Mold Growth Requirements

Growth Requirements

It's not easy to simplify the growth requirements of 80,000 complex organisms, but suffice to say, our indoor environments provide suitable conditions for many types of fungi.

Granted, there are fungal organisms that require specific light/dark cycles, temperature, nitrogen/oxygen levels, nutritional constituents, moisture levels, etc. for optimum growth and reproduction, however, our focus is on the types of fungi likely to occur in indoor environments.

For many of these organisms growth can be simplified to three significant factors;

• Mold Spores

• Organic Food

• Water

envirochex uses the 'Mold Growth Triangle' to illustrate this relationship and serve as a simplified overview of growth requirements.  When a mold spore locates a suitable food source with sufficient moisture, growth occurs.  Simple as that.
 

Growth Cycle

Growth starts when the Mold Triangle is formed...

Spores provided with sufficient moisture and suitable food form microscopic, branched, thread-like filaments called hyphae (hi-fee).  Continued hyphal growth forms a mass called a mycelium (my-sill-ee-um) which often becomes visible within 24-48 hours.  

As the organism grows, reproductive spores are formed and released as part of it's continuing life cycle.

Mold Spores

Most molds reproduce by forming spores that disperse into the air in search of more food and moisture (a reproductive activity similar to seed dispersal from plants).  Due to the diversity of mold in our environment, outdoor air normally always contains some level of these airborne spores.

A few types of spores are visible and anyone who has disrupted a puffball has witnessed millions of spores being released into the atmosphere.  However, most filamentous mold spores are microscopic; therefore, invisible to the naked eye.

It is not uncommon to find hundreds or even thousands of mold spores per cubic meter of outdoor air.  Some mold types like Cladosporium, produce light and buoyant spores that aerosolize easily which is one factor in Cladosporium being recovered in outdoor air tests over 90% of the time.

Other types of mold, like Stachybotrys, do not easily go airborne; therefore, their spores are not frequently recovered in outdoor air tests.  When actively growing, Stachybotrys spores are typically a wet, sticky mass that is not easily aerosolized.  Some believe this organism's spores are like a cocklebur that hitchhikes on insects and rodents rather than traveling by air. 
 

Numerous factors (i.e. spore desiccation or other physical damage) influence a spore's ability to grow and comparative testing has shown a large percentage of airborne spores to be non-viable.

It is important to recognize that spores retain many of their adverse health characteristics regardless of their ability to reproduce.  In other words, non-viable spores are still allergens, contain toxins, etc.  This trait not only has significance on exposure to molds, but also greatly influences testing methods.
 

Most fungal spores range from 1 to 100 microns in size with many types between 2 and 20 microns.  People with good vision may see 40-80 micron particles unaided, but below that range, magnification is necessary.

To put things in perspective, you could place over 20 million five micron spores on a postage stamp!

This small size has numerous impacts on dealing with mold.  They are so tiny they infiltrate our environments with air and are essentially invisible so cleaning them up without special equipment and procedures is next to impossible (its tough to clean up things you can't see).
 

Scanning Electron Microscope Image of
Stachybotrys Spores

There is little we can do to stop nature's production of airborne mold spores; however, we can prevent mold from growing indoors.  When these organisms are allowed to amplify in a closed indoor environment, they can release millions of spores causing indoor levels to reach concentrations that are hundreds of times higher than outdoors...levels that can be detrimental to even non-sensitized individuals. 

Airborne mold spores are particles and generally settle out with time but can be disrupted and re-aerosolized.  They may also just sit quietly waiting for food and moisture.
 

Totally eliminating mold spores from our indoor environments is virtually impossible without extreme measures of air and access control. 
 

For molds, the food of choice is organic matter (things that are or once were living).  Fungi will consume anything organic, yet many types have selective appetites.

Building materials including wood, paper, natural fabrics, leather, and even the starch in wallpaper paste are common examples of dead organic matter preferred by filamentous molds.  

Grasses, plants, trees, etc. provide examples of living organics for parasitic fungi and animal or human tissue provides a source of living nutrients for many pathogenic fungi.
 

By absorbing nutrients and obtaining carbon and energy necessary for growth from organic sources, fungi are classified as absorptive heterotrophs.  

During the process of enzyme release and nutrient absorption, molds also produce volatile organic compounds (VOC's).  Because they are produced during processes essential to growth, VOC's are termed Primary Metabolites.  (Since they are generated from a microbial source, some references list them as microbial VOC's or mVOC's.)

Representing a wide range of organic chemicals, VOC's are responsible for the 'moldy/musty' odor associated with fungal growth.
 

Mold growing on glass, ceramic tile, metals, or other inorganic materials is not obtaining nutrients from these substrates.  In these cases, mold is feeding on microscopic organic matter that is on the surface or trapped in tiny pores of the material.  

Bath-tile mold is an example:  Mold is typically consuming organic dust, dirt, debris, skin flakes, body oils, soap scum, etc., and the ceramic tile is simply a foundation for the colony.
 

Water is critical to life-forms, even fungi.  Without water, growth and reproduction simply will not occur.  

The amount of water for optimum growth varies by species and is also influenced by environmental factors such as temperature and the types of nutrients available.  

Some species can grow at relatively low moisture levels, levels that would not be considered 'wet' in ordinary terms (think of mold growing on bread).  Other types of fungi require significant amounts of moisture for optimum growth.
 

Dealing with water in liquid form is relatively straight forward.  Most everyone recognizes that liquid water moves by forces of gravity causing 'water to run downhill'.  But there are other forces which warrant consideration including hydrostatic pressures, capillary actions, and differences in the partial pressures of water vapor.  In simple terms, water will move from 'wet towards dry' in an attempt to reach equilibrium between these forces.

Note that water does not have to be liquid in order to saturate a substrate.  Water vapor, a frequent cause of mold growth, is naturally absorbed by most porous materials.  Moisture in air (humidity), or water vapor migrating through substrates is often slow developing and may be more difficult to assess.  Note that most building materials including concrete, brick, mortar, grout, drywall, wood, etc. are porous and do allow water vapor to pass.
 

Examples of water activity ranges are shown below.  For the types of mold compatible with indoor environments, few are considered capable of growing below A_w = 0.65. 

Quick response in drying all materials impacted by plumbing breaks, rain intrusion, etc. and controlling humidity in areas with organic substrates is of paramount importance.
 

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