FIGURE 1. Isopleth of germination – Aspergillus Niger.

Time is always valuable, but sometimes it’s more costly to hurry past items of concern. The author passes along some lessons from his expert witness experience, offers a primer on mold as the enemy, and relays some measures that might’ve kept things out of court.

An engineer, as an expert witness involving litigation arising from mold indoors, must be able to serve as an expert for either the plaintiff or the defendant. The challenge is to develop a convincing argument which considers the evidence and technical analysis to support a professional judgment either in favor of or against the parties involved.

Mold spores are ubiquitous and can become a major health problem indoors when they germinate, experience growth and, especially, when sporulation occurs. The foregoing is directly affected by the indoor climate (by air conditioning) if it supports one or more phases of the life cycle of a mold.

Overview of Mold and the Indoor Climate

Mold originates out-of-doors, produces spores, which become airborne and, eventually, enter indoor spaces. The spores, which are similar to seedlings, may germinate and grow indoors, repeating the mold life cycle. A common representative size of a mold is about 10 microns (0.0002 inches). A new life cycle originating from a mold spore requires a substrate containing nutrients, a suitable temperature, moisture, and oxygen. Normal indoor dust of an organic nature is minimally sufficient as a nutrient. Cotton or wool fabrics, wallpaper, the cellulose in wood and painted surfaces are all very adequate nutrients.

An overwhelming number of mold species will commence to grow above 32°F (0

FIGURE 2. Diagram of a common HVAC system.

Biological Aspects of Mold

Fungi are recognized as heterogeneous, non-photosynthetic, plant bodies. Eighty to 90% of the dry weight of a fungal cell is comprised of polysaccharides.

The fungal cell obtains nutrition entirely from organic sources such as living plants, cellulose (wallpaper, wood), paint, etc.

The germination of mold from a spore requires moisture in the form of free water, referred to as “water activity” by microbiologists. Once estab-lished, however, moisture in the surrounding air can be absorbed by the mold. A minimum rh of 75%, and primarily 80% to 95%, is required for growth and sporulation by many mold species. Xerophylic fungi are abundant in indoor dust where they facilitate the ingestion of human skin scales by dust mites -  which are yet another occasional source of allergy.

It is interesting that the germination and growth of fungi (mold) is affected by the simultaneous effects of both temperature and rh. As an example, hours for growth of Aspergillus Niger is as shown in Figure 1. The shape and the spacing of the isopleths would be similar, yet, different for other genera and species. As noted in the footnote (2) of Table 1, % rh (expressed as a fraction) and water activity, aw are practically interchangeable.

Fungal spores and conidia, which are similar to the seeds of flowering plants, range in size from 3 microns (0.00004 in.) to 200 microns (0.008 in.). Such spores and conidia are liberated into the atmosphere in the billions and remain airborne to be carried over large distances (miles) by prevailing winds. Nature has rendered spores extremely suitable for reproduction, survival, and dispersal. Spores are, typically, very dry and have minimal metabolic activity. It is suspected that some components on the surface of a spore may be relevant to the allergic reaction experienced by humans.

Among the common mold genera Cladisporium, Alternaria and Aspergillus are typical allergic irritants. Molds are able to quickly produce enormous quantities of an asexual spore (conidia). Threshold concentrations for allergic symptoms are estimated to be as low as 100 spores/m_ for Alternaria and as much as 3,000 spores/m_ for Cladisporium. In late summer and autumn, concentrations of such spores easily exceed these values out-of-doors.

Fungus that is observable to the eye is made up of a filamentous mass of hypha, the basic element which commences from a germinating spore. The mass of hypha, which constitutes the filamentous mass of the mold, is called the mycellium. The seedlings in the form of spores constitute the final phase of growth from a fruiting body in preparation for reproduction by the spores and conidia.

Molds are also the sources of some diseases in animals and humans - in addition to the known causes of allergic reactions. The secondary metabolites called micotoxins are produced chiefly by spoiled food. Moreover fungi in active growth produce volatile organic compounds (VOC), which are typified by a musty or moldy odor. The presence of an odor from a VOC, as above, is a most important indicator of the presence of mold and is annoying; however the possible affects of the VOC on health risks are uncertain.


Free water and moisture (e.g., as in humid air) are often the most critical elements in the life cycle of a mold and deserve special attention. The moisture content of wood, for example, is expressed as a percentage of the oven dry weight. Such expressions of moisture content do not represent the availability of free water necessary for the germination or growth of mold. Unbound water, by contrast, is expressed by microbiologists as water activity aw. It is expressed as

              Vapor pressure in the substrate
 aw =     Vapor pressure in the ambient atmosphere

Some representative aw, minimums, required to initiate growth of fungi are listed in Table 1.


Ventilation, as outdoor air containing moisture, is a building code mandated requirement for both humans and animals. During the warm seasons of the year the moisture content of outdoor air is, typically, elevated above the moisture content (humidity ratio) acceptable for indoor human comfort. An indoor level of rh in the range of 50% to 60% is often the design objective for indoor comfort and constitutes a safe margin below the minimum values of rh which would initiate the growth of mold.

