Why Are Biocides Not Recommended for Mould Remediation?

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The economic importance of biocides cannot be overemphasized.

The world demand for these chemicals is projected to increase by 5.6% annually to $5.9 billion in 2006. Questions always arise as to whether biocides should be used or not in indoor mould remediation projects.

This article is an attempt to explain why biocides may not be recommended for indoor mould control.

First, what are biocides?

Biocides are Toxic

Biocides are Toxic

Biocides are a broad class of inorganic chemicals and sometimes of biological origin intended for control of pests, insects, bacteria, fungi (moulds) and other micro organisms in non agricultural sectors. They are similar to plant protection products (commonly known as pesticides).

Biocides have a wide range of applications such as:

  • disinfectants or sanitizers used in hospitals, restaurants, homes, laundries, carpets and drinking water for infection control,
  • preservatives for materials/products sold in tins (e.g. paint); paper; textiles; leather, metal working fluids; timber and wood composites and other natural and man-made materials,
  • antifoulants for inhibiting growth of organisms on the hulls of ships, on fishing nets and lobster pots,
  • pesticides used to control/kill pests such as rodents, insects, etc.

How are biocides classified and how do they control mould growth?

We shall discuss biocides with respect to control of moulds in indoor environments. Biocides differ in chemical class and general and specific modes of action. A biocide’s mode of action can be described in general or specific terms. A biocide with broad spectrum activity would be effective against a large variety of moulds. Copper based biocides are examples of broad-spectrum biocides. Some biocides may have a very narrow spectrum of activity (i.e., they are effective against a single or a few moulds).

Alternatively, a biocide may affect a broad range of moulds but by only a specific mode of action. A biocide’s mode of action can also be described as protectant or as an eradicant. Protectant biocides may be applied to prevent mould spores from germinating or penetrating the material to be protected. They must be applied before the mould spores have a chance to germinate.

Protectants generally are not effective once mould growth has occurred. Eradicant biocides can kill mould that has already infested the material. Lime sulphur is an example of eradicant biocide that acts by killing moulds on contact. Biocides that kill mould are said to be fungicidal. Some biocides only inhibit mould growth rather than kill them and are said to be fungistatic. Fungistatic biocides must be applied repeatedly to suppress mould growth.

Biocides kill or prevent growth of micro-organisms in a number of ways. Broad spectrum biocides may act at several sites within the cell such as the cell wall, cell membrane or other cell contents. Their action may include disruption of cell membrane, inactivation of a broad range of enzymes, denaturing of cell proteins and coagulation of cell contents. Selective biocides have specific effects on cells such as inhibition of cell growth.

Are biocides recommended for indoor mould control?

Biocides are generally not recommended for indoor mould control primarily due to health concerns. However, careful selection and use of these products may be needed in some situations, for example where immuno-compromised workers are involved or to eliminate infectious agents from grey/black water contamination.

Current mould remediation guidelines such as those by the Canadian Construction Association, the Environmental Abatement Council of Ontario (EACO) and the New York City Department of Health also do not recommend use of biocides in mould remediation. Also, the U.S. Environmental Protection Agency (EPA) does not “approve” biocides for mould remediation applications and cautions against using disinfectants and sanitizers in ventilation systems.

Some reasons why biocides are not recommended for indoor mould control or remediation include:

  • Allergenic spores and mycotoxins do not require the mould to be alive to be a health hazard.

It is widely recognized that moulds do not have to be alive to cause allergic, toxic, or inflammatory responses to individuals at risk. While application of biocides may stop further mould growth, there is additional health risk due to biocide application. The best approach to limit mould exposure is to reduce the level (amount) of mould by using other recommended methods and subsequently controlling factors that favour growth.

  • Biocides may not be completely effective against indoor moulds

Microbial growth on building materials may be controlled by using various biocides. However, different microbial genera have been shown to have considerable variation in their sensitivity to biocides. The variation in efficacy of biocides against different micro-organisms suggests that it may not be possible to completely prevent the microbial growth on building materials and thus their incorporation should be carefully considered and tested.

Being an enclosed system, the indoor environment may not allow application of protectant biocides due to health concerns as discussed above. For most eradicant biocides to be effective against mould, they must get into contact with the mould. However, since mould is capable of growing deep inside their substrates (in this case building materials), it may be difficult to ensure the biocide has come into contact with the entire mould. Only the mould on the surface of the infested material would be killed. A recent study indicated that incomplete control of Stachybotrys chartarum resulted to production of spores of higher toxicity than those spores from untreated mould.

What effects to human health and the environment are produced from biocides exposure?

Most biocides that would be effective against mould are highly toxic and if used in indoor environments may pose serious health effects to the occupants. The risk to human health and the environment of continuous use of some biocides in indoor environment is not well documented and could be very high.

Biocides are a hazard to aquatic life

Biocides are a hazard to living things

Human exposure to biocides applied in indoor environments may occur through dermal, inhalation or ingestion routes. Symptoms of biocide poisoning may include headaches, vomiting, stomach aches and diarrhoea.

Children, because of their particular physiological and developmental factors would be particularly vulnerable to biocides. Other population groups at risk include the elderly, the immuno compromised and the chronically ill.


Biocides are definitely useful in many areas but their use in indoor mould remediation is limited by their toxicity. In situations where one is dealing with pathogenic moulds, use of biocides may be recommended. However, before using any biocide for indoor mould control one would need to consider several factors including:

  • the occupants’ health status and risk of exposure
  • the short and long-term health effects to the occupants
  • exposure control procedures and their effectiveness
  • the efficacy of the biocide to the target mould
  • potential for personnel and environmental harm.

Additional information

  • To see guidelines for interpreting non-viable air sample results click here.
  • To see guidelines for interpreting viable air sample results click here.

To get hands-on experience on the application of these guidelines register for one of our mold & bacteria online training programs today!

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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.

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