by Patrick J. Rafferty, MSPH, CIH

The Problem

Mold is everywhere.  We have lived with it for millennia.  It grows on tree leaves, in potted plants, on old bread, and it turns milk products into fine cheese.  But mold is also in the headlines.  It’s in our basements, in our air conditioning systems, and may be hidden in the walls and ceilings of our homes and offices.  Buildings are being evacuated, schools closed, and homes bulldozed.  Naturally, litigation abounds.

It is widely recognized that mold growth in the indoor environment can cause allergic reactions in persons who are allergic to mold, and that it can trigger asthma attacks in persons who suffer from this chronic condition.  But it wasn’t until 1994 that indoor mold grabbed the public’s attention, when Dr. Dorr Dearborn, a pediatric pulmonologist in Cleveland observed ten cases of babies with unexplained bleeding in their lungs.  Dearborn and others suggested that  this condition (“acute idiopathic pulmonary hemorrhage”) was possibly related to mold and tobacco smoke in the babies’ homes; however a subsequent investigation by the Centers for Disease Control concluded that the link was not backed by sufficient evidence from which to draw conclusions.

The fear in Cleveland was that the babies may have been affected by toxic substances produced by some molds.  Some species of fungi produce chemicals known collectively as mycotoxins, which give the species a competitive edge against other microorganisms.  There are over 200 recognized mycotoxins, the health effects of which are highly variable yet poorly understood, and are said to range from immediate toxicity to certain cells to cancer.

Because of the potential for health, comfort and aesthetic concerns, building owners and managers should work to prevent moldy conditions and to investigate and correct them if they are suspected.

Conditions That Lead to Mold Growth

Mold growth in buildings is frequently the result of an accidental release of water into the built environment.  Releases can be sudden, as in a sprinkler release, pipe burst or flood, or they can be chronic, such as water infiltration in a sub-grade space or roof or window leaks.  Mold can grow on wet porous building materials (e.g., gypsum wallboard, carpeting and ceiling tiles) or furnishings (e.g., upholstered furniture, cubical dividers, and documents) if they are not dried out immediately.  According to the US EPA, if porous building materials and furnishing are not dried completely in 24 to 48 hours, they are likely to create conditions favorable for mold growth.

Another cause of mold problems in buildings is poor design, installation, operation or maintenance of ventilation systems.  One common problem involves cooling coils found in air-conditioning units.  These coils condense moisture out of the air, which should be collected and disposed by a drainage system.  When the drainage system is improperly designed, installed or maintained, water accumulates and the porous insulation in the unit becomes wet and moldy.  Another common problem is that systems are not designed or operated to control humidity in the occupied space, and excess humidity condenses on cold walls, causing chronic moist conditions and leading to mold growth.

What is Mold?

Before discussing the means by which mold growth is identified and removed, it is important to understand some basics in the science of mycology.  The term “mold” is a layman’s term for microscopic filamentous fungi.  Let’s take that one word at a time.  Every one is familiar with macroscopic fungi, such as mushrooms prepared for the table.  Mold is in a sense the microscopic version of a mushroom.  Like mushrooms, most molds consist of a fruiting body (the most obvious part of the organism), a root system, and very small seeds known as spores.  Filamentous refers to the extensive root system that microfungi send into whatever material they are growing on so they can soften and digest the material.  Fungi refers to organisms that are saprophytic, meaning that they use organic matter created by other organisms as a food source.

Mycologists (scientists who study mold) have estimated that there are hundreds of thousands of species of mold, each having its own preference for moisture, temperature and food source.  Under the right conditions nearly any organic matter can serve as a food source for some fungi.  Mold comprises a significant part of the world’s biomass, and grows on all live plants, dead plant and animal matter, and in soil.  As spores are blown about by the wind, they are almost always found in indoor and outdoor air, and they are a normal component of house dust.

Buildings are unavoidably maintained at a temperature conditions appropriate for the growth of most fungal species.  Almost all buildings contain organic material that is of interest to at least a few species of fungi, such as gypsum wallboard, ceiling tiles, and books.  Most buildings also contain accumulations of dust on which mold can feed.  Fortunately all molds need a significant amount of moisture in order to grow.  Thus, it is only by controlling the moisture content of building materials and furnishings that mold growth can be prevented.

