There are three prime sources of contamination through which the integrity of a cleanroom might be breached. First, there is the human element. While many processes might be automated, many situations remain whereby people are essential to the process. Controlling this source of contamination is available by a variety of methods, but human error can never be ruled out. This may occur at a variety of levels from the supply and cleaning of protective clothing, to the manner in which it is worn, and any other stipulations dependent upon the human factor.
The second source is the equipment within the room or items brought into the room, and the surface areas such as floors, walls and ceilings. Contamination can arise at any stage of the process, either as a result of human error or the failure of the cleaning regime to ensure that all surfaces remain contaminant-free.
Finally, there is the source of the air supply into the cleanroom. Research over the past decade has confirmed that air conditioning systems can be as much a source of contamination as a means of resolving it. The constant monitoring of the air in cleanrooms to ensure that they conform to the designated classification should enable the standard to be maintained or any possible deterioration detected at the earliest opportunity and so provide sufficient time for remedial action to take place.
The development of a system capable of providing adequate control of all of these risk factors has occupied the minds of many involved in research and development. Research on the use of ozone, ultraviolet systems, and biocides have attracted much attention.
Although the discovery of how to produce synthetic ozone dates back to the last century, it is only in the last few years that significant progress has been made in the development of ozone generators. Ozone is now widely used in the treatment and disinfection of water and, in limited circumstances, for the control of airborne bacteria.
Ozone is a triatomic form of oxygen and is created when ordinary air or oxygen is brought into contact with a high voltage electrical discharge. The oxygen (O2) molecules disassemble and re-combine to form ozone (O3). It is non-selective and will oxidise anything it comes into contact with. It works by attacking the structural integrity of micro-organisms, making it impossible for resisting strains of bacteria to develop and is effective in both wet and dry environments. In addition to being a very powerful bactericide and viricide, ozone is also a strong bleaching agent.
Being only moderately stable, ozone in the absence of oxidisable substances will revert back to stable diatomic oxygen. This means that ozone has to be produced on site rather than being transported then stored in large quantities. However, because of its oxidising action, ozone may not be suitable for use in all environments. Long term use has shown that it can cause the rusting of metalwork and perishing of rubber. For people, long-term unprotected exposure can result in bronchial problems. It must therefore be used cautiously and in clearly defined circumstances.
Ultraviolet (UV) is part of the electromagnetic spectrum between visible light and X-rays and kills micro-organisms by destroying their DNA. Short-wave UV is generated when an electrical current passes between two electrodes at either end of an arc tube (or within a UV lamp), irradiating the surrounding air.
UV lamps can also be fitted to laminar flow cabinets, and UV disinfection tunnels can be installed through which products being transferred into a cleanroom may pass. UV surface treatment systems can also be fitted to conveyors or within filling and packing machines to prevent residual contamination.
UV systems are also being used as part of the heating, ventilation air conditioning (HVAC) system whereby it disinfects incoming and recirculated air to the filters. However, its effectiveness will depend on where it is located in relation to the air filter. If upstream of the filter the equipment is in danger of becoming covered in particulate matter itself and hence require cleaning in order to remain effective, and if downstream it is not in a position to protect the filter from incoming micro-organisms. Moreover, unless the light can achieve a 100% kill rate the live micro-organism will still become imbedded in the filter or remain elsewhere in the air. It is also debatable as to how far UV could effectively penetrate the most efficient HEPA filters to ensure that all micro-organisms are eradicated. Finally, the effectiveness of this method must be balanced against the cost of installation and maintenance.
A third means of controlling bacterial contamination is through the application of a biocide. The range of micro-organisms against which a biocide might be effective may vary, along with how it actually operates upon that micro-organism. Some biocides simply inhibit the growth of the bacteria, fungi, yeast and mould, while others have the capacity to kill the micro-organism on contact. The latter work by penetrating the cell wall thereby totally eradicating the micro-organism. The advantage of a biocide of this type is that the micro-organism is denied the capacity to develop resistance to the biocide.
The development of biocides capable of eradicating a wide range of micro-organisms has provided the ability to effectively treat the most vulnerable elements of an HVAC system where contaminants can collect and breed, namely in the ductwork and on filters.
