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Where It Can Grow

It is normal to find some mold spores in the air and a few mold spores on some surfaces. Unless large numbers of spores become airborne, there is usually little problem. However indoor mold growth on surfaces and materials is abnormal. Mold growth occurs when mold spores settle on a surface at the proper temperature, with appropriate food sources, and with the proper amount of moisture. An adequate amount of moisture in materials is typically the limiting factor preventing mold growth as well as the easiest to control.

Mold growth in homes and other buildings is an indicator of excess moisture intrusion and accumulation often caused design or construction defects, ineffective maintenance and repairs, and occasionally occupant activities. Obvious sources are floods, roof leaks, problems with drainage or plumbing and include occupant-generated sources. A less obvious source of moisture is the effect of temperature gradients (temperature differences), especially in locations where relatively warm and moist air comes in contact with relatively cool surfaces. These conditions can cause water vapor to condense on building surfaces, just as it does on a glass of ice water on a warm, humid day.

Most molds must get their food from the environment, living and feeding on dead organic matter. Outdoors, molds are very important in decomposing organic materials and recycling nutrients. Indoors, many building components and contents contain materials that are excellent food sources for mold, such as wallpaper glue, some paints, greases, paper, textiles, and wood products. Indoor dusts may contain fibers, dead skin cells, and other organic matter that can serve as a food source for mold when adequate moisture is available.

Temperature also affects mold growth. Different types of mold have minimum, optimum and maximum temperature ranges for growth. Many fungi grow well at temperatures between 60 and 80 degrees Fahrenheit, which are also ideal temperatures for human comfort. In addition, as mentioned above, temperature gradients often produce the moisture needed for mold growth.

In the summer, when air-conditioning is in use, mold growth can occur in buildings where the cooling systems are oversized, undersized or poorly maintained. Unplanned air flow in buildings can also create conditions favorable to mold growth. A competent heating and air conditioning contractor should be able to address these issues.

In the winter, when buildings are heated, mold often grows in cold, uninsulated exterior windows and walls, including uninsulated closets along exterior walls where building surfaces are generally cold relative to the indoor air temperature.

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