Aspergillus: Should It Worry You?

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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.

Aspergillus: Should It Worry You If You Find It in Your Home or Office?

Mold pictures: Aspergillus fumigatus

Aspergillus fumigatus

MBL is commonly asked if these types of molds are dangerous. In reality, species of Aspergillus affect our lives in several ways:

  • Biodeterioration. Species of Aspergillus are capable of utilizing an enormous variety of organic material for food because of their ability to produce a large number of enzymes. Under humid conditions, Aspergillus species grow on leather and cloth fabrics, reducing their value and imparting musty odour to shoes and clothing. Several species are frequently found on grains and exposed foodstuffs where they cause decay and subsequent loss of food.
  • Medical Effects. 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. The best known toxic metabolites of A. fumigatus are mainly fumigaclavines A, B, C, and D, spinulosin and tremorgenous toxins, e.g. verruculogen. A. niger strains produce toxic metabolites belonging to malformins A1, A2, B1, B2, C and oxalic acid. On building material, strains of A. versicolor produce the toxic class 2A carcinogenic sterigmatocystin.
    • 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. Other species of Aspergillus associated with Aspergillosis are A. flavus, A. niger and A. terreus. 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 hypersentivity pneumonitis known as malt worker’s lung.

Occurrence of Aspergillus species in indoor environments

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.

Species of Aspergillus have been isolated from damp walls, wallpaper, PVC/paper wall covering, gypsum board, floor, carpet and mattress dust, upholstered-furniture dust, acrylic paint, UFFI, leather, HVAC insulations, filters and fans, humidifier water, shoes, leather, bird droppings and potted plant soil, plastic and decomposing wood.


Microorganisms in home and indoor work environments: Diversity, Health Impacts, Investigation and control. Flanning Brian, Samson, Robert A., and Miller, David J (Ed.), Tayler and Francis, 2001.Atlas of Clinical Fungi, Second Edition. G. S. de Hoog, J. Guarro, J. Gene, and M. J. Figueras. Centraalbureau voor Schimmelcultures, 2000.

Summerbell RC, Staib F, Dales R, Nolard N, Kane J, Zwanenburg H, Burnett R, Krajden S, Fung D, Leong D.Ecology of fungi in human dwellings. Journal of Medical and Veterinary Mycology 1992;Suppl.1:279-85.

Millner PD, Bassett DT, Marsh PB. Dispersal of Aspergillus fumigatus from sewage sludge compost piles subjected to mechanical agitation in open air. Applied and Environmental Microbiology 1980;39:1000-9.


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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at
Dr Jackson Kung'u

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