A Rational Approach to Hard Surface Disinfectants

The process of hard surface disinfecting is an integral component of a comprehensive cleaning protocol. Its use and purpose is twofold: First, to lower the initial bioburden of a contaminated surface and render it “safe”, generally from a public health perspective. Secondly, as a cleaning adjunct to ensure that the final bioburden is several orders of magnitude lower than can be reasonably expected by the cleaning processes. This is the purpose for which disinfectants are most often used in clean room applications.


Disinfection, by its classical definition describes a process that eliminates many or all microorganisms on inanimate objects with the exception of bacterial endospores. Although disinfectant chemistry and environmental microbiological techniques have broadened this definition, it nonetheless remains part of a cleaning process. Even with a renaissance in the art and science of disinfecting, and particularly with the advent of the clean room, this portion of the cleaning process remains somewhat of an enigma. Oftentimes we do not set priorities on either prevention or on the objectives of cleaning when a disinfectant agent is involved. To this end, it is safe to say that no other economic poison is so poorly understood, oftentimes so over-used and otherwise so universally inappropriately applied as a hard surface disinfectant. Product testing of hard surface disinfectants for labeling requirements does not necessarily mimic conditions in the field, and yet we rely on these agents to grant us freedom from harm or litigation without assessing their role as other than a single component within an environmental microbiological process.

Our expectations of hard surface disinfectants are sometimes unrealistic. For instance, we somehow expect disinfecting agents to provide a “degree” of sterility as a remedial measure; with zero risk of recovering a viable microorganism from the disinfected surface. We deal in probabilities of survival and deny that any survival is permitted. The “cidal” activity plots of disinfecting agents on semi-log charts do not have a “zero” value, yet we insist on zero chances of survivability; if survivors are found, it is considered a “failure”. We concern ourselves with the survival of an occasional spore, but assess the need for Disinfection as a function of expediency, realizing that we do not have to destroy all spores. And finally, we disinfect objects to the nth degree, only to expose them to a contaminating environment and blame the process if a viable organism is found.

All of this does not imply that hard surface disinfectants are bad things or that our expectations are totally unrealistic. Quite the contrary; they are a most valuable tool when used within the proper context of cleaning. Disinfecting agents, when used appropriately can significantly reduce the risk of carrying a viable microorganism on the cleaned and treated surface. Of greatest importance however, is to consider the primary objectives of contamination control. The first objective is to keep the contaminants out. This means that every effort to prevent initial contamination of the hard surface will determine the final bioburden. The second objective is to eliminate the contaminants that found their way to the hard surface, in spite of all preventive efforts. This objective characterizes Disinfection. What this means in terms of actual practice is the following: As a rule of thumb, cleaning will reduce the microbial numbers by approximately one order of magnitude. Applying a disinfecting agent to the cleaned surface under “standard conditions”, will further reduce the bioburden by an additional five orders of magnitude; this is the number most disinfectants must meet in AOAC testing protocols, assuming a contact time of 10 minutes at a recommended use concentration. Therefore, the more effort and care put into the initial cleaning process with a hard surface, the greater is the probability of not recovering a viable microorganism after treatment. For this reason, it is imperative to establish cleaning objectives and priorities. In other words, determine how clean is clean before using a disinfecting agent.

Not all hard surface disinfectants are the same. Aside from the numerous chemistries involved, ranging from a multitude of complex phenols, quaternary ammonium compounds, and the halogens, to various simple alcohol formulations and hydrogen peroxide at the other end of the spectrum, all hard surface disinfectants are applied to an already clean surface. Compatibility with that surface as well as with the cleaning agents that were used to prepare that surface, is an important consideration, which is not often overlooked. What we rarely consider however, is the potential for leaving behind an unwanted residual on the treated surface. The residual of both the actual disinfecting agent and the “inert ingredients” that make up the finished product can compromise overall contamination control efforts. For instance, if the disinfectant requires removal after its application, is the process of removing what is left of the chemical, re-depositing unwanted microbes? Consider also that most manufacturers of liquid Disinfectants use only a softened (non-DI) and minimally filtered (generally 20µ to 50µ) water as the inert ingredient. Will the deposition of salts and/or other particulates have a deleterious effect? If the answer to these questions is in the affirmative, consider minimizing the use of liquid disinfectants in favor of concentrating on the initial cleaning protocols, augmented by a program of aggressive microbiological monitoring until reasonable levels of confidence are established. There are numerous disinfectants available that are tailored for the clean room industry, that is: particulate-free. But, as with any disinfectant, it is not a substitute for an initial and comprehensive cleaning program.

We also often forget that hard surface disinfecting agents are a class of economic poisons, which are controlled in much the same manner, and under the same set of regulations as herbicides and pesticides. That which is toxic to microbes, may also be toxic to other biological systems. It is therefore prudent not to overkill with a disinfectant. Rather, select a disinfecting agent that demonstrates the lowest level of toxicity. For instance, if the goal is to eliminate vegetative bacteria and fungi, it is not necessary to use a toxic agent that will kill mycobacterium such as M. tuberculosis or bacterial spores. A disinfectant selected against an organism of known resistance, will be effective against organisms lower on the resistance scale. Other considerations in choosing a disinfectant include the number and type of organisms to be destroyed as well as numerous environmental conditions such as temperature, pH of the substrate, and realistic contact times. Physical or chemical factors in the surrounding environment being treated, such as compatibility with other cleaning agents, stability and solubility of the disinfecting compounds also deserves equal consideration. In this science, “one size does not fit all” and selecting a single disinfectant for all applications may actually hinder the cleaning process or add an additional cost-factor with little or no benefit.

In selecting a disinfecting agent, read beyond the manufacturers’ literature. After all, they need to sell product in an unbelievably competitive market. While no one will readily admit to problems encountered with these compounds, virtually everyone will crow with a success. Don’t believe everything you read or hear; myths and legends abound. For instance, there is no evidence in the scientific literature that microbes develop resistance to hard surface disinfectants. Most of these products kill by coagulating protein in one form or another. The building blocks of life have not advanced to the stage where this mode of action is reversible. What generally happens when the bioburden seems to rise is either ennui on the part of the workers in handling the product in current usage or a change in the soiling type and lowered solubility with the disinfectant. It is also a fact that most disinfectants are not wholly compatible with surface-active agents such as detergents (there are notable exceptions) and cleaning may not be as effective in removing the initial soiling if a combined product is used. Remember too that all microbial decay takes place in a logical manner. The portions of the number or organisms that are destroyed are constant with time. Natural decay occurs anytime when the microbe is removed from its environment. Therefore, “lethal” versus “natural’ decay is a matter of degree. The results you get in comparison testing may be nothing more than a natural phenomenon under the guise of a lethal treatment. Finally, disinfectants do not require pre-sterilization. After all, they are designed as cidal agents. If the disinfecting agent is capable of supporting target organisms and requires additional bio-cidal treatment before use, one should seriously question its efficacy as a hard surface disinfectant and select one with enhanced cidal capabilities.

In summation, hard surface disinfectants are an integral part of a cleaning protocol. Their use and efficacy are predictable and their selection should be based on good science. Establishing well-defined microbiological end points through cleaning; with toxicity, economy and ease of use as relevant factors, will make that selection a valuable tool in an overall program of contamination control.


Reprinted by permission of R.W.Powitz & Associates, PC.

Forensic sanitarian Robert W. Powitz, Ph.D., MPH, RS, CFSP, is principal consultant and technical director of Old Saybrook, CT-based R.W. Powitz & Associates, a professional corporation of forensic sanitarians who specialize in environmental and public health litigation support services to law firms, insurance companies, governmental agencies and industry.


For more than 12 years, he was the Director of Environmental Health and Safety for Wayne State University in Detroit, MI, where he continues to hold the academic rank of adjunct professor in the College of Engineering. He also served as Director of Biological Safety and Environment for the U.S. Department of Agriculture at the Plum Island Animal Disease Center at Greenport, NY.


Among his honors, Powitz was the recipient of the NSF/NEHA Walter F. Snyder Award for achievement in attaining environmental quality, and the AAS Davis Calvin Wagner Award for excellence as a sanitarian and advancing public health practice. He is the first to hold the title of Diplomate Laureate in the American Academy of Sanitarians, and also is a Diplomate in the American Academy of Certified Consultants and Experts and with the American Board of Forensic Engineering and Technology.


Dr. Powitz welcomes reader questions and queries for discussion in upcoming articles, and feedback or suggestions for topics you'd like to see covered can be sent to him directly at [email protected] or through his website at www.sanitarian.com.