Facts About Mold


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For more than a decade, mold has been in the news. People are talking about the effect on population health and damage to the building. But what are the risks and issues?

The available science on molds and their potential health effects remains under study, but considerable progress has been made. The U.S. Centers for Disease Control and Prevention (CDC), the Institute of Medicine of the U.S. National Academy of Sciences, the World Health Organization and Health Canada all agree that living or working in a building with mold damage results in increased risk of respiratory disease. Although there are several guidance documents available, there are no accepted national or international standards for mold investigation, evaluation or remediation. AIHA, however, has worked to translate the advice from the previously mentioned government agencies into state-of-the-art inspection and sampling protocols, such as AIHA’s Recogni​tion, Evaluation and Control of Indoor Mold​ book, also known as the Green Book. If properly used, these methods are suitable for assessing hidden contamination and directing essential visual inspections. For health outcomes, there are no available exposure assessment methods that can provide useful information for individuals. This is primarily due to the fact that each person’s response to mold exposure is unique.

The scientific complexities surrounding this issue would be a huge challenge but the truth is that other, less scientific, difficulties dwarf them. Media attention on this topic often creates emotionally charged circumstances, making scientific and professional judgment, as well as reasoned dialogue on this subject, very difficult. In some instances, building owners have been known to ignore or dismiss potentially serious problems. Many indoor air quality (IAQ) problems have nothing to do with mold, and buildings seldom have only one indoor environmental quality problem. It is essential to consider multiple sources of building IAQ problems instead of focusing on just mold concerns. In other instances, building occupants or public officials armed with mold sampling reports of dubious quality have reacted with alarm to potential threats, making risk communication very difficult.

The information on this page represents a consensus statement by a group of experts about important aspects of the state of the science. The guidance offered is practical information based on years of experience addressing mold issues, and this document does not claim to be a definitive or comprehensive position statement. Because it is not comprehensive, it should always be used in conjunction with other existing guidance documents, as well as professional judgment by qualified consultants and public health officials.

Public and occupational health practice is rarely an exact science. Prevention always poses the challenge of making tough and often costly decisions with incomplete information or understanding.

The Facts about Mold: For Everyone

What is mold?

The term “mold” is a colloquial term for a group of filamentous fungi that are common on food or wet materials. This includes the green Penicillium species that produces penicillin, and fungi that spoil our bread, fruit, cheese and crops. Most of these are Ascomycetes that produce a lot of spores.

The majority of the molds that grow on damp building materials are found in the soil and are adapted to grow on a wide variety of materials. Outdoors, molds live in the soil, on plants, and on dead or decaying matter. There are thousands of species of mold and they can be any color. Different mold species are adapted to different moisture conditions ranging from very wet to just damp. Many times, mold can be detected by a musty odor. Live spores act like seeds, forming new mold growths (colonies) under the right conditions. All of us are exposed to a variety of fungal spores daily in the air we breathe, both outdoors and indoors.

How mold gets into a house or building

Mold and fungal spores occur naturally outdoors, where fungi are the earth’s most important recyclers. Indoors, mold needs moisture to grow; it becomes a problem only where there is water damage, elevated and prolonged humidity, or dampness. Common sources of excessive indoor moisture that can lead to mold problems include:

  • flooding from surface waters (i.e., overflowing rivers) or from severe storms;
  • roof leaks from damaged or missing roofing materials, ice dams or blocked gutters;
  • storm-driven rain through window frames, exterior walls or door assemblies;
  • leaking pipes, sewer back-ups or overflows;
  • damp basements or crawl spaces due to a high water table or poorly managed rainwater drainage; and
  • condensation on cold surfaces.

How to prevent mold growth

The key to preventing and stopping indoor mold growth is to control excessive moisture and condensation. Keeping susceptible areas in the home clean and dry is critical. In general, mold will not grow indoors without water, dampness or excessive moisture.

