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ArticleTechnical Article

What Germs Are We Killing? Testing and Classifying Disinfectants

By Allen Rathey

A single bacterium can become a million bacteria in just eight hours and — in an era of MRSA and other public health concerns — disinfectant products are also multiplying: germicidal sales are projected to be more than $1 billion this year with 8,000 disinfectant products now registered with the U.S. Environmental Protection Agency (EPA).


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The EPA closely regulates germicidal product claims, particularly where the target germs cause human illness. Since all germ killers are considered pesticides, the EPA requires testing to certify manufacturers' claims for safety and effectiveness.


EPA has set up test protocols for three disease-causing or pathogenic germs: Staphylococcus aureus (or "Staph"), Salmonella choleraesuis, and Pseudomonas aeruginosa. Disinfectants that kill all three of these bacteria are called "Hospital disinfectants." Products that kill only Staph and Salmonella are called "General disinfectants," and products which kill either one or the other — Staph or Salmonella — are referred to as "Limited disinfectants."


Product effectiveness is gauged by its ability to kill certain benchmark microbes. For example, if a germicide kills Pseudomonas, it's assumed that all weaker microbes will also be killed. There are no specific EPA-certified claims for killing some known disease-causing germs, but it's commonly assumed that if Pseudomonas dies, so does a slew of less-virile microbes.


Mycobacterium tuberculosis (the bacterium that causes Tuberculosis) — even tougher to kill than Pseudomonas — is another and more demanding benchmark.


Interestingly, even though Tuberculosis is not transmitted by surfaces, it is used as a benchmark for the effectiveness of hard surface disinfectants. According to the U.S. Centers for Disease Control (CDC):


Potency against Mycobacterium tuberculosis has been recognized as a substantial benchmark. However, the tuberculocidal claim is used only as a benchmark to measure germicidal potency. Tuberculosis is not transmitted via environmental surfaces but rather by the airborne route. Accordingly, use of such products on environmental surfaces plays no role in preventing the spread of tuberculosis.


However, because mycobacteria have among the highest intrinsic levels of resistance among the vegetative bacteria, viruses, and fungi, any germicide with a tuberculocidal claim on the label is considered capable of inactivating a broad spectrum of pathogens, including such less-resistant organisms as bloodborne pathogens (e.g., HBV, HCV, and HIV). It is this broad-spectrum capability, rather than the product's specific potency against mycobacteria, that is the basis for protocols and regulations dictating use of tuberculocidal chemicals for surface disinfection.

Label Deciphering

Specialized terms on germicidal product labels describe conditions of use that improve or impair disinfectant effectiveness. Here's what they mean.


Dwell Time: Many disinfectants work best — and live up to germicide label claims — when allowed to work for several to ten minutes before wiping or rinsing.


Organic Load: Disinfectants are deactivated by many organic soils — for example, blood serum, protein, food and body waste — and may require two applications to disinfect. The first removes the bulk of any organic soiling, the second application disinfects.


The statement "effective against stated germs in the presence of 5% serum or 5% organic load or bioburden" acknowledges that the product will still work well in the presence of a small amount of organic matter, but reminds end-users of the need for an initial cleaning with disinfectant to remove the worst soil. The statement "kills germs on precleaned environmental surfaces" is basically reiterating the point.


Water Hardness: Many disinfectants don't work well in hard water. The newest quats work fairly well in hard water, so a typical quat formula label might read "effective in 400 parts per million (PPM) hard water."


PPM (parts per million) Active Ingredients: This is an indicator of disinfectant strength. The more PPM of active ingredients indicated after the product is properly diluted ("in use dilution") often the more effective it is.

Slime Wars

Simply applying a proven disinfectant and allowing proper dwell time will not always kill the bacteria present, for example, under the rim of toilet bowls. Bacteria can produce a protective slime or biofilm which prevents disinfectants from getting in. Such surfaces need to be agitated with a brush or scrubber (or penetrated by heat from a steam vapor device) to break up the slime barrier.


According to Science News Magazine, "Scientists found Pseudomonas [embedded in slime] could survive in bottled iodine solution for up to 15 months."

The Right Mix

Getting the dilutions correct is not only important for proper disinfection--it is a legal requirement. That's why labels typically read, "It is a violation of federal law to use this product in a manner inconsistent with its labeling." The use and claims of disinfectants are regulated by the EPA under the Federal Insecticide, Fungicide and Rodenticide Act. To comply with that, it's important that disinfectants be used only as specified on the label and manufacturer's literature.

What Germs Are We Killing? Testing and Classifying Disinfectants:  Created on July 4th, 2005.  Last Modified on January 21st, 2014