Investigation of Mold Problems

Identifying and correcting mold problems in buildings is of vital interest not only to the occupants, but to employers, building owners and managers, insurers and lenders.  An investigation should be started if a characteristic mold odor or visible mold is observed, if water intrusion is noted, or if occupants report mold-related health effects such as allergies and asthma that may be related to the building.

A mold investigation should be designed to determine the cause and extent of the problem, and to develop a strategy for correcting the problem and preventing it from happening again.  The investigation begins by gathering background information to identify the possible causes of mold in the building.  This may include interviews with current and former occupants, property management and maintenance personnel, contractors who work on building systems, and in some cases, persons involved with design and construction of the building.

This is followed by a detailed physical inspection, known as an “informed inspection” because it concentrates on those areas with known or suspected water impact or other mold-related problems based on interviews.  An informed inspection may include looking into hidden areas, such as cavities behind walls and above ceilings.  The inspection consists of a detailed visual observation, by a trained eye, aided with a flashlight and perhaps a magnifying glass, of materials that may house mold growth.  Frequently, mold growth can be seen by the naked eye.  However, in some cases the naked eye might not be able to see mold growth, such as on carpet and other textured surfaces.  In these cases, samples may be collected for confirmation, as discussed later.

The air conditioning system should also be physically inspected for the presence of visible mold and conditions likely to lead to mold growth.  In the case of water inundation or flooding, the observations of persons who witnessed the flooding are important in understanding which materials were impacted by water. In addition, moisture measurements may be made on porous materials to delineate the extent of water impact.

Temperature and humidity measurements of the air in the occupied space may be collected during the investigation.  Similarly, if historical data are compiled and maintained, these may be of interest in evaluating the performance of the air conditioning systems in maintaining appropriate humidity levels in the building.  Surface temperature measurements of walls and other surfaces can assist in the evaluation of potential condensation on these surfaces.

Sample Collection

In some cases, samples will be needed to complete the investigation.  One type of sample is the tape lift sample, which is collected by placing a piece of cellophane tape on a surface containing discoloration suspected of being mold.  This type of sample is collected to see if what appears to be mold to the naked eye can be confirmed by a trained mycologist using a microscope in a laboratory.  This evaluation can usually identify the mold to the genus level, and can indicate the presence of spores, fruiting bodies, or roots.  Thepresence of fruiting bodies or roots suggest mold growth on the material, whereas the finding of only spores may suggest that the spores have settled on the surface from some other source.

A second type of sample is a bulk sample, in which a piece of building material or furnishing suspected of housing mold growth is cut out and sent to a laboratory.  In the laboratory, the sample can be either inspected under the microscope as with the tape lift sample described above, or subjected to culture analysis.  In culture analysis, the sample is placed in a growth medium (agar) and incubated for about a week.  At the end of the incubation period, the agar is inspected by a trained mycologist, who lists the fungal species observed and estimates the number of organisms present.  The result of this observation is usually expressed in colony-forming units per gram of material, as a general indication of the amount of fungal material present.  Samples that show large numbers of organisms (of the order of a million or more colony-forming units per gram) or those that show a single organism indicate that the material is moldy.

Wipe (or “swab”) samples are collected by some investigators; however this type of sample is difficult to interpret.  Wipe samples are collected by wiping a smooth surface with a cotton swab, which is then placed in growth media.  After incubation, the media is evaluated to determine the fungal species observed and the number of colony-forming units observed per square inch wiped.  In general, samples that show large numbers of organisms or the presence of a single organism suggest that the surface is moldy.

Samples of settled dust are sometimes collected to see how much and what types of fungi are in the dust.  This is important, for example, in areas with heavy fungal growth on walls or in air-conditioning systems, where the spores may have accumulated on surfaces.  Dust samples are subjected to culture analysis and are reported as colony-forming units per gram of dust.  The results are evaluated to determine whether the fungal content of the dust is different from that which would be expected under normal (not moldy) building conditions.  Both the number of colony-forming units and the relative abundance of species are important considerations in interpreting the results.