Three main factors contribute to condensation of water on building surfaces:

  • Relative Humidity: Condensation occurs when the air is saturated with water and it cannot hold any more moisture. For example, steam generated from bathroom showers or from cooking can fill up the air with moisture, which will then condense into drops of water on cooler surfaces, such as mirrors and windows. Where possible, localized sources of humidity, such as clothes dryers, should be directly vented to the outdoors. To lower indoor humidity during warm, humid weather, air conditioners and/or dehumidifiers should be used. In chronically damp areas such as basements or crawlspaces, it is often recommended that dehumidifiers be used to maintain humidity levels below 60 percent.
  • Temperature: Warm air holds more moisture than cold air. Condensation occurs when warm humid air comes into contact with a cold surface and the moisture condenses into water. This can often be seen on single-pane windows, where water condenses and then runs down, causing the wood frames and sills to rot and the wall under the windows to blister. Condensation can occur on exterior walls, particularly north-facing walls, if they are not properly insulated. Other chronically cold surfaces, such as cold water pipes, should be covered with insulation to help prevent condensation.
  • Poor Ventilation: Indoor humidity can build up if there is not enough ventilation and exchange of indoor and outdoor air. Where there is little or no air movement, such as behind dressers and cabinets, surfaces can remain cooler than surrounding areas, which can lead to increased condensation and mold growth. It is recommended that the area be ventilated and the occupants use exhaust fans (vented to the outdoors) to remove moisture from high-humidity areas, particularly in bathrooms, kitchens and laundry areas. Furniture should be moved slightly away from walls so that air can freely pass behind it. Air should be allowed to circulate between rooms and regularly ventilate to remove humid air. Fans should be used as needed.

Other things that can be done are to clean and repair gutters regularly, make sure the ground slopes down and away from the home’s foundation and keep air conditioner drip pans and drain lines clean. In addition, in air conditioned buildings in hot and humid climates, vinyl wall coverings on the interior sides of exterior walls should not be used, as these materials can trap moisture, resulting in mold growth underneath them.

In the case of floods or leaking pipes, any standing water should be promptly removed and water-damaged materials should either be dried out and cleaned, or removed and replaced. Porous materials that are wet for more than 48 hours are likely to produce mold growth and should be discarded. In instances where the water damage is extensive, it is recommended that professional help, such as a commercial restoration company, be consulted.

Can mold spores contain toxins?

Yes. Some of these fungi produce toxic metabolites (mycotoxins), and almost all molds that grow in the built environment can produce triple helical glucan, both of which are toxic to lung cells. Many studies in appropriate laboratory animals have demonstrated that very low exposures of these compounds can result in inflammation. The health effects of breathing mycotoxins indoors are not well understood and they continue to be studied. This research is done to better understand why epidemiological studies consistently show increased asthma among occupants of damp buildings not associated with atopy.

Some studies have shown that in agricultural settings, occupational exposure to fungi that produce mycotoxins on grain may result in significant exposures to their toxins. However, it is important to not relate human exposures to mycotoxins in agricultural settings with those exposures that can occur in the built environment (homes and offices). Exposures to mycotoxins in agricultural environments can be at much higher airborne concentrations, and these levels can result in systemic exposure. These agricultural exposures tend to be fungi that do not generally occur in buildings, such as Aspergillus flavus (aflatoxin) and Fusarium graminearum (deoxynivalenol).


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The use of thermal imaging has become pretty common in the restoration industry. Detecting a temperature difference behind a wall can save big bucks and time by avoiding the need for demolition, and give remediators an early idea where the source of a water problem may lie. In essence, a thermal imaging camera is like a second set of Superman-style eyes. However, while Superman could physically see through a wall, IR cameras simply show you the surface temperature difference - but often give the illusion you are seeing through a surface. It takes knowledge about thermography to truly know what all those colors on the screen mean.

Michael Stuart has been working with thermal imaging technology for the last decade. Today, he is a thermographer for Fluke, a company specializing in thermal imaging cameras. Stuart feels as a product expert, he’s better able to connect with potential customers. Until the last decade or so, thermal imaging technology was out of the reach of the general contractor. But now it’s a rather widely-adopted skill.

“I like to do it, see it, and feel it for myself,” Stuart said. Thermographers who truly understand the science are able to determine things a newbie tech with a thermal imaging camera in hand would not.

John Anderson is a thermographer with FLIR, another company specializing in this technology. He has nearly 25 years of experience in the building and construction industries, and like Stuart -- a wealth of knowledge on thermal imaging and infrared technology. Anderson compares trying to find moisture in a room without an infrared or thermal camera like walking around with a blindfold.

“You might get lucky and find the wet spots, or you may not,” he said. “Thermal cameras serve to guide the user to the cold spots usually associated with moisture.”

For restorers, there are several key areas where thermal imaging can help. Most commonly, thermal imaging is used to find the source of a roof leak, condensation issues when something within a building fails, leaking pipes, and other similar water issues or air flow issues. It could also be used at a fire damage job – tracing the mess left by the sprinklers or fire hoses.

