MOLD, MOISTURE, AND YOUR HOME
Why is mold growing in my home?
Can mold cause health problems?
Mold is everywhere! Mold is a natural part of the outdoor environment. In fact, it’s a necessary part of the outdoor environment. Mold is just one of many microorganisms responsible for decomposing or breaking down organic compounds outdoors. Dead plant and animal matter is decomposed by these microorganisms and returned to the soil. Without mold and other microorganisms, we would be overcome with waste. So, mold is an abundant and useful part of the outdoor environment.
Mold grows as branching threads called hyphae. These hyphae elongate, increasing in length. The hyphae intertwine and increase in size until eventually the mass of branching thread-like hyphae can be observed as a small mold colony (Figure 1). The small colony becomes visible to the unaided eye when it grows to a diameter of 1 mm or so. Upon examination with a microscope one would observe small oval to spherical structures, varying in size, scattered amongst the thread-like hyphae (Figure 2). These are spores, the means by which mold reproduces. Mold spores can also lie dormant for months or even years until conditions become favorable for reproduction, a means by which mold survives. These small structures become airborne, often traveling long distances before settling to the ground. When conditions are right, the spores will germinate and grow, and the reproduction cycle repeats.
Often, the mold produces a fruiting structure, where a large number of spores are produced in clusters (Figure 3). A single mold colony can produce thousands and even millions of spores that serve as a way for the mold to reproduce and survive. Different kinds of mold have a unique spore morphology, meaning a unique shape and size that is characteristic to the kind of mold. This unique spore shape can be used to identify the type of mold producing the spores. Depending on their morphology and the environmental conditions, some mold spores become airborne more readily than others, giving some species of mold greater mobility than others. Mold spores in the air outdoors enter our homes when we open windows and doors; we bring them into homes on our clothing and on our shoes. Even in new homes that have never been occupied, air sampling would detect these mold spores in the air. Normally, the mold spores in the home settle to the floor and don’t cause any problems. Mold counts in the home are, under normal conditions, much lower than outdoor counts. Thus, allergists tell their patients to stay indoors during certain times of the year because of high outdoor mold spore counts. They will often advise that it is better to close the windows and turn on the air conditioner rather than cool the home by opening up the windows and letting the mold spore laden air into the home.
So, while the outdoors may have mold spore counts and other bioaerosols that are high enough to cause an allergic reaction in sensitive individuals, the indoor counts are normally dramatically less, lessening a sensitive individual’s exposure to the mold spores, and thus minimizing their allergic reaction. The presence of relatively small numbers of mold spores indoors is not unusual and the spores normally settle out and cause no further problems. Problems arise, however, if excess moisture is present. In the presence of sufficient moisture, the mold spores will germinate, resulting in mold growth, spore formation, more mold growth and more mold spores. With time, mold spore counts indoors increase, and are responsible for many indoor air quality problems, particularly for those with allergies and asthma or those that have a suppressed immune system.
Mold spores will begin to grow within 24 to 48 hours after the environmental conditions are conducive to growth; however, early growth is only discernable when is observed with the help of a microscope. After sufficient time, light mold growth can usually be discerned as a slight discoloration of the material it is growing on (Figure 4).
As the mold continues to grow, it will become more visible, and heavy mold growth is recognized as such by the average observer (Figure 5). So, while the presence of mold spores is considered normal at low counts, MOLD GROWTH INDOORS IS NOT NORMAL. There should be no significant mold growth occurring in indoor environments.
Why is mold growing in my home?
While the presence of mold spores is considered normal at low counts, mold growth indoors is not normal. A small amount of occasional mold growth indoors, however, is probably inevitable. It’s not all that uncommon to observe small amounts of mold growth on the refrigerator or freezer door seal, in the bathroom, or below kitchen or bathroom sinks that can usually be remediated quickly by the building occupants or janitorial or custodial staff. However, mold growth over larger surfaces or entire walls or living areas is much more difficult to remediate properly and usually requires the help of a professional. In some situations, large areas of the building may have mold growth that goes undetected for long periods of time. This would include areas of the building that people seldom go in, such as the crawl space or attic; or hidden mold that often occurs behind walls or in or below carpeting.