Air samples are sometimes collected as part of an investigation to determine if hidden sources of mold are present in a building.  It should be noted that this technique, as normally applied, is not able to positively rule out the absence of hidden mold growth; however, it can sometimes identify mold growth where it is otherwise not apparent.

The simplest type of air sample is the spore trap technique, in which a known quantity of air is drawn across a sticky surface.  Spores in the air adhere to the sticky surface, which is later examined under a microscope to determine the genus of fungi and the number of spores in a given volume of air (usually a cubic meter).  A more sophisticated approach is the viable impaction technique, in which sampled air is drawn directly onto a growth medium on a Petri dish where it is impacted.  The medium is incubated and examined to determine the number of colony-forming units and to characterize the fungal species that have grown.  The results are expressed in terms of colony-forming units per cubic meter of air.

It should be noted that the spore technique measures the concentration of all fungal materials present, regardless of their ability to reproduce.  The viable impaction technique measures only those fungal materials capable of reproducing.  Air sample results are interpreted by comparing the results of samples collected indoors with those collected outdoors on the same day.  When the results of indoor air samples show higher concentrations than the outdoor air, or different species, an indoor source of mold is suggested, and further investigation or remediation may be warranted.

Remediation of Mold Problems

Once an investigation has identified the nature and extent of a mold problem and its cause, a strategy for correction and remediation should be developed.

The first objective is to “fix the leaks” or correct the moisture problem that led to the mold growth in the first place.  This is the single most important objective of a remediation effort.  This might be as simple as fixing a roof leak or correcting a drainage problem.  In other cases, architects or mechanical engineers may need to investigate to get to the root cause of the moisture problem.

Next, moldy porous materials, such as carpeting, ceiling tiles, insulation and gypsum wallboard, are removed and discarded.  In some cases they may be laundered, dry-cleaned or, in the case of irreplaceable art objects or books, thoroughly cleaned by a conservation specialist.  Once the moldy materials have been removed, the area is inspected to see if there are signs of additional mold or water damage revealed during the demolition process.  Any new mold found must also be removed.

The removal of moldy materials must be done without spreading fungal debris into other areas.  This is normally done by using specific work practices and engineering controls designed for this purpose.  Work practices that minimize the release of spores consist of the careful handling and disposal of moldy materials.  Engineering controls range from contact paper used to cover a sheet of wallboard during removal to the construction of high-level isolation and decontamination systems.  Workers removing mold are protected by personal protective equipment such as respirators, gloves and disposable coveralls.

Next, other porous materials in the area that may have accumulated fungal debris are thoroughly cleaned using techniques appropriate for the material.  This typically involved vacuuming with vacuum cleaners equipped with High-Efficiency Particulate-Air (HEPA) filters. Nonporous surfaces that may have been affected by mold growth or surfaces on which spores may have settled are cleaned, usually by HEPA vacuuming and wet wiping.  Some contractors use a bleach or biocide solution, but the effectiveness of these is not always known, and the solution may damage surfaces or leave a hazardous or unpleasant residue.  Biocides are not normally used in ventilation systems.

The effectiveness of the mold removal and surface cleaning is usually assured by a knowledgeable third party, who verifies that the moldy materials are removed and that the controls and work practices are implemented as planned.  In addition, a final visual inspection is conducted to assure that visibly moldy materials and visible dust have been removed.  The visual inspection can be followed by the collection of dust samples, which are weighed to determine whether or not dust has been removed.  This can be supplemented by mycological analysis of the dust sample.

It is important to keep in mind that the objective of remediation is not to try to sterilize the building.  The goal of the remediation project is to reduce the levels of microorganisms to background levels; that is, the levels found in normal buildings and building materials.

A final verification step frequently used is air sampling.  Air samples can be collected either immediately after the remediation project, or after the area is repaired or re-built.  There is considerable debate among health professionals as to what types of samples should be collected and how results should be interpreted.  Some professionals compare the number and types of colony-forming units indoors with those outdoors; others use the concentration of a single species if a single species dominated the moldy materials in the first place.

The type of samples collected is a matter of debate and some importance.  What is important is that the source of moisture or water incursion is identified and corrected, and that moldy materials and spore-laden settled dust is removed prior to re-occupancy.