Biocides and Antimicrobials

Q: Why is it that people sometimes refer to the chemicals used to kill mold and bacteria as biocides, and at other times antimicrobials? I thought that there was a difference between the two.

A: The IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration currently refers to those chemicals that are used to kill microorganisms as biocides, and those that retard or suppress growth as antimicrobials. Biocides are sometimes referred to as bactericides or fungicides when they are designed to kill bacteria and fungi, respectively. In addition, antimicrobials are sometimes referred to as bacteriostats or fungistats when they are designed to inhibit growth of bacteria or fungi.

If you break down the term biocide into its prefix and suffix, you come up with "bio-," which means life and "-cide," which means kill. They are life killers. The suffix "-stat" means stasis or static. Often, the products referred to as fungistats only inhibit microbial growth and do not necessarily kill fungi. They maintain a static environment.

These definitions are consistent with the American Conference of Governmental Industrial Hygienists (ACGIH) publication titled "Bioaerosols: Assessment and Control." Note the following quotes from this publication:

"Biocides are toxic chemicals or physical agents capable of killing or inactivating one or more groups of microorgan-isms, that is, vegetative bacteria, mycobacteria, or bacterial spores; vegetative fungi or fungal spores; parasites; or vi-ruses." (16.1.1)

Biocides tend to be aqueous solutions (e.g., alcohols (ethyl, isopropyl), hydrogen peroxide, aldehydes (formaldehyde, glutaraldehyde), phenolic compounds, quaternary ammonium compounds (cationic detergents), halogens (chlorine, iodine, and bromine compounds)). Biocides are generally aqueous, and once they are applied or evaporate, they are no longer effective.

"Antimicrobial agents are chemical formulations incorpo-rated into or applied onto a material or product to suppress vegetative bacterial and fungal growth as it occurs (Table 16.1). Such compounds may be used to retard micro-bial growth in potential sources. Typically, antimicrobial agents are incorporated into products during manufacture (e.g., carpet material, ceiling tiles, and air filters). Additionally, antimicrobial agents are often included in products (e.g., paints, coatings, and sealants) that are applied to vari-ous building and equipment surfaces." (16.1.2)

"Bacteriostatic agent: chemical agent that suppresses or retards bacterial growth on direct contact with the treated material.

Fungistatic agent: chemical agent that suppresses or retards fungal growth on direct contact with the treated material." (Table 16.1)

Antimicrobials are often metallic compounds that continue to leave a residue that has prolonged effectiveness. Some are chemically bound so that they can be added as ingredient in paint, plastic or fiber production. Some antimicrobial fibers are used in garments and flooring materials.

Another well-respected publication, "Disinfection, Sterilization, and Preservation" by Seymour S. Block, has similar definitions. While the book uses similar terms, it also refers to biocides as antimicrobial in several instances. Here, a biocide is defined as "a substance that kills all living organisms, pathogenic or nonpathogenic." A bacteriostat is defined as "an agent, usually chemical, that prevents the growth of bacteria but does not necessarily kill them or their spores." Finally, an antimicrobial agent is defined as "any agent that kills or suppresses the growth of microorganisms."

This last definition is somewhat consistent with the way that the Environmental Protection Agency refers to biocides and antimicrobials. Interestingly, the EPA also refers to both biocides and antimicrobials as pesticides. On the EPA Web site at, under the heading, "What are Antimicrobial Pesticides?" it states that "Antimicrobial pesticides are substances or mixtures of substances used to destroy or suppress the growth of harmful microorganisms whether bacteria, viruses, or fungi on inanimate objects and surfaces." In some states you might be required to have a pest control applicator's license in order to apply these products legally in your customers' homes.

There is currently an effort to change the way that we, as an industry, refer to these chemicals. You may see that there is a change in the IICRC standards to using the EPA terminology. While I understand the desire to change to the EPA terminology, I do not look forward to these terms being substituted into the standards. They are more confusing to the reader than the terms that we have used until now. If there is a change to the new terminology, we will be referring to quaternary ammonium chloride - quats - as a biocide and as an antimicrobial or pesticide. On the other hand, there are those that will argue that the change to the EPA terminology will make it more consistent and ultimately less confusing. Time will tell.

Now, to answer your question: either term can be used. Whenever you read a document, look for any definitions that are provided in the text. Always read the document with these definitions in mind. If there are no definitions, look for the function of the chemical to determine its class.

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