Control of airborne and surface microorganisms and airborne particles in a cleanroom is achieved through the physical operation of the heating, ventilation, and air conditioning system and cleaning and disinfection techniques.1
Even with these factors taken into account, contamination can still occur. One area of concern is the entry of personnel2 and the movement of equipment into and out of cleanrooms.3 Traditional ways to control these activities have centered on gowning techniques and the cleaning of equipment. To accomplish this, cleanroom mats are frequently used to remove particles from footwear and from trolley wheels.
Research undertaken at an independent laboratory—and described in this article—evaluates the performance of temporary adhesive mats and semi-permanent polymeric flooring in retaining contamination from footwear, and particle generation from removing layers of the adhesive mats.
The dispersal of particles, including microorganisms, in turbulent flow clean areas occurs relatively easily.4 After a period of time, any particles present will be either removed from a clean area through the air-handling system, or will be deposited onto a surface as a result of gravity, convection, or diffusion. Most particles that land on surfaces will eventually become re-suspended into the air and thus represent a contamination concern. One common way in which re-dispersal happens is from people walking across floors.5
Special flooring commonly is used at cleanroom entrances to reduce the level of contamination from the transit of materials and people. The flooring is designed to remove a significant level of the particles carried on footwear and equipment. Major risk areas are changing rooms and air locks.6
This cleanroom flooring typically is in the form of temporary or semi-permanent mats. Temporary mats are typically adhesive and are commonly called sticky mats. Semi-permanent mats are made of polymeric material.
Adhesive mats consist of layers of plastic film coated with an adhesive and are attached to the floor; the mat is ‘sticky’ when a foot comes into contact with the surface. The mats are disposable and after a period of use, the top layer is removed from the stack and discarded. The removal of this top layer generates airborne particles that are dislodged from the surface of the mat.
Permanent mats have a polymeric surface manufactured from a material and are deposited onto a non-conductive substrate surface and become bound to the surface through electro-static forces. The mat remains permanently tacky and the flooring is designed to retain any particulate contamination that comes into contact with its surface.
Differences between types of mats
Given the range of adhesive-based, peel-off disposable flooring produced by different companies, it is not surprising that the adhesive capabilities, and hence the ability of the flooring to reduce the number of particles carried on footwear, varies. There have been few published studies into the effectiveness of adhesive cleanroom flooring, despite its long history and widespread use.7 For polymeric flooring, studies have shown the effectiveness of the flooring in reducing large numbers of microorganisms.8-10 However, no major studies looked at the ability of adhesive mats or polymeric flooring to remove particles.
In 2012, a study into the particle retention of mats, and the particle generation from adhesive mats when the top layer is removed, was undertaken at an independent laboratory. The study examined:
• The level of particles captured on a cleanroom mat.
• The level of particles that typically remain on shoes and overshoes after a person stepped onto and then off a cleanroom mat.
• The level of particles released into the air when the top layer from an adhesive mat is removed.
In order to examine these different conditions, six different types of adhesive mats were compared with two pieces of polymeric flooring—one newly fitted piece and one that had been in place for one year.
The study was conducted in an ISO Class 7 cleanroom. To simulate a level of use, 10 different people walked across the mats. Mats were assessed in the clean state (no footsteps on the mat); in the semi-dirty state (10 footsteps on the mats); and in the dirty state (20 footsteps on the mats). For the peeling test, mats were peeled either slowly or rapidly.
The study was carried out for each type of mat and polymeric flooring for people wearing overshoes and for uncovered shoes. Each person took one step. In order to create ‘worst case’ conditions, the steps on the mats and flooring were overstrikes. Particles were assessed using an optical particle counter fitted with a surface sampling probe to measure particles deposited onto the mat surfaces, and with an alternative probe for measuring airborne particulates.
The cumulative particle size examined was the one of most interest for pharmaceutical and healthcare facilities: particle counts of 0.5 µm and larger.
To obtain the different conditions required (slow peel, semi-dirty mats; fast-peel, dirty mats; and so forth), many replicate experiments were required. The cleanroom was given time to “clean up” in between each measurement and the surface particle counter probe was sanitized in between each measurement.
Results from walking across the different mats
For the first part of the study, the level of surface particles remaining on shoes and overshoes was examined before and after personnel had walked across each type of mat. Figure 1 displays the average results of the level of particles from shoes before an individual has walked across a mat and then afterwards. The adhesive mat types are coded 1 to 6. The polymeric flooring, assessed as new flooring and year-old flooring, is abbreviated as ‘polym’.
Figure 1: Surface particle counts from shoes measured before and after walking across a mat.
Click for larger image
Figure 1 indicates that the level of particles on shoes measured before an individual stepped on the mat was reduced after the individual had stepped onto the mat. The graph indicates that a far greater reduction was seen for the polymeric flooring compared with the six adhesive mats. The adhesive mats reduced the particle level by 20% to 50%; the polymeric flooring reduced levels by approxi-mately 80%.
Figure 2: Surface particle counts from overshoes measured before and after walking across a mat.
Figure 2 displays the equivalent mean results of particle levels from overshoes. Figure 2 indicates that the level of particles from the overshoes measured before an individual walked across the mat was reduced after the individual stepped onto the mat. The reduction was ranged from 13% to 45% for the adhesive mats and was approximately 80% for the polymeric flooring.
Figure 3: Surface particle retention by flooring type (uncovered shoes).
Surface particle etention
Given the differing levels of particle removal between the adhesive mats and the polymeric flooring, the levels of particles retained by the different flooring types also varied, with the polymeric flooring retaining far greater numbers of particles. This is reflected in Figure 3, (uncovered shoes) and Figure 4 (overshoes).
Figure 4: Surface particle retention by flooring type (overshoes).
Particle count generation from mat removal
The third part of the study examined the level of particles released from removing the top layer from each adhesive mat. As discussed above, the adhesive mats were studied