Within an indoor environment, excess moisture is a necessary precondition for microbial growth. Factors such as temperature, nutrient availability, and oxygen level influence what mold and bacteria will grow. But without dampness, no growth can occur—it’s just too dry.
The level of moisture is described by a measure called water saturation. Water saturation—which is also expressed as aw—is a technical term to quantify the moisture of a building material. aw is defined as the ratio of the vapor pressure exerted by water in the material over the vapor pressure exerted by pure water.
Molds and bacteria have requirement water saturation levels for growth. For a given mold, there is a minimal requirement for aw that, below which, it will not grow. In most cases, the mold requires more than the minimal aw to grow optimally and to produce mycotoxins.
Water saturation is a helpful concept for classifying molds because they grow and thrive at different levels of water saturation. For reference, here are some aw values for context:
- 0.61 — the minimal level at which fungi grows
- 0.61-0.8 — growth conditions for xerophilic, or “dry,” molds
- 0.8-0.9 — growth condition for slightly xerophilic molds
- > 0.9 — the minimal level at which bacteria and water-loving (hydrophilic) molds grow.
As one researcher wrote,
“Properly identified fungi can tell a story. Fungi are no different from plants. Water-loving plants, such as water lily or cattails, grow only in water or swamps. On the other hand, cacti grow in dry arid environments. Identifications of moisture-loving fungi suggest the environment was wet or water damaged.”
With sustained water-damage, there is usually a progression of growth starting from the xerophilic molds and progressing to the hydrophilic molds. Due this sequential growth, the xerophilic molds are also known as primary colonizers, the slightly xerophilic molds are called secondary colonizers, and the hydrophilic molds are termed tertiary colonizers.
Each round of colonization may feature harmful molds that can cause inflammation, toxicity, and other types of injury. Here are fungal examples of colonizers:
- Primary colonizers Penicillium, Aspergillus, Wallemia
- Secondary colonizers Alternaria, Cladosporium, Ulocladium
- Tertiary colonizers Chaetomium globosum, Fusarium, Memnoniella echinata Stachybotrys chartarum, Trichoderma
Secondary and especially tertiary colonizers may indicate moisture or chronic condensation in a building. High water saturation may be associated with greater human hazard because there is sufficient moisture to promote growth of all three types of colonizers, as well as enough water to support their mycotoxin production.
Moreover, particularly dangerous new players enter the game at high water saturations.
Stachybotrys, also known as “toxic black mold,” is hydrophilic and makes dangerous toxins such as tricothecenes which causes lung bleeding in infants.
Bacteria, another Kingdom of microbes, start their growth at the same water saturations as the hydrophilic molds. Actinobacteria proliferate within damp buildings which is significant because they are more inflammatory than all molds, even Stachybotrys.
The microbes may be battling each other for wet turf, but they collectively contaminate the air and synergistically amplify the human health hazard.