Classification Guide to Common Types of Mold


APRIL, 2017

What is Mold?

Molds are simple, microscopic organisms that can grow virtually anywhere, both inside buildings and outdoors. Mold colonies can grow inside damp or wet building structures. And mold spores are a common component of household and workplace dust.

Health effects from exposure to mold can vary greatly depending on the person and the amount and type of mold present.

Regardless of the type of mold, it should be treated as a potential health hazard and should be removed from homes and workplaces.

Common types of mold found in homes and businesses include:


Acremonium is a widespread mold currently believed to contain about 100 species. Most species of this mold exist as saprophytes, being isolated from dead plant material and soil. Some species are parasites of plants and animals capable of causing serious infections. A few species are widely used for producing pharmaceuticals.

Health Significance:
Acremonium rarely cause disease in humans. However, infection with Acremonium has been described in immunocompromised patients. It can cause fungal maxillary sinusitis. In medical literature, it has been reported as the cause of pulmonary infections and infections of the cornea and nails in individuals with weak immune systems. There are three main species of Acremonium associated with human infections: Acremonium falciforme, A. kiliense, and A. recifei. The 3 species are biosafety level 2 fungi. Some species have been reported to be allergenic while some are known to produce mycotoxins.


The mold Alternaria is a well recognized allergy causing fungus.  Alternaria spores can be detected from spring through late fall in most temperate areas, and can reach levels of thousands of spores per cubic meter of air. Alternaria spores can be at their highest concentrations during dry, windy conditions that are ideal for the spores to become airborne. Alternaria is currently comprised of about 40-50 species. It is commonly isolated from plants, soil, food, and indoor air. One of the species, Alternaria alternata, has been isolated from numerous kinds of organic materials in damp situations, including textiles, stored food, canvas, cardboard and paper, electric cables, polyurethane, jet fuel, sewage and effluents.

Health Significance:
A. alternata is recognized as an important allergen with airborne spores and mycelial fragments being responsible for the allergic symptoms in individuals with rhinitis or bronchial asthma. Alternaria sensitivity can also lead to severe and potentially fatal asthma. Studies have shown that up to 70 % of mold-allergic patients have skin test reactivity to Alternaria. It has also been shown that prolonged heavy exposure to A. alternata spores and mycelial fragments mimics that of other allergens such as cat dander and dust mites. It has also been recorded as an opportunistic pathogen causing skin diseases particularly in immunocompromised patients such as the bone marrow transplant patients.



The mold Aspergillus has close to 200 species and varieties. Aspergillus is widely distributed from the arctic region to the tropics. Aspergillus species are frequently found in air and soil. As concerns indoor air quality the most important species are Aspergillus fumigatus, Aspergillus flavus, Aspergillus clavatus, Aspergillus niger, Aspergillus versicolor. It is generally believed that the amount of airborne spores of Aspergillus spp. in indoor air is higher than outdoors at any given time. In the home, the amount of spores in the air is significantly increased when cleaning is carried out mechanically, for example, when carpets are vacuum cleaned. Some studies on prevalence of Aspergillus species in indoor environment documented A. fumigatus in kitchens and bathrooms, A. versicolor and A. repens in mattresses and carpets, A. versicolor and A. fumigatus in basements, A. fumigatus, A. niger and A. flavus in flower pot soil, A. versicolor and A. fumigatus in various pad materials.

Health Significance:
As concerns health, the most important species of Aspergillus are Aspergillus clavatus, A. flavus, A. fumigatus, and species from the group of A. niger, and A. versicolor. These molds have been classified by some authorities as being toxigenic or pathogenic and instant action is required when they appear in occupied indoor environment.

⦁  Mycotoxins. Aspergillus species produce toxic compounds, the most well known being aflatoxins. Aflatoxin is a class 1 carcinogen produced by strains of Aspergillus flavus and Aspergillus parasiticus.

⦁ Opportunistic Pathogens. Aspergillus fumigatus is an animal and human pathogen causing a group of diseases commonly referred to as Aspergilloses. Aspergillosis of the lungs is believed to be the most serious of these diseases and is quite common in birds and various mammals including humans. Aspergillus niger has been reported as causing ear, nose and lung infection particularly in immuno-compromised individuals.

