Multnomah Falls, Columbia River Gorge. This forest that is always wet from the mist of the falls in the background is a perfect environment for mold — nature's recycler. |
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What is Mold?
Molds (and mildew) are fungi. Fungi are neither plant nor animal but, since 1969, have their own kingdom. The fungi kingdom includes such wonderful organisms as the delicious edible mushrooms, the makers of the "miracle drug" penicillin and the yeast that makes our bread rise and our fine wines ferment. Biologically, all fungi have defined cell walls, lack chlorophyll and reproduce by means of spores. Approximately 100,000 species of fungi have been described and it is estimated that there are at least that many waiting to be discovered. The vast majority of fungi feed on dead or decaying organic matter – they are one of the principle agents responsible for the natural recycling of dead plant and animal life.
The most common fungi are ubiquitous within our environment and we are constantly exposed to them. For the most part, however, diseases caused by these common fungi are relatively uncommon and are rarely found in individuals with normally functioning immune systems.(1) Nonetheless, mold has recently experienced high profile press coverage. There are a variety of inflammatory press reports concerning lawsuits over air quality in homes, courthouses and other buildings; parental concerns regarding school classroom environments; home insurers refusing to cover mold damage; and widely distributed news reports on so-called "toxic mold." But don't panic. Mold can be managed effectively in most cases and this guide will help you do that.
Critical Requirements
There are 4 critical requirements for mold growth – available mold spores, available mold food, appropriate temperatures and considerable moisture. The removal of any one of these items will prohibit mold growth. Let's examine each requirement, one-by-one.
Magnified sporangiophore of the Pilobolus — "the shotgun fungus." The small dark knobs at the top of each stalk contain the spore sacks that are "shot" up to 6 feet into the air toward the sunlight by this unique fungus. |
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Mold Spores. Ranging in size from 3 to 40 microns (human hair is 100-150 microns), mold spores are ubiquitous – they are literally everywhere. There is no reasonable, reliable and cost-effective means of eliminating them from environments that humans inhabit. So, trying to control mold growth through the elimination of mold spores is not feasible.
Mold Food. If all three other requirements are met, almost any substance that contains carbon atoms (organic substance) will provide sufficient nutrients to support mold growth. Even the oil from your skin that is left when you touch an otherwise unsuitable surface, like stainless steel, or the soap residue left from a good cleaning will provide sufficient nutrients to support the growth of some molds. And many of the most common materials found in homes like wood, paper and organic fibers are among the most preferred of mold nutrients. Thus, eliminating mold food from your environment is a virtually impossible task.
Appropriate Temperatures. Unfortunately, most molds grow very well at the same temperatures that humans prefer. In addition, anyone who has cleaned out their refrigerator quickly realizes that temperatures close to freezing are not cold enough to prevent mold growth and temperatures that are much warmer than humans prefer, like those of the tropics, will grow abundant quantities of mold. Therefore, it is not feasible to control mold growth in our home environment through the control of temperature.
Considerable Moisture. Most molds requires the presence of considerable moisture for growth. Obviously, the word "considerable" is key here. The mycologists (fungi scientists) refer to "water activity" when describing the required conditions for mold growth. The various species of mold have different water activity requirements. A material's "water activity" is equivalent to the relative humidity of the air that would be in equilibrium with the material at that material moisture content. The vast majority of mold species require "water activity" levels that are equivalent to material equilibrium moisture contents corresponding to relative humidities of at least 70%. In fact, the great majority of serious, large mold outbreaks inside buildings occur where porous, cellulose-type materials have literally been kept wet by liquid water or sustained condensation.
Human beings prefer humidities that are below the critical relative humidity for mold growth. Thus, of the four basic requirements for mold growth, moisture availability is by far the easiest mold growth requirement to control in environments that humans like to inhabit. As you will see from the remainder of this guide, and from the vast majority of the literature on mold control, the consensus regarding effective mold control strategies consists of the combination of reducing the availability of moisture and killing and removing active mold growth colonies.
Determining If You Have Mold
Spoiled Florida orange. |
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Common household molds have a characteristic "musty" or "earthy" smell,
somewhat like the forest floor deep in the woods. Growing colonies of mold
can also be visually observed in many cases. Most people are familiar with
moldy bread or mold growth on cheese or other food products that have been
kept too long, so the "green fuzzy" characteristic of most mold
growth is familiar. And those who have lived in Florida have heard the expression "green
shoe syndrome" which refers to the fact that mold is particularly fond
of leather products left unused for periods of time in dark humid places.
Note the pink "splotch" at the bottom-center of this photo. It is the telltale warning sign that there is a likely mold "bloom" behind the vinyl wall covering. |
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Although most active mold colonies appear greenish to black (typical of mold growing on bathroom tile grout) in color, the characteristics of mold colonies growing behind vinyl wall covering in buildings takes on very different characteristics. These mold outbreaks typically result in pinkish to yellowish staining of the wall covering. They are quite important because they indicate serious, detrimental moisture accumulations within the gypsum wallboard behind the wall covering that can not be removed by your air conditioning or dehumidification systems. Where these problems appear, they usually require the assistance of a professional equipped with pressure measurement and other diagnostic equipment to determine the source(s) of the moisture causing the problem.