The role of ductwork in promoting the growth of bacteria, fungi, yeasts and moulds has been exhaustively researched. The research, however, did not commence with any real enthusiasm until the mid-70s when building inhabitants in the USA were beginning to present with a series of upper respiratory symptoms on a scale hitherto unseen. These symptoms and clusters of malaise were collectively known as Sick Building Syndrome, although it was not until the 1980s that it received clinical recognition rather than being regarded as a psychosomatic condition. As the research developed, attention turned to establishing the source of the contamination and, as Sick Building Syndrome was most prevalent in buildings with HVAC systems, this began to concentrate on the system itself and initially the cleanliness of ductwork.
Research established that ductwork could support and even promote bacterial and fungal growth, particularly in moist conditions. In 1991 Ahearn et al showed that Cladosporium could adhere to dry ducting, penetrating the paintwork with minimum available moisture. By 1992 there were regulations in Britain requiring attention to be paid to the cleanliness of ductwork, and on January 1st 1996 Health & Safety Regulations concerning Ventilation System Maintenance and Cleaning came into force.
The other weak link in an HVAC system is the air filters, which did not attract attention until relatively recently. In 1995 Noble et al documented volatile organic compound (VOC) production by fungi colonising filters, postulating a role in Sick Building Syndrome. Further evidence of the role of filters in supporting the growth of other micro-organisms quickly followed, along with evidence that the microbial colonisation of filters could constitute a hazard before the pressure loss across the filter reached the level at which the filter should be replaced.
The development of biocides, and in particular those that could be applied to ductwork and impregnated in filters should be considered a significant step forward in bacterial control for two reasons.
First, the agent of bacterial control is integral to the system and not a separate item of equipment, such as an ozone generator or UV system. The risk of equipment failure or inability to operate due to circumstances beyond the control of the immediate personnel would not arise. The second advantage arises from cost control. Filters have to be purchased for an HVAC system regardless as to whether they are impregnated with a biocide or not. While those impregnated with a biocide may be slightly more expensive, depending on the quality of filter purchased the opportunities to reduce energy costs could become available. The time required to clean ductwork and range of chemicals required for that process could also be reduced.
The research and development of biocides suitable for impregnation into plastics and in applications such as coatings on walls, floors and ceilings and certain types of equipment further extends the scope for bacterial control within the cleanroom environment.
While there is interest by the pharmaceutical sector in these applications, it is not surprising to find that there is also concern about selecting this means of control. These concerns fall into three areas.
The first concern relates to the constituent parts of the biocide and whether it would be appropriate for use in a pharmaceutical environment. In the case of the Bactiguard(R) range of air filters and coatings products, this active ingredient is based on food-safe compounds approved by FDA, EPA and MAFF. The reason for this, as with the development of other biocides, is that their primary use has been in the food processing industry. The impregnation of the biocide into air filters at the manufacturing stage rather than applied after manufacture reduces the risk of the elements becoming separated. Similarly, the ability to mix the biocide with paint for use as a coating on ductwork, or on walls, floors and ceilings ensures that it becomes part of the structure. In some instances biocides have been found to permeate porous structures, such as concrete floors.
The effectiveness of the biocide raises the second concern. While laboratory testing will allow for a degree of confidence, it can only be through on site testing that these results can be proven in real-life scenarios. Claims should be treated with caution, and test data always requested.
Finally, there is concern over the longevity of the treatment. This can only be accurately measured through ongoing testing and the continual bombardment of the surface with the micro-organism against which it is being tested. In the case of certain applications, such as floor coatings, this will be a lengthy process and definitive answers will not be available for many years to come. The situation regarding air filters, however, is more certain given that the filters have a limited lifespan by virtue of their primary function.
Identifying the weak links in the chain of bacterial control is essential to maintaining the integrity of the cleanroom and to developing new solutions to be placed at the disposal of cleanroom managers. It is unlikely that any single solution would be applicable to all circumstances, and that a combination of the methods would probably be the correct route.
A version of this article appeared in the June 1999 edition of Pharmaceutical Manufacturing and Packing.