Change rooms in need of a facelift
Traditional change room designs do not control contamination but merely dilute the concentration levels in the area.
By K.F. McCann
It is probable that every hour, somewhere on this planet, a contamination control miracle occurs. This must be true since traditional cleanroom change areas are obviously designed to control only sentient particles. Consider the following design and operational parameters in a traditional operational Class 100 facility. It will most likely boast full ceiling coverage of HEPA filters with floor or sidewall returns and air change rates approaching 700 per hour. Its humidity and temperature are rigidly monitored and controlled, walls, floors and other surfaces are fabricated from specially selected materials which will not generate particles and will dissipate static electricity. Materials and personnel entering this environment are required to be contamination free. The basic job to be performed by this exotic environment is to control particulate, a function aided by the stringent controls placed upon every object entering the environment. It could be argued that this job is relatively easy since theoretically no contamination is introduced into the area from outside.
Consider the function of the change room. Grossly contaminated (by cleanroom standards) personnel enter on the “dirty” side of the room where various garment donning protocols are observed preparatory to the transition to the clean side of the room. These procedures usually require physical contortions across a “clean boundary bench” at the same time manipulating boots, gloves and suits, etc., according to the facilities` specific requirements. The now appropriately dressed and presumably particle-free individual is the “clean zone” and ready to enter the cleanroom after passing through an air shower. In many facilities the change room is also the exit from the cleanroom creating an additional burden on the system. Since the “product” of the change room is personnel compatible with the cleanroom environment, it must follow that much more “work” has to be done by the change room contamination control system than the cleanroom system due to the large quantities of particles generated by personnel in “street” clothing within the confines of the change room.
The magic line
Most change rooms are relatively small areas with supply HEPA filters typically covering less than 50 percent of the ceiling. The rooms are separated into “dirty” and “clean” with a magic line or bench defining the transition. Non-unidirectional air supply systems used in clean environments achieve their design goals by dilution of the particle burden in the air and, in fact, often redistribute particles from areas of high concentration by the necessarily turbulent flow patterns created. Therefore, by inference, since most change areas are of this design and the “barrier” between clean and dirty is the magic line, the airborne particulate must be able to detect the presence of this barrier and avoid crossing into the clean zone! This type of system requires a large degree of cooperation and training on the part of the particles, indeed the perceived need for air showers at the entry into the cleanroom would indicate that uneducated and untrained particles are indeed finding their way across the barrier into the clean zone and onto clean garments.
As the primary interface between the cleanroom and the outside world the change room is the most important element in the contamination control system. It appears, historically, to have been treated as an afterthought to the more glamorous production areas of the cleanroom.
The functions of the cleanroom and change room are fundamentally different and require different approaches to achieve particulate control. Typical cleanrooms are designed to achieve a uniformity of cleanliness over a defined area. This area may have zones of differing cleanliness requirements but the cleanliness levels targeted within each zone are usually uniform. A change room is often treated as a lower class non-unidirectional or turbulent flow cleanroom. This is an inappropriate approach since cleanroom garments are packaged and stored in a cleanroom compatible environment, the change room air system must be designed to remove the potential for particulate generated by un-gowned personnel contaminating the gowned individual and the cleanroom. The air flow characteristics inherent in a non-unidirectional design are counterproductive to this basic function of the change room and will often contribute to the migration of particles into the clean zone.
The particulate burden in a change room is unique to its function. If a typical cleanroom can be considered digital, i.e. clearly defined clean vs. dirty or inside vs. outside, then the change room is an analog environment which transitions from dirty to clean as you proceed from change room entry to cleanroom entry. The cleanroom is designed to control particulate generated at any location within its boundaries; the change room should be designed to control and contain the very large quantities of particulate generated at the entry as well as the (presumably) decreasing burden generated as the cleanroom access is approached.
Design considerations
The change room is an ideal candidate for a unidirectional horizontal flow design, with the flow from the cleanroom entry to the change room entry. The room should be long and narrow providing a gradual change from dirty to clean with traffic flow in one direction only. An air shower may be useful at the entry to the change room where the removal of loose particles on street clothing can reduce the incidence of transported contamination. The use of air showers at the entry into the cleanroom is of limited value since submicron particles are not readily removed from clothing by the air turbulence and larger particles should not be present in significant numbers if the gowning procedures are effective.
Strategic placement of cleanroom garments along the length of the change room would almost eliminate protocol errors and particulate transfer as personnel move toward the cleanroom, since all particle generation will be directed away from the cleaner areas toward the entry to the change room where less clean procedures are taking place. This gradual change from dirty to clean is far more compatible with most gowning procedures since it is not usually possible to don boots, gloves, hoods, masks and gowns simultaneously while at the same time avoiding contact with contamination sources and other personnel as is required in traditional change room environments.
The use of external garment storage, mechanical shoe brushes, shoe and hair coverings outside the entrance of the change room can further enhance the gradual transition to cleanroom compatibility. Sticky mats and floors can also be used in one or more locations along the change room to reduce the potential for tracking contamination into the controlled areas.
Air volume requirements for this design are relatively modest since the air is traveling along the smaller cross sectional area. Air velocities in excess of 100 fpm can be achieved with air change rates that appear very low when compared to vertical unidirectional flow cleanrooms. A separate exit from the cleanroom is mandatory although a common area at the street exit is not a problem.
Any reduction in the potential for particle migration into the cleanroom will allow the cleanroom systems to be more effective. Traditional change room designs do not control contamination but merely dilute the concentration levels in the area. This unsatisfactory condition should be addressed with specific systems designed to control particle migration and not simply exist as a lower class transition to the cleanroom. CR
Kevin McCann is a NEBB-accredited supervisor for cleanroom performance testing and an NSF-accredited biohazard cabinet field certifier who has over 20 years of industry experience.