Practices That Will Minimize Mold Growth
The following practices will help minimize the growth of molds inside homes located in hot, humid climates like Florida's. Mold growth on the outside of homes is not covered here and readers are cautioned that these practices may not be applicable in other climates.
The following table of results from
experiments that were conducted by the Florida Solar Energy Center
illustrate the relative humidity impacts of both thermostat set
point temperature and the position of the fan mode switch. (2)
Impact of Indoor Set Temperature
and Fan Operation Mode on Interior Relative Humidity* |
Indoor T (oF) |
Avg RH % (Fan= Auto) |
Compressor Run Time Fraction |
Avg RH% (Fan=On) |
80 |
55% |
0.27 |
65% |
77 |
60% |
0.35 |
70% |
76 |
67% |
0.60 |
70% |
71 |
74% |
0.80 |
75% |
* AC operated alternatively for at least one week in each mode. |
The above wall diagrams from detailed computer simulations that model the combined impacts of heat, moisture and air transport (3) illustrate the importance of this air flow phenomena. The wall on the left bounds a space that is pressurized with respect to the outdoors and the one on the right bounds a space that is depressurized. The 2 Pa (Pascal) pressure gradient is very, very small — there are 101,325 Pa in one atmosphere. Clearly, it is the direction of the pressure gradient rather than its magnitude that is critically important here. You very much want your home slightly overpressurized in hot, humid climates so that dry, cool indoor air is pushed out of the home through the walls (figure on left) rather than have hot, humid outdoor air sucked into the home through the walls (figure on right). Fortunately, it is relatively easy to pressurize a home — all that is necessary is that slightly more air be brought into the home than is exhausted. This normally requires a positive mechanical ventilation system.
Things that may cause space depressurization in homes:
- Exhaust
fans (bathroom, kitchen, attic, crawlspace, etc.)
- Cloths
dryers
- Supply duct leaks
- Insufficient return
air pathways due to interior
door closure
The highest relative humidities in your home are likely to occur during mild weather when your air conditioner is not needed during the day. In Florida, the outdoor relative humidity reaches very near 100% on most nights, regardless of daytime temperatures. If your home is open to the outdoors during these periods, the materials in your home will adsorb moisture from this very humid air, again regardless of the temperature. If this moisture is not removed during the following day, the "water activity" of the materials in your home can stay at or above 75% for extended periods and mold is likely to grow on these surfaces. If you have RH levels exceeding 70-75% for extended periods, and find it difficult to control mold growth on surfaces in your home in spring, fall and winter, you may need to invest in some type of dehumidification system and should consult with a qualified mechanical system expert for advice.
Note the pink "splotch" at the bottom-center of this photo. It is the telltale warning sign that there is a likely mold "bloom" behind the vinyl wall covering. |
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If room doors are kept open, there will be sufficient return air pathways. However, if rooms doors are closed, the rule-of-thumb is that there should be about 50 square inches of "free" air transfer area for each 100 cfm (cubic feet per minute) of supply air to the room. In this case, the term "free" means a simple, clear hole in the wall between the room and the remainder of the home. If, for appearance and privacy reasons, this hole is to be covered by grilles on each side of the wall, then the overall return air pathway area needs to be increased by about 40% to account for the air flow resistance of the grilles, or about 70 square inches per 100 cfm of supply air flow.
It is best practice to either have bathroom vent fans interlocked with the light switch so they do not get left on or have them switched by a manual timer that will shut them off after a period of time, or control them by humidistat.
Recommendations to Control Humidity in Vacant Florida Homes
Luckily, FSEC has done a study sponsored by Florida Power and Light Company on how to treat vacant Florida homes so that humidity levels are controlled:
http://www.fsec.ucf.edu/en/publications/html/fsec-cr-1487-04/
Fundamentally, within that report, two strategies are recommended to control moisture:
1) Place a dehumidifier on a timer in the home so that it runs for 3-4 hours per day. Often it is best to drain the condensate from the dehumidifier to a sink. Leave the AC system set off with this strategy, although ceiling fans on low speed might be set to circulate the air in rooms. Doors to rooms should be left open.
2)
Using a programmable thermostat, set the air conditioner to 74 F from 3-6 AM. This operates the cooling system constantly during the early morning hours when indoor coil temperatures will be achieved resulting in good moisture removal. Set up the thermostat to 85 F during the rest of the day. This appears to work well in a majority of homes and under a wide range of weather conditions, although any significant duct leakage must be repaired.
Additional Resources
The authors of this web page are Philip Fairey, Subrato Chandra and Neil
Moyer of the Florida Solar Energy Center.
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Footnotes:
1. Readers are cautioned that there is not full consensus among the medical and health science community on the potential health consequences of exposure to molds. Individuals with compromised immune systems, allergies, asthma, other lung diseases or other medical problems should consult their physician before physically contacting or disturbing any mold growths.
2. M.K. Khattar, M.V. Swami and N. Ramanan, "Another Aspect of Duty Cycling: Effects on Indoor Humidity," FSEC-PF-118-87, ASHRAE, 1987.
3. Swami, M.S., Lixing Gu and Philip Fairey, 2001. Appendix K, Moisture Analysis and Condensation Control in Building Envelopes, Heinz R. Trechsel, Ed., pp.183-184, ASTM Stock No. MNL40, ASTM International, West Conshohocken, PA.