A Practical Approach to the Selection of Cleanroom Disinfectants

Tim Sandle

Chartered Biologist

This article provides guidance for the selection of disinfectants to be used for the bio-contamination control of cleanrooms and controlled environments. Control is important for product and patient safety. The article focuses on factors contributing to microcidal and microstatic properties, including concentration, contact time, and application methods.

Cleanrooms play an important role in hospitals, from special environments for the preparation of medicines in pharmacies to providing clean air zones for operations. Cleanrooms are designed with special air filters (high efficiency particulate air) to provide ‘clean air’, have positive pressure differentials to prevent the ingress of less clean air, and have strict entry and clothing requirements for personnel. Nevertheless cleanroom surfaces can become contaminated with microorganisms, transported in from consumables and equipment or shed from personnel. Surfaces pose a risk if they harbour high numbers of bacteria and fungi as such microorganisms can be readily transferred. Thus, an important part of contamination control within a cleanroom requires the use of cleaning and disinfection agents. A second risk arises within hands. Hands, whether gloved or ungloved, are one of the main ways of spreading infection or for transferring microbial contamination and the use of hand disinfectants is also part of the process of good contamination control (1).

Disinfectants used on cleanroom surfaces and for hand sanitization need to be of a high quality and be effective at killing microorganisms. However, there are a multitude of disinfectants in the marketplace. This range and choice of disinfectants can make the selection process difficult. This article sets out to provide a practical approach for the selection of disinfectants for use in cleanrooms.

Disinfectants vary in their spectrum of activity, modes of action and efficacy. Some are bacteriostatic, where the ability of the bacterial population to grow is halted. Here the disinfectant can cause selective and reversible changes to cells by interacting with nucleic acids, inhibiting enzymes or permeating into the cell wall. Other disinfectants are bactericidal in that they destroy bacterial cells through different mechanisms including causing structural damage to the cell; autolysis; cell lysis and the leakage or coagulation of cytoplasm (2).  Within these groupings the spectrum of activity varies with some disinfectants being effective against vegetative Gram positive and Gram-negative micro-organisms only while others are effective against fungi. Some disinfectants are sporicidal in that they can cause the destruction of endospore forming bacteria (these are the most difficult forms of microorganisms to eliminate from cleanroom surfaces). However, a chemical agent does not have to be sporicidal in order to be classed as a 'disinfectant' or as a 'biocide'. (3)

Ensuring that surfaces are regularly disinfected and that the numbers of bacteria present are kept to a minimum is of great importance. However, disinfectants are only effective when used in conjunction with a detergent. This is because most disinfectants have poor cleaning ability and will not easily penetrate ‘soil’ (dust, grease and dirt). So surfaces must be rinsed frequently with a detergent and then disinfected at frequent intervals.

Hand sanitisation is also of importance. Personnel carry many types of microorganisms on their hands and such microorganisms can be readily transferred from person to person or from person to equipment or critical surfaces. Such microorganisms (including Staphylococcus, Micrococcus and Propionibacterium) are either present on the skin not multiplying (transient flora) or are multiplying microorganisms released from the skin (residential flora). For critical operations, some protection is afforded by wearing gloves. However gloves are not suitable for all activities and gloves, if not regularly sanitized or if they are of an unsuitable design, will pick up and transfer contamination.

Selection of disinfectants
There are many different types of disinfectants for use within the cleanrooms (4). The range of disinfectants presents a somewhat bewildering choice for the cleanroom user. Disinfectants have different spectrums of activity and modes of action. A range of different factors needs to be considered as part of the process of selection including the mechanism of action, and also efficacy, compatibility, cost and with reference to current health and safety standards (5). For this, the cleanroom manager should have a rationale or policy in place for selection.

The key factors: 15 points for success

There are fifteen key factors for disinfectant selection. These are:

