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Occupational & Environmental Epidemiology


New fact sheet External link on mold and moisture!

Molds and mildew are terms used to describe a fungi that may appear as a fuzzy-looking growth on the surface of organic materials in damp conditions, both outdoors and indoors. Molds may be gray, black, green, yellow, orange or various other colors, and may have a velvety or wooly texture.

Like other fungi, molds release tiny spores in order to reproduce. Mold spores continually waft through the air, both indoors and out-of-doors.

Molds are a natural part of the environment, but mold growth indoors is an abnormal condition resulting when there is excessive moisture infiltration and accumulation, particularly if the moisture problem remains undiscovered or unaddressed.

There is no practical way to eliminate all mold and mold spores indoors. However, indoor mold growth can be controlled by controlling moisture.

Health Effects of Indoor Mold

Exposure to mold can occur when airborne mold particles, mostly spores and fragments, are inhaled. We breathe in mold spores and fragments every day, indoors and out. Usually these exposures do not present a health risk. However,  health problems may result when people are exposed to large amounts of mold, particularly indoors.  Inhaling excessive quantities of airborne mold particles, fragments or spores may lead to allergic illness, trigger asthma, cause respiratory infections, or may bring about toxic effects from certain chemicals in the mold cells. Following are descriptions of health problems that can be caused by exposure to mold.

Allergic Illness

When mold cells are inhaled and land in the respiratory tract, the body's immune system's response to those invading cells can cause allergic illness. The immune system tries to destroy the mold as it would an agent, like a flu virus, that might cause infection. In about 10 percent of people in the U.S., the immune system overreacts and causes the allergic response resulting in symptoms such as runny nose, scratchy throat and sneezing. Most of us know this allergic illness as "hay fever" or "allergic rhinitis."


Asthma is a disease that affects the lungs and with symptoms such as wheezing, breathlessness, chest tightness, and coughing.  Asthma attacks often at night or early in the morning, but can also occur at any time.   During an asthma attack the airways partially close causing breathing difficulties ranging from mild (such as a dry cough) to life-threatening (inability to breathe).

In an asthma attack, the airways that carry air in and out of the lungs become inflamed.   Often inflammation is the result from inhaling allergens or irritants.  The inflammation causes the muscles around airways to tighten, narrowing the airways and creating less flow in and out of the lungs.  Inflamed cells in the airways make more mucus than usual further restricting air flow.

 According to the Centers for Disease Control and Prevention (CDC), one in 12 adults and one in 11 children in the United States have asthma. More than half of asthmatics have respiratory allergies, often to mold. Molds can trigger asthma episodes in sensitive asthmatics. Learn more about asthma in North Carolina.

Respiratory Infection

Some mold species can cause respiratory infection when the live mold invades the tissues of the lungs or respiratory tract. Generally, infections are not a significant risk for healthy people, but can be dangerous for individuals with severely weakened immune systems. Two infections, Histoplasmosis and Cocidicimycosis can be a risk for healthy people.  These diseases occur when people are exposed to enormous numbers of mold spores during activities such as cleanup of bird or bat roosts and other activities that disturb reservoirs where large numbers of mold spores reside.

Toxic Effects

Very large doses of certain molds, when inhaled or ingested, can result in poisoning caused by toxins, called mycotoxins, in the mold cells. The evidence on whether an individual can inhale enough toxins from inhaling spores or fragments from indoor mold growth is inconclusive or unavailable.

One particular type of mold that has been highlighted in the media is Stachybotrys chartarum (also known as Stachybotrys atra), commonly referred to as "black mold". Stachybotrys is a greenish-black mold that grows on materials with high cellulose content (drywall, wood, paper, ceiling tiles) that are chronically wet or moist. It is one of several molds that can produce mycotoxins under certain environmental conditions. The health effects of breathing mycotoxins are not well understood, but we do know that most molds can present some health risks, such as allergic reactions. Therefore, any mold growth in a building should be cleaned up, regardless of the type of mold.

How to Clean Mold

The first step in addressing any mold growth problem in a building is identifying and correcting moisture source(s) (see Where Does Mold Grow?). If moisture problems are not corrected, then any mold cleanup or removal that takes place will most likely be only a short-term solution; at some point the mold growth will recur. It is critical to control moisture at the beginning, during, and at the end of a mold-growth removal project.

One of the most common misconceptions about mold is that it can be removed by spraying the surfaces with products such as disinfectants, biocides or cleaners. That will not take care of the problem because the allergenic and toxic properties of mold are not removed by using such products. Whether mold spores and other parts of the mold are viable (living) or nonviable (dead) when they get into the air, they still present a health risk to exposed individuals.

While disinfectants and biocides may kill mold spores and take away their ability to reproduce, these products should not be used alone in addressing a mold-growth problem. Either the mold must be completely removed from the affected material, or the mold-contaminated material must be completely removed from the building.

In determining which materials can be cleaned and what should be removed, the two important factors are how porous (absorbent) the material is and how extensive the mold growth is. Generally, non-porous materials (such as metals, glass and hard plastics) and semi-porous materials (wood, plaster and concrete) that are visibly moldy but structurally sound can usually be cleaned and reused. Moldy porous materials (carpeting, wallboard, ceiling tile, wallpaper, fabric, upholstered furniture, mattresses) should usually be discarded, since they absorb and hold moisture, may be internally moldy, and cannot be completely cleaned and thoroughly dried.

Cleanup and mold removal activities can expose people to mold particles and other hazards, so it is important to wear protective equipment and follow procedures safely. For complete instructions, see:

Air duct systems in buildings can also become contaminated with mold. Air duct systems can be constructed of bare sheet metal, sheet metal with exterior or interior fibrous glass insulation, or made of entirely out of fibrous glass (ductboard). If mold growth has occurred on fibrous glass or other porous surfaces, then effective cleaning will not be possible and the ductwork and/or insulation will need to be discarded. Mold growth on metal ductwork may be cleaned and disinfected following the instructions for non-porous materials. For additional details on addressing air duct cleaning see Should You Have the Air Ducts in Your Home Cleaned?, External link a publication of the Environmental Protection Agency (EPA).

For Additional Information