Bio-Contaminants in Water-Damaged Buildings

Aug 21, 2017

The following is a chapter I submitted for a workshop on water-damaged buildings, with a target audience of property managers.  It is a good primer on the contaminants found in water-damaged buildings.

Microorganisms are all around us.  In fact, they are in and on our bodies as well!  The vast majority of these organisms are harmless to humans, with many even having beneficial effects.  The human cells in your body are outnumbered by the bacterial cells in your body 10 to 1.  And some speculate that your human cells are outnumbered 100 to 1 when compared to viruses.  These mostly good microorganisms help our bodies function and are often referred to as our “microbiome”.

Even buildings can have a microbiome.  Normal types of bacteria and fungi are found in the air, dust, and surfaces throughout.  Many of these microorganisms are normal and expected to be found in homes, offices, schools, and other buildings. However, when water damage occurs, the indoor microbiome can change and have a negative effect on health.  

This chapter will describe the three most important microorganisms: fungi (mold), bacteria and viruses.

Fungi (Mold)

Mold belongs to the taxonomic kingdom of fungi. Fungi are not plants because they are unable to make their own energy via photosynthesis.  That means fungi don’t need light to grow.   Besides mold, other fungi include mushrooms and yeast. Fungi is a scientific term, whereas “mold” is more of a layman’s term.  Mildew is a term with several disparate definitions but is essentially the same thing as mold.

There are over a million different species of fungi with an estimated 30,000 types of mold. Mold gives off tiny spores, which are similar to plant seeds.  Once these spores land on ideal environmental conditions, they can grow into a colony (similar to how a seed grows into a plant).  Mold spores are ubiquitous in outdoor and indoor air, but because they are microscopic, you cannot see them.  They are typically about 2-10 microns in diameter, which is about 10 times smaller than the width of a human hair.  Even a normal home, office or school will commonly have more than 1,000 spores per cubic meter.

What do mold spores need in order to grow?  There are two primary environmental conditions: nutrients and moisture.

Mold needs nutrients to grow.  Just like there are enzymes in our digestive tract to break down food, each mold species has unique enzymes too.  Because of their enzymes, some molds prefer to grow on leather, whereas others prefer to grow on paper, leaves, drywall, soil, dust, or other organic materials.  Mold can grow in every building because the indoor environment is filled with organic materials.  Mold can even grow on an inorganic surface such as concrete by using nutrients from the paint or the dust accumulation on the surface.

Mold also needs moisture to grow. When organic materials are dripping wet, mold can certainly grow.  But what about environments where there is no liquid water, just high humidity?  There is a wide variety of humidity preference between the different species.  Relative humidity (RH) is used to measure moisture in the air and “equilibrium relative humidity” (ERH) is used to describe moisture within a material or substrate.  When a substrate such as drywall is in a room with 80% RH, it will generally approach an ERH of 80%.  Surfaces above 60% ERH are at risk for mold growth, but the vast majority of species require an ERH above 80%.

When a spore lands on a surface with the right nutrients and moisture, it will germinate, grow into a colony, and give off more spores.  That can happen as quickly as 24-48 hours for some species when conditions are ideal.  Therefore, it is important to dry materials quickly.  If mold grows but then dries out, it will stay dormant until more moisture comes back.  If the surface is dry for long enough, the colony and its spores will eventually die.  However, dead mold is still potentially allergenic and toxigenic.

The primary health concerns about mold are allergy and respiratory symptoms, such as asthma and cough.  Although there are no reliable numbers, it is estimated that about 15% of the US population has an allergy to at least one mold type.  Beyond these effects, mold can cause infections in immunocompromised individuals. These rare infections may happen in a hospital or nursing home, but some fungal infections can affect even healthy individuals (e.g. Valley Fever).  The most controversial effects of mold are the mycotoxins that some species can produce. The toxins are not released as a gas but are present on the colony and travel on the spores. There is a wide range of opinion regarding the exact effects of mycotoxin inhalation. Lastly, there are musty odors produced during growth that also have an effect on human health, although the effects are not well understood.