Possible Contributions of HVAC for the Growth of Mold

The typical system of air conditioning for comfort is designed to control space temperature. The engineer/designer simply verifies that the resulting indoor rh is at an acceptable level - typically 50% to 60% at design conditions. The interior drybulb temperature and rh are seldom, if ever, investigated at other than the aforementioned “design conditions.”

If an A/C system is not designed to control indoor dewpoint and thereby limiting the rh in the space, then it is prudent to examine various non-design (“part load”) conditions to discover any possibility of rh levels approaching 100%. Recall that high rh, intended or not, could lead to propogating the entire life cycle of a mold. If conditioned air is inadvertently or deliberately allowed to pass through the attic, it could load yet to another problem - deposition of free water (condensation) on cold surfaces during the winter season. Exhaust of room air through the attic in cold climates is generally not recommended.

At the chamber immediately downstream of an AHU, during the various seasons, obtains very approximately the following conditions:

    a. Summer, all zones: 56° to 58°, 80%  to 90% rh
    b. Winter, interior zones: 56° to 58°, 30% rh (+)
    c. Winter, perimeter zones: 70°(+), 30% rh (+) [summer with
    dehumidifying coil; winter, no humidification]
    d. Winter all zones: 56° to 100°, 50% to 100% rh (+) [with in-duct
    steam/water humidification]

Systems a. and d., especially, require careful review to preclude conditions which would lead to a mold problem. System b. and c. do not automatically exclude having a mold problem, they are simply less probable than systems a. and d. A common A/C system employed with central station air handlers, packaged air conditioners, fancoils (heat pumps, classroom ventilators), etc., is shown in Figure 2.

Part-Load Analysis

It is extremely important that the designer of an air conditioning system in which rh is indirectly achieved - at full load - also investigate the rh that could result for a combination of indoor activity and outdoor climate at part load.

During the course of a recent litigation involving the growth of mold indoors, an analysis (investigation) demonstrated that the indoor rh would rise to 100% at part load conditions. Investigation and engineering analysis should have been performed for part load conditions of the A/C system being considered prior to releasing the plans and specifications; instead, that analysis was performed for the plaintiff following a consequential mold infestation, to support a million dollar claim for damages. The above refers to example number 3, which follows.

Examples of Mold Growth Observed

Here are several examples of mold growth, and the suggested solutions which would have diminished the probability of that growth.
  • Observation - Mold was observed to be growing, very visibly, on the surface of aluminum foil-faced insulation inside the plenum within sight of the dehumidifying coil of an air conditioner.
Suggested solution - Disinfect the entire surfaces of the air handler, plenum and ductwork - back to the air filter bank - install air filters of at least a MERV8 at 70% efficiency or opt for a MERV12 at 90% efficiency. Eliminate all standing water leading to drain.
  • Observation - Mold was observed to have suddenly blossomed at the inside surface of a 911 facility on the surface of a perimeter insulated concrete block wall. The mold growth had been caused by faulty controls of an in-duct steam humidifier.
 Suggested solution - Disinfect the concrete block wall and repair or replace the high limit in-duct humidistat which had failed to cycle the humidifier off at the high limit setpoint.
  • Observation - Refer to the part-load analysis, above, which was a classroom ventilators system having a throwaway air filter (probably less than a MERV1).
Suggested solution - Eliminate the ventilation function of the classroom “ventilator” and instead, provide outdoor air ventilation from a dedicated 100% outdoor supply system. The engineer would then obtain adequate filtration of mold spores and would need to coordinate the control of the outdoor air supply with the automatic control of the classroom “ventilator” - transformed into a room air recirculating air conditioner.


The possible growth of mold at the interior of a building, owing to the indoor climatic conditions of the HVAC system needs to be very carefully investigated employing competent engineering analysis. The proximate cause of the indoor climate, which could result in the occurrence of mold, must be established and avoided in the design of the HVAC system. This becomes especially imperative when litigation owing to mold growth takes place. The time and costs of engineering analysis before the design of an HVAC system is conceived and completed is orders of magnitude less costly than the financial costs of mold problems after the HVAC system has been installed. ES


  1. Christensen, C. M., Molds, Mushrooms And Mycotoxins, University of Minnesota Press 1975: 5, 11.
  2. Etzel, Ruth A. et. al, “Acute Pulmonary Hemorrage in Infants Associated with Exposure to Stachybotrys atra and other Fungi,” Pediatric Adolescent Medicine, Vol. 152, August 1998:737-762.
  3. Hens, H., Operating Agent (representative) Sourcebook, International Energy Agency, March 1991.
  4. Graveson, Suzanne, “Fungi as a Cause of Allergic Disease,” Allergy, 1979, (34):135-154.
  5. Lstiburek, J., “Understanding Attic Ventilation,” ASHRAE Journal, April 2006:36-43.
  6. Yang, Chen S., Eckardt Johanning, “Airborne Fungi and Mycotoxins,” Manual of Environmental Microbiology, ASM Press, Washington, 2002:839-852.