⦁ Allergic Reactions. Many species of Aspergillus produce dry, hydrophobic spores that are easily inhaled. Due to their small size, about 70 % of spores of A. fumigatus are able to penetrate into the trachea and primary bronchi and close to 1 % into alveoli. Inhalation of spores of Aspergillus is a health risk. Aspergillus clavatus is allergenic causing the occupational hypersensitivity pneumonitis known as malt worker’s lung.


It is a commonly encountered species in wet buildings. Aureobasidium pullulans is both a soil and leaf fungus. In indoor environments, it is very common on wet wood and window frames, in floor, carpet, and mattress dust, damp walls, and in humidifier water. It is occasionally found on a wide range of stored foodstuffs and cereals (such as wheat, barley and oats). Sometimes it occurs on meat in cold stores as it can grow at very low temperatures.

Health Significance:
Aureobasidium pullulans is associated with occupational disease in wood processing. It has also been isolated from human skin and nails. Strains of Aureobasidium pullulans are used in various standard tests for resistance to microbial degradation.


In indoor environments the most common species of Chaetomium is Chaetomium globosum. C. globosum is frequently isolated in water-damaged buildings and produces two mycotoxins called chaetoglobosins A and C when cultured on building material. Presence of Chaetomium species in indoor environment is a sign of serious water problem.

Health Significance:
Species of Chaetomium are known to produce mycotoxins but to what extent these toxins contribute to poor indoor air quality or affect human health is not documented. However, injection of chaetoglobosin A in rodents has been shown to be fatal at relatively low doses. In medical literature some species have been reported to cause disease in immuno-compromised individuals. Species that have been reported to cause invasive human disease grow well at 35 to 37oC, and those with a predilection for the central nervous system often display growth at up to 42 to 45oC.


It includes about 40 species naturally found in soil, on decaying plant material and as plant pathogens. Several studies conducted in Europe and North America have shown that Cladosporium spores are present in the outdoor environment throughout the year. However, concentrations are very low in winter. In summer, daily peaks may range from 2,000 to 50,000 spores per cubic meter of air. The concentration of Cladosporium species in indoor air is influenced by outdoor concentrations and indoor growth sources. Cladosporiumis very common on wet building material (e.g., gypsum board, acrylic painted walls, wood, wallpaper, carpet and mattress dust, HVAC fans, and wet insulation in mechanical cooling units).

Health Significance:
Species of Cladosporium are not human pathogens except in some cases of immuno-compromised patients. However, Cladosporium species have the ability to trigger allergic reactions to sensitive individuals. Prolonged exposure to elevated spore concentrations can elicit chronic allergy and asthma.


Penicillium is a group of molds found everywhere world-wide. It is the mold that saved millions of lives by producing the first ever known modern antibiotic, the penicillin. Penicillium causes food spoilage, colonizes leather objects and is an indicator organism for dampness indoors. Some species are known to produce toxic compounds (mycotoxins). The spores can trigger allergic reactions in individuals sensitive to mould. Therefore, the health of occupants may be adversely affected in an environment that has an amplification of  Penicillium.

Health Significance:
Penicillium species other than  P. marneffei are commonly considered as contaminants but they are also known to produce mycotoxins. For example, P. verrucosum  produces a mycotoxin, ochratoxin A ,  which is damaging to the kidney (nephrotoxic) and could be cancer causing (carcinogenic). The production of the toxin usually occurs in cereal grains at cold climates but has been isolated in buildings contaminated with Penicillium. Other mycotoxins include patulin, citrinin, and citroviridin among others.


Growth occurs when there is moisture from water damage, excessive humidity, water leaks, condensation, water infiltration, or flooding. Constant moisture is required for its growth. It is notorious as a mycotoxin producer that can cause animal and human mycotoxicosis. Evidence has accumulated implicating this fungus as a serious problem in homes and buildings and one of the causes of the “sick building syndrome.”  The fungus can be hidden in the ceiling, walls or floors with no or little visible evidence within the interior of the room. The spores, however, can contaminate the interior of the room through holes and cracks in the building materials (aided by negative pressure) or be transported via the air handling system.

Health Significance:
Toxic black mold causes serious symptoms and health problems such as mental impairment, breathing problems, damage to internal organs and sometimes even death. The main groups of symptoms toxic black mold causes are: mental and neurological symptoms, respiratory symptoms, circulatory symptoms, vision and eye problems, skin problems, immune system problems, reproductive system problems, tiredness and discomfort.