Fungi possess a great variety of forms, but most exist as microscopic filamentous cells called hyphae (hypha = singular).  Under the right environmental conditions, hyphae will grow into an intertwining network of cells called the mycelium (myceilia = pleura).  While some fungi such as mushrooms produce large fruiting bodies, the spore-bearing structures of most microscopic fungi are not necessarily visible to the naked eye.   Typically, only dense concentrations of mycelia and spores are readily visible as discrete colonies.  Visual detection is aided by the presence of pigments found in spores and hyphae of certain fungal species.  These pigments are the primary reason for  the variety of colors we associate with fungal growth such as that seen on moldy bread.   Actually, the color of most fungi is influenced by nutrient source and colony age.  For example, mold growing on gypsum board may change from brown to yellow as growth progresses.  The same species may appear pink or red on wallpaper or other textiles. 

Fungi are collectively referred to as eukaryotic, absorptive, heterotrophs.  Fungi differ from bacteria in that they are slightly larger in size and they store their genetic material (DNA) within a nucleus ("eukaryotic").  Fungi, like bacteria, obtain nutrients by absorbing soluble substances across the cell surface ("absorptive").  And similar to most bacteria, fungi obtain their carbon and energy source from organic materials ("heterotrophs").   

A typical mold life cycle:

Spores - Fungi produce spores for dispersal and increased survival.  Spores may remain dormant for years - even under extreme conditions. 

Spore Germination - Germination is triggered by specific environmental and biological conditions.  Moisture and nutrients are especially important.

Hyphal Growth - Like plant seeds that produce roots, germinated spores will form thread-like cells called hyphae, which release enzymes for the degradation and absorption of nutrients. 

Spore Formation - Spores are typically produced by specialized spore-forming structures that often rise above the hyphae.  Spore production is affected by environmental variables such as temperature, light, oxygen, and nutrients.  

Fungi are found in virtually all man-made and  natural environments.   There are well over 100,000 species of fungi distributed worldwide.    Most of these species are obligate saprobes, which means they feed on dead organic matter such as leaves, wood, and other plant & animal tissues.  Fungi can also feed on a variety of building materials and textiles including gypsum board, insulation, rubber, wood, carpet, adhesives, paper, leather, and cardboard.  Fungi will also grow on paint, solvents, fuel, and many other industrial products.  In short, almost any organic substance may be exploited by fungi.

 What is a "mold"?

The term "mold" (sometimes spelled "mould") is used to denote many different types of fungi.  For example, there are the plasmodial slime molds, the cellular slime molds, water molds, and the everyday common mold.   When used in the scientific context "mold" refers to a filamentous fungus having a life history that includes 1) hyphae or mycelia, 2) spore-bearing structures called conidiophores, and 3) conidia (spores).  Mold is often used as a synonym for fungi; however, this is technically incorrect and the term should be confined to reference those organisms with the above characteristics.  Still, in the common vernacular, mold persists and indeed you'll find that we use it here as a synonym for fungi, especially when referring  to indoor contaminants.  

What is "mildew"?

Mildew is another common name for fungi.  As with mold, there are actually several different types of fungi that are referred to as mildews, including the powdery mildews and the downy mildews - both of which are groups of plant pathogens.  The common mildew growing on bathroom tile is actually mold (i.e. a fungus that consists of the mycelium, conidiophores, and conidia).

Fungi as Indoor Contaminants

Fungal spores are common in outdoor and indoor air; so it is virtually impossible to avoid at least some exposure.   The problem arises when fungi become established on building materials - either within habitable spaces or within building assemblies such as wall cavities, crawlspaces, and attics.  The result is considerably higher exposures to airborne spores, cell fragments, and secondary products such as volatile organic compounds and mycotoxins.   If wood rotting species are present, fungal growth may also pose serious consequences to the structural integrity of wood-based materials, including entire wall and roof assemblies. 

Preventing fungal contamination requires a basic understanding of the environmental factors responsible for mold growth.  The most important of these factors are moisture, nutrients, temperature, oxygen, and pH.  Although all of these factors determinine the type and extent of mold proliferation, experts agree that moisture is the most critical variable.   Mold is usually not a problem unless high moisture levels create suitable conditions for spore germination, nutrient absorption, and subsequent vegetative growth.   Once established, colonies may continue to grow as long as moisture and nutrient requirements are met.  Under less favorable conditions, hyphae and spores may go dormant only to re-emerge once suitable conditions reappear.        

Since mold requires water to grow, it is important to prevent excessive moisture in buildings. Several mold-related problems have been linked to changes in building construction practices since the 1970s, which resulted in tightly sealed buildings with diminished ventilation.   Other moisture problems may result from roof leaks, landscaping or gutters that direct water into or under a building, or unvented combustion appliances.  Delayed or insufficient maintenance may contribute to moisture problems in buildings. Improper maintenance and design of building heating/ventilating/air-conditioning (HVAC) systems, such as insufficient cooling capacity for an air conditioning system, can result in elevated humidity levels in a building.

What are the health effects of indoor fungi?

As recent as 2004, The Institute of Medicine completed a comprehensive review of the medical literature and found that the current evidence links mold to a variety of adverse health effects: Institute of Medicine Press Release.   Exposures were associated with asthma, coughing, wheezing, upper respiratory tract symptoms, and hypersensitivity pneumonitis.  Insufficient information exists to establish a clear association with a wider array of symptoms such as fatigue and neuropsychiatric disorders.   The study was sponsored by the Centers for Disease Control and Prevention. The Institute of Medicine is a private, nonprofit institution that provides health policy advice under a congressional charter granted to the National Academy of Sciences.

It is important to recognize that exposed individuals show remarkable variability in their susceptibility to mold.  This one one of several reasons why there are no federal standards or recommendations for airborne concentrations of mold or mold spores.