Although there are many species of mold, there are a few common types of which it is important to be aware. The following are water damage indicators which are uncommon in normal buildings, but common where there is chronic dampness:

  • Stachybotrys (sometimes referred to as “black mold” or “toxic mold”)
  • Chaetomium (pronounced K-toe-me-um)
  • Ulocladium
  • Fusarium
  • Trichoderma
  • Acremonium

Furthermore, some species of the following types can indicate water damage:

  • Aspergillus
  • Penicillium
  • Cladosporium


Bacteria belong to an entirely different kingdom than mold.  Their cells are unique in that they lack a nucleus.  Unlike other organisms, bacteria can be found in extreme environments from the tops of mountains to the bottom of oceans.  Although it is estimated that there are over 10 million species of bacteria, the vast majority do not cause human illness.  

The following are examples of building-related diseases caused by bacteria:

  • Legionnaires’ Disease
  • MRSA
  • Tuberculosis
  • Digestive tract and respiratory disease from fecal bacteria

With water damage, the primary concern is fecal bacteria, which are those species found in sewage.  Pathogenic fecal bacteria include:

  • Escherichia coli
  • Enterococcus
  • Fecal Streptococcus
  • Clostridium difficile
  • Leptospira
  • Helicobacter pylori

Besides the risk of infections from these fecal bacteria in sewage, certain bacteria produce endotoxins.  These toxins are found in the cell wall of Gram-negative bacteria, which can be commonly found in sewage.  When breathed in, endotoxins can affect respiratory function.  It is important to note that even when Gram-negative bacteria are killed, their endotoxins may not be diminished.  Therefore, it is critical to clean flood-damaged areas, not just use disinfectant.  

When areas are impacted with sewage, it is important to verify that fecal bacteria has been properly removed prior to reoccupancy.  The highest standard is to check surfaces for DNA related to fecal bacteria.  Culture plate methods may provide a “false negative” because the remnant, dead bacteria doesn’t grow on a petri dish (although its DNA could be detected).  Performing clearance testing puts people at ease when re-occupying a room impacted by raw sewage.


Viruses are the most unique microorganisms to be found with water damage.  Viruses do not have the means to reproduce by themselves.  They invade cells and commandeer the machinery to reprogram the cell to produce more viruses.  They can be 100 times smaller than a single bacteria cell as they generally range in size from .02 – 0.4 microns in diameter.  

There are a few pathogenic viruses associated with water damage:

  • Hepatitis A
  • Enteroviruses
  • Norovirus
  • Rotavirus

Viruses are very difficult to measure, so their impact on sewage-damaged properties is not fully understood.

Assessment and Remediation

When there is water damage, it is common to hire an industrial hygienist to assess environmental concerns before and after the remediation process.  The industrial hygienist can perform several functions, including:

  • Confirm mold growth or presence of other organisms
  • Identify the types present
  • Investigate the underlying moisture source
  • Determine the extent of the problem
  • Develop a scope of work for the remediation contractor to follow
  • Perform clearance testing when remediation is complete to verify the work was successful

Some states require that an assessment is performed both before and after remediation by a party independent from the contractor.  Although Illinois does not have such a requirement, it is still a good idea.

When selecting an industrial hygienist to assess biological contamination, look for certifications such as the Certified Industrial Hygienist (CIH) which has the most rigorous education, experience, and testing requirements.  Other acceptable certifications are from the organization American Council for Accredited Certification (ACAC).  Experience and knowledge are also important factors when selecting an industrial hygienist, as not everyone has a specialty in biological contamination.  


It is important to act quickly when there is water damage.  Delays can turn a small water problem into a large mold project.  If a water loss leads to biological growth, it is advisable to hire an industrial hygienist to evaluate the problem prior to remediation taking place.  Mold and sewage problems must be taken seriously as sensitive individuals may experience symptoms from the exposure. Being proactive can quickly resolve an issue that might otherwise lead to a drawn out lawsuit.