been
recovered from a variety of habitats, but are especially common as saprophytes on decaying vegetation, soils, stored
food, feed products in tropical and subtropical regions. Some species are parasitic on insects, plants and animals,
including man. Species within this genus have reported Aw’s (water activities) between 0.75 – 0.82. All of the species
contained in this genus should be considered allergenic. Various Aspergillus species are a common cause of extrinsic
asthma (immediate-type hypersensitivity: type I). Acute symptoms include edema and bronchiospasms. Chronic cases
may develop pulmonary emphysema. Members of this genus are reported to cause a variety of opportunistic infections
of the ears and eyes. Sever pulmonary infections may also occur. Many species produce mycotoxins which may be
associated with disease in humans and other animals. Toxin production is dependent on the species or a strain within a
species and on the food source for the fungus. Some of these toxins have been found to be carcinogenic in animal
species. Several toxins are considered potential human carcinogens.

Cladosporium sp – (Hormodendrum sp.) – Aw (water activity) in the range of 0.84 to 0.88. Most commonly identified
outdoor fungus. The outdoor numbers are reduced in the winter. The numbers are often high in the summer. Often
found indoors in numbers less than outdoor numbers. It is a common allergen. Indoor Cladosporium sp. may be
different than the species identified outdoors. It is commonly found on the surface of fiberglass duct liner in the interior
of supply ducts. A wide variety of plants are food sources for this fungus. It is found on dead plants, woody plants, food,
straw, soil, paint and textiles. It can cause mycosis. Produces greater than 10 antigens. Antigens in commercial extracts
are of variable quality and may degrade within weeks of preparation. Common cause of extrinsic asthma (immediatetype
hypersensitivity: type I). Acute symptoms include edema and bronchiospasms, chronic cases may develop
pulmonary emphysema.

Penicillium sp – Aw (water activity) 0.78 – 0.88. A wide number of organisms have placed in this genera. Identification to
species is difficult. Often found in aerosol samples. Commonly found in soil, food, cellulose, and grains (17, 5). It is also
found in paint and compost piles. It may cause hypersensitivity pneumonitis and allergic alveolitis in susceptible
individuals. It is reported to be allergenic (skin) (7, 17). It is commonly found in carpet, wallpaper, and in interior
fiberglass duct insulation (NC). Some species can produce mycotoxins. Common cause of extrinsic asthma (immediatetype
hypersensitivity: type I). Acute symptoms include edema and bronchiospasms, chronic cases may develop
pulmonary emphysema.

Stachybotrys sp – Aw (water activity) – 0.94, optimum Aw (water activity) – >0.98. Several strains of this fungus (S. atra, S.
chartarum and S. alternans are synonymous) may produce a trichothecene mycotoxin- Satratoxin H – which is poisonous
by inhalation. The toxins are present on the fungal spores. This is a slow growing fungus on media. It does not compete
well with other rapidly growing fungi. The dark colored fungi grows on building material with a high cellulose content
and a low nitrogen content. Areas with relative humidity above 55% and are subject to temperature fluctuations are
ideal for toxin production. Individuals with chronic exposure to the toxin produced by this fungus reported cold and flu
symptoms, sore throats, diarrhea, headaches, fatigue, dermatitis, intermittent local hair loss, and generalized malaise.
The toxins produced by this fungus will suppress the immune system affecting the lymphoid tissue and the bone
marrow. Animals injected with the toxin from this fungus exhibited the following symptoms: necrosis and hemorrhage
within the brain, thymus, spleen, intestine, lung, heart, lymph node, liver, and kidney. The mycotoxin is also reported to
be a liver and kidney carcinogen. Affects by absorption of the toxin in the human lung are known as pneumomycosis.
This organism is rarely found in outdoor samples. It is usually difficult to find in indoor air samples unless it is physically
disturbed. The spores are in a gelatinous mass. Appropriate media for the growth of this organism will have a high
cellulose content and a low nitrogen content. The spores will die readily after release. The dead spores are still allergenic
and toxigenic. Percutaneous absorption has caused mild symptoms.


Virtually everyone has some type of mold or another somewhere in their home. Although not all types are toxic, it is
sometimes difficult to distinguish types without lab testing. Black molds can develop from water seepage, and while
toxic mold is less common than other mold species, it is not rare. For that reason, it is imperative to treat and remove all
molds as if they are potentially harmful. Regardless of the type of mold found, a home containing mold is not essentially
a healthy home.