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Spores need three things to grow into mold:

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  • Nutrients: Cellulose is a common food for spores in an indoor environment. It is the part of the cell wall of green plants.
  • Moisture: Moisture is required to begin the decaying process caused by the mold.
  • Time: Mold growth begins between 24 hours and 10 days from the provision of the growing conditions. There is no known way to date mold.

The main problem with the presence of mold in a building is, the inhalation of mycotoxins. Molds may produce an identifiable smell. Growth is fostered by moisture. After a flood or building leak, mycotoxin levels are higher in a structure, even after it has dried out.

Food sources for black molds in buildings include cellulose-based materials, such as wood cardboard, and the paper facing on both sides of drywall, and all other kinds of organic matter, such as soap, fabrics, and dust containing skin cells.

If a house has mold, the moisture may be from the basement or crawl space, a leaking roof, or from plumbing pipes behind the walls. 

People residing in a house also contribute moisture through normal breathing and perspiration. Insufficient ventilation can further enable moisture build-up. 

Visible mold colonies may form where ventilation is poorest, and on perimeter walls, because they are coolest, thus closest to the dew point.

If there are mold problems in a house only during certain times of the year, then it is probably either too air tight, or too drafty.

Mold problems occur in airtight homes more frequently in the warmer months, yet occur in drafty homes more frequently in the colder months (when warm air escapes from the living area into unconditioned space. 

If a house is artificially humidified during the winter, this can create conditions favorable to mold. Moving air may prevent mold from growing since it has the same desiccating effect as lowering humidity. 

Molds grow best in warm temperatures, 77 to 86 °F (25 to 30 °C), though some growth may occur anywhere between 32 and 105 °F (0 and 35 °C).

Removing one of the three requirements for mold reduces the chance of growth, yet may not eliminate new growth of mold. These three requirements are:

  1. Moisture
  2. Food source for the mold spores (dust, dander, etc.)
  3. Warmth (mold generally grows best in warmer temps, although the cold may not kill or restrict mold life.

  HVAC systems can create all three requirements for significant mold growth. 

The A/C system creates a difference in temperature that may cause condensation to occur. The high rate of dusty air movement through an HVAC system may create ample sources of food for the mold. 

And finally, since the A/C system is not always running - the ability for warm conditions to exist on a regular basis allows for the final component for active mold growth.

Because the HVAC system circulates air contaminated with mold spores and sometimes mycotoxins, it is vital to prevent all three of the environments required for mold growth. 

An effective return air filtration system, that eliminates up to 99.9% of dust accumulation (as compared to 5% elimination by typical HVAC air filters), are available.

These newer filtration systems usually require modification to existing HVAC systems to allow for the larger size of electrostatic 99.9% filters. However, thorough cleaning of the HVAC system is required before usage of high efficiency filtration systems will help. 

Once mold is established, the mold growth and dust accumulation must be removed. 

Insulation of supply air ducts helps to reduce or eliminate the condensation that ultimately creates the moisture required for mold growth. This insulation should be placed externally on the air ducts, because internal insulation provides a dust capture and breeding ground for mold.

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