Contaminated HVAC Ducts: Fungal Growth And Its Role In Indoor Air Problems


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As Indoor Environmental Quality (IEQ) professionals focus increasing attention on fungal growth and its role in indoor air problems, some people are looking at how fungi contaminate duct and duct liner. While most professionals advocate removing contaminated duct liner, recent studies indicate that this isn’t always done — and that contamination is more variable and harder to predict than previously thought.

One recent study, which looked at hundreds of duct liner samples taken from “problem buildings,” found that nearly half of the samples were contaminated. This presumably means that contaminated duct liner material was left in place until it either caused or contributed to ongoing IEQ problems — problems severe enough to trigger an investigation. The conventional wisdom is that dirt + moisture = fungal growth. However, another study by a major US environmental laboratory showed that some duct liner supported growth with just moisture. The same study also indicated wide variations in growth among different brands of similar insulation, as well as between many liners of the same brand, making it difficult to tell under what conditions building managers should consider removing the liner.

Bare Metal

In all fairness, even unlined ducts will support microbial growth if water and nutrients are present, leading some advocates of duct liner — most notably the North American Insulation Manufacturers Association (NAIMA) — to claim that the task is to focus on keeping ducts dry and clean, rather than pointing fingers at the lining material.

NAIMA, and others, point out that the insulation provides sufficient benefits to outweigh the dangers. However, this doesn’t mollify others in the indoor air community.

Growth on Various Liners

Another recent study examined the potential for fungal growth on various types of duct liner under different humidity conditions. The researchers looked at how the duct liners performed at what would normally be considered high humidity situations — between 85% and 97% relative humidity (RH). They studied the materials in high humidity, when wetted, soiled, and clean.

The materials they studied included:

Fiberglass duct liner- (FDL) A and FDL-C: >44%-98% fibrous glass, 1%-18% urea polymer of phenol and formaldehyde or urea-extended phenol-melamine-formaldehyde resin, <0><1><1% formaldehyde; and
Fiberglass insulation: 90%-95% refractory ceramic fiber, 0%-10% phenol formaldehyde.

New materials were purchased from commercial vendors. The researchers also studied used materials taken from noncomplaint buildings. The used materials were similar in appearance to the new materials, but researchers couldn’t determine their origin. First, the researchers looked at all five samples after placing them in an environmental chamber at 97% humidity and measuring microbial growth. Then, they wetted the samples to see what effect th

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