When the Curtain Falls
Mold is a form of fungus that is ubiquitous on earth; there are many species. Mold plays a vital role in biodegradation and the decay of organic materials in the biosphere. As part of their consumption of organic carbon, molds secrete enzymes and acids that degrade organic material. Prolonged exposure to molds can cause material damage and can render items unusable. Also, some mold species create toxins that if ingested or inhaled can injure humans. Mold spores are always in the air and are deposited onto surfaces as the air moves. Should a mold spore land in a location that contains organic carbon and sufficient moisture it will begin to grow and reproduce. The reproduction and spread of mold are further aided by darkness, relative humidity in excess of 60%, and temperatures between 77°F and 86°F. EDT Engineers often become involved when the question becomes what created the environmental conditions that promote mold growth? Where did the water come from?
As the pandemic has resulted in the shuttering of public venues, many owners of properties have turned off the lights and left the spaces conditioned with an eye toward energy (cost) savings. This has left the spaces dark, and unobserved for long periods of time. One such venue was a large movie theater complex. The complex was vacated at the order of the state and was uninhabited for months. On a random walk-through, personnel found biological growth blooming on upholstery and carpeting. The cause of the biologic growth was a mystery since the theater had no history of elevated moisture in the past and had never experienced biologic growth when occupied.
Figure 1 is a schematic of a typical air conditioning system with an economizer. Commercial air conditioning systems are designed with a minimum amount of fresh air to maintain indoor air quality. Some commercial systems employ a fixed damper system that discharges between 10 and 15 percent of the space air and fans make up the air with fresh air (outside air). Many systems also blend in additional fresh air when the outside air temperature is low, providing free cooling. The system in the theater was of this type, and on cool nights in the summer, it would draw large amounts of very humid air into the theater. Because the theater was unoccupied, the temperature in the space was not controlled for comfort, but rather for energy savings and the cooling coils ran very little during the day, increasing the time and severity of the high-humidity events. This provides a dark, moist environment that creates conditions for biological growth, but what about a food source?
Theatergoers like snacks, and snacks tend to be spilled.
The photographs show that the biological growth is active in areas of past food and drink spills. While the owner had cleaned these fabrics in the past, there was sufficient organic carbon residue in these locations to support biologic growth.
It is easy to see the patterns of past spills on the upholstery and carpeting in the theater. So in this case, by increasing the space temperature, and the amount of humid outside air, the perfect situation for biologic growth was present in the theater.
The US Environmental Protection Agency (EPA) recommends that to control mold in commercial buildings that the relative humidity (RH) be controlled to less than 60%, with ideal RH in the range of 30 – 50%. EPA further recommends that air handlers be well maintained with free-draining drip pans and regular inspection of internal components for cleanliness. Per the publication “Mold Remediation in Schools and Commercial Buildings”, attached as an appendix, “The key to mold control is moisture control”.
Owners of large public venues should heed the guidelines of the EPA with respect to controlling moisture inside commercial buildings. A plan for controlling humidity in these buildings should be part of the long-term shutdown. It is permissible during times of limited occupancy to disable fresh air systems, and it might serve owners well to disable economizers during the summer months where high humidity exists outdoors.
 Economizers reduce the need for cooling by introducing cool outside air to air-conditioned spaces, saving energy.
About the Author
Rod Turk, P.E. is a Consulting Engineer in our Charlotte, NC Office. Mr. Turk provides consultation in the areas of assessment of industrial and utility incidents. These assignments include damage assessment, root cause investigation, valuation of loss, and project management for repairs. You may contact him for your forensic engineering needs at email@example.com or (704) 523-2520.