Why is mold growing in these areas? It’s because the environmental conditions are providing all the essential things needed for mold to grow and thrive. The things mold needs to grow are: air, food, and moisture. Air and food are found in abundance in a home, but usually not moisture. The abundance of air is obvious. A source of food is also abundant as mold can grow on a variety of materials found in homes: wood, paper, carpeting, paint, clothing, and even many synthetic materials, although in the latter case, it is actually dust and other organic material that the mold is growing on, not the substance itself. Homes are usually dry, however, and the lack of moisture in a home usually keeps the mold spores from germinating and growing. If enough moisture is present, however, the mold spores will germinate and grow, eventually becoming visible to the unaided eye. Mold growth inside a home is an indicator of a moisture problem.
If mold is growing indoors, there is an excess of moisture. If the source of moisture is localized, the area impacted by the mold growth is usually small and can be fixed relatively quickly and easily. However, there is another source of moisture in homes that if not controlled can impact much larger areas of the home. It’s the moisture in the air, often measured and reported as the relative humidity (RH). Conditions favoring mold growth will usually be absent when the environment is kept at RH of <60%. When the RH is between 60% and 65%, mold growth is possible, and when the RH exceeds 65%, mold growth is probable. The further the RH exceeds 65%, the more extensively and quickly mold will grow. Industry guidelines suggest that the relative humidity in a home should be kept between 30% and 50% for limiting mold growth and providing optimum comfort for the occupants.
test testing tested images/final_-fig3.jpg pp
Can mold cause health problems?
The most common effect mold has on individuals is an allergic reaction. Exposure to mold spores by susceptible (sensitized) individuals through either physical contact or inhalation may cause immediate or delayed reactions, including hay fever-type symptoms such as sneezing, runny nose, red eyes, and skin rash. Spores and fragments of mold can produce allergic reactions in sensitive individuals regardless of whether the mold is dead or alive. Mold produces allergens, and all molds have the potential to cause health effects.
Because repeated exposure to mold has the potential to increase sensitivity, non-allergic (non-sensitized) individuals may become sensitive to mold and notice increased reactions to mold through repeated exposure.
Molds can also trigger asthma attacks in individuals who are allergic to molds. The irritants produced by molds may also worsen asthma in non-allergic people.
A more comprehensive section dealing with mold’s health effects is listed on our Health Effects page.
How do I remove the mold?
Although mold growth can be symptomatic of a larger problem, such as a moisture or humidity problem, leakage, and poor indoor air quality, not all mold growth needs the same degree of professional attention. In fact, there are simple home methods or inexpensive store-bought solutions for cleaning mold. Below are descriptions of different levels of mold contamination, with guidelines and recommendations for when to seek professional judgement on the mold in your home.
Small Area – Total surface area affected is less than 10 square feet (ft²).
For small areas, minimum personal protective equipment is needed. Small areas such as these can be cleaned by the homeowner, building occupant, or maintenance personnel, and do not require the services of a professional mold remediator. 
To clean, warm, soapy water or a detergent solution can be used. Scrub as needed. Minimal personal protective equipment would include an N-95 paper respirator, gloves, and goggles. The N-95 respirator covers the mouth and nose and has a filter efficiency of 95% or greater. It can be purchased at most hardware stores.
Although soapy water is usually sufficient for this type of mold removal, cleaning solutions are commercially available from the big box retailers and are specially formulated for removing mold. An excellent kit that can be purchased from Home Depot is the “Mold Armor” Mold and Mildew Stain Removal kit. It is a hydrogen peroxide-based, environmentally-friendly cleaning process that successfully kills the mold and brightens the surfaces that had mold growth.
Medium area – Total surface area affected is between 10-100 ft²
Use professional judgment for an area of this size. Consideration should be given to the amount of mold and the size of the contaminated area, and the potential for exposure and contamination of the clean areas. If there is light mold growth, limited personal protective equipment is required. If a plumbing leak has been ongoing for years in a wall cavity and the mold growth is extremely heavy, the potential for personal exposure and/or contamination of adjacent areas increases, and it may be necessary to increase the level of protection to the “remediator” and to provide some level of containment to keep the mold from spreading to non-contaminated areas. Personal protective equipment will range from an N-95 respirator, gloves, and goggles to a half-face respirator with a HEPA filter.
Large area – Total surface area affected is greater than 100 ft².
Large areas increase the potential for exposing occupants or the remediator to mold during the remediation. A professional mold remediator should be consulted, and must use full personal protective equipment and full containment to keep from spreading the mold spores to adjacent non-contaminated areas.
See our Mold Removal page for more extensive reading on reasons to remove mold and methods that are often professionally employed.