“Any number of things can get moisture into those building materials, and that just creates another set of nightmares if it’s not taken care of properly,” explained Stuart.

He lives in the Seattle, Washington area, where moisture problems are abundant.

“A large portion of the old homes in the Seattle area don’t always perform that well,” said Stuart. “There are a lot of moisture problems because we just didn’t know how we were building things. The reasons for moisture can be very, very different, but the end consequence is moisture in the space, which can deteriorate the structure, and trigger mold growth.”

Thankfully, skilled restorers with a thermal imaging camera can detect problems early, before the black, moldy spots appear. However, that temperature difference does not always indicate moisture or water.

“It could be cool air from outside,” Stuart warned. “It requires some skill to determine if it’s one thing or another.”

“For example, some thermal imaging devices allow restorers to add a pin probe or surface moisture meter. That will give a clearer indication if a temperature change behind the wall is a gap in insulation, or a moisture problem. It can essentially back up what your IR camera is seeing.

“The main thing restoration contractors need to know about the science of infrared cameras is these cameras measure the heat radiating from an object, plus heat reflecting off that object from the area around it.” Anderson explained.

Anderson explains until the colder areas associated with moisture evaporation are then verified with a traditional moisture meter, one should never blindly assume that cold equals moisture.

Stuart says the cameras are “just a dumb tool, another passive set of eyes that see something in a different wavelength than our own.” After a while, however, a restorer’s trained eyes will notice patterns based on materials, construction techniques, and other factors that show up on a thermal imaging screen.

Plus, once you identify the problem and begin the remediation process, an infrared camera and imaging moisture meter can help monitor the process.

“The user can quickly see what areas need additional attention, redirect fans or dehumidifiers, and measure the process as it happens,” said Anderson.

Make sure YOU are the trained set of eyes

Proper education is key. Stuart says it’s up to trained professionals to train the end user so they don’t see a restorer with a thermal imaging camera in hand and think it’s magic. Anderson agrees, reinforcing that a lack of proper training can result in false or incorrect inspection reports.

He recommends at least one person in every organization utilizing thermal imaging cameras have a level one qualification (or higher) from the American Society for Nondestructive Testing (ASNT). The organization offers three levels of qualification by way of about 30 hours of class time, hands-on training, field time, and even application-specific training.

“For people who are really serious about it, try to get your level one qualification if you can,” Stuart said. “If not, at a bare minimum, have someone in your organization with a level one who can then be serving as the go-to person, and all the other folks in the organization.”

Stuart recommends everyone else in the organization at least have a thermal imaging for building applications course under their belts.

Basically, restorers need to understand how their camera operates and its limitations, basic theory and mechanics behind the infrared energy and what that temperature difference means, and knowledge of building structure to make proper diagnoses.

“You can get pretty good at it in a pretty short amount of time,” Stuart said. But in the same breath, he said going out and buying a cheap product without training will result in a disservice to your company and your customers.

Be aware of cheap alternatives

You’ve likely seen apps boasting thermal imaging technology, but Stuart compares that option to playing a Beethoven symphony on your cell phone, versus though a Bose surround-sound audio system.

Anderson warns of the same thing. He says the so-called thermal and infrared camera apps for your smart phone can’t actually see heat. Instead, these apps are design filters that change the colors produced by the camera.

However, some companies that make thermal imaging cameras do offer a separate camera attachment that would run through their own downloadable app. Just be aware that while this might be a good option for the initial step of adding thermal imaging to your tool kit, the quality won’t be as good as a standalone device.

“You don’t want a mechanic using tools from the five and dime, or tools bought out of a truck,” Stuart said. “You get what you pay for. With a lot of the consumer’s electronics-type stuff that’s popping up in the marketplace, the performance just isn’t there for restorers to properly diagnose what’s going on. As resolution and power decrease with the price, you lose the ability to see nuances.”

Once you have all the right knowledge to back up what you see through your thermal imaging camera, the sky is really the limit.

When the job is done, pull that camera back out and show the client or adjuster the new temperature reading – dry, fixed, good as new.


Black Mold

The news media and some contractors often refer to “black mold” or “toxic black mold.” It is usually associated with Stachybotrys chartarum, a type of greenish‐black mold commonly associated with heavy water damage. Not all molds that appear to be black are Stachybotrys. The known health effects from exposure to Stachybotrys are similar to those caused by other common molds, and ​again in high exposure situations (as in agriculture), are known to be associated with severe health effects in some people. Such exposures seldom, if ever, occur in buildings except during remediation activities by people not taking appropriate precautions.