  • The type of disinfectant. Disinfectants can be divided into two groups: non-oxidising and oxidising. Non-oxidising disinfectants include alcohols (which disrupt the bacterial cell membranes); aldehydes (which denature bacterial cell proteins and can cause coagulation of cellular protein); amphoterics (which have both anionic and cationic character and possess a relative wide spectrum of activity); phenolics (some phenols cause bacterial cell damage through disruption of proton motive force); and quaternary ammonium compounds (QAC) (which cause cytoplasm leakage and cytoplasm coagulation through interaction with phospholipids). QACs are among the most commonly used disinfectants. Oxidising agents have a wider spectrum of activity than non-oxidising disinfectants and can damage endospores. However, they pose greater risks to human health. This group includes: halogens like iodine, peracetic acid and chlorine dioxide (6).
  • A disinfectant must have a wide spectrum of activity. This refers to the ability of the disinfectant to kill different types of microorganisms and microorganisms which are in different physiological states.
  • Many disinfectants are capable of killing vegetative microorganisms only and will not work against bacterial or fungal spores. Periodically, for disinfecting surfaces, a sporidical disinfectant should be used (such as one a monthly or quarterly basis). Many sporidical disinfectants are oxidising agents. This requirement influences the type of disinfectant purchased. However, sporicidial disinfectants tend to have greater health and safety considerations and some, particularly chlorine based disinfectants, are aggressive to certain types of surfaces (especially stainless steel) and will cause discoloration and abrasion. To avoid this, the residue of the disinfectant should be removed by wiping with sterile water or alcohol.
  • The disinfectant must have a rapid in action with an ideal contact time of less than ten minutes. The contact time is the time taken for the disinfectant to bind to the microorganism, traverse the cell wall and membrane and to reach its specific target site. The longer the contact time, then the longer the surface needs to be left for prior to use. For the contact time the surface needs to remain ‘wet’.
  • Often two disinfectants are used for regular disinfection, and are often used in rotation (for premises which are inspected by the European Medicines Agency this is a Good Manufacturing Practice requirement). Where two disinfectants are used the disinfectants selected must have different modes of action. The argument for rotating two disinfectants is to reduce the possibility of resistant strains of microorganisms developing. Whilst the phenomenon of microbial resistance is an issue of major concern for antibiotics there are few data to support development of resistance to disinfectants (7).
  • Some disinfectants require certain temperature and pH ranges in order to function correctly. One type of disinfectant, for example, may not be effective in cleanroom which is set at a cold temperature (below 10oC) and many disinfectants have not been validated by the vendor to show that they work at temperatures below 20oC.
  • With effective disinfection surfaces must be cleaned with detergents first. Some disinfectants are not compatible with certain detergents. In such circumstances detergent residues could neutralise the active ingredient in the disinfectant. Before selection a check should be made that the disinfectant is compatible with the detergent used. This is normally overcome by the use of neutral, non-foaming detergents.
  • Some disinfectants leave residues on surfaces. Whilst this can mean a continuation of an antimicrobial activity, residues can also lead to sticky surfaces and or the inactivation of other disinfectants.
  • Different disinfectants are not compatible with all types of surfaces. The disinfectants must not damage the material to which they are applied to and can cause corrosion, discoloration For more aggressive disinfectants a wipe down using water or a less aggressive disinfectant like an alcohol is sometimes necessary in order to remove the residues (8).
  • The disinfectants used should have been validated by the manufacturer. There are a series of European Standards for disinfectant validation for bactericidal, fungicidal and, if appropriate, sporicidal and viriucidal activity (9).
  • The presentation of the disinfectant is an important choice, whether as a pre-diluted preparation in a trigger spray or as a ready-to-use concentrate or an impregnated wipe.
  • The disinfectants selected must be relatively safe to use, in terms of health and safety standards. Here the main concern is with operator welfare. A related concern is the impact upon the environment, especially in the way that waste disinfectant solutions are disposed of.
  • The cost of the disinfectant is also a factor to consider; especially it is to be used over a large surface area.
  • Certain high grade cleanroom activities require disinfectants to be sterile (such as aseptic preparation areas). For this disinfectants can be purchased which have been sterile filtered (through a 0.2 mm filter) and are provided in gamma irradiated containers with outer wrapping.
  • Hand sanitizers fall into two groups: alcohol based, which are more common, and non-alcohol based. The most commonly used alcohol based hand sanitizers are Isopropyl alcohol or some form of denatured ethanol (such as Industrial Methylated Spirits), normally at a 70% concentration. The more common non-alcohol based sanitizers contain either chlorhexidine or hexachlorophene. For hand sanitizers used on skin, these must not cause excessive drying and be non-irritating (10).

Carrying out such a review, based on the above factors prior to purchasing a disinfectant, does not guard against the incorrect use of the disinfectant within the cleanroom. Any disinfectant will only be effective if it is used at the correct concentration and by wiping the disinfectant into the surface.

Disinfectants are of great importance for controlling the microbial population in cleanrooms. However, the selection of the most appropriate disinfectants to use is not straightforward. This article has examined some of the key criteria for the selection of disinfectants. Whilst selection is important, disinfectants must be applied and used appropriately. Given that the objective of the disinfectant is to kill microorganisms and to reduce the surface bioburden then the real test of whether a disinfectant is efficacious, is with the numbers of microorganisms present. This can be assessed by undertaken periodic viable microbiological environmental monitoring using surface techniques like contact plates and swabs.  Further evidence as to how effective a disinfectant is can be shown with the types of microorganisms recovered (the ‘microflora’). Finally, the selection of disinfectants should not be thought of as a one-off decision; it must remain part of the on-going quality reviews undertaken by cleanroom manager.

Author BIO:
Tim Sandle is a chartered biologist with over twenty-five years experience of microbiological research and biopharmaceutical processing. Dr. Sandle Tim serves on several national and international committees relating to pharmaceutical microbiology and cleanroom contamination control. Dr. Sandle has written over two hundred book chapters, peer reviewed papers and technical articles relating to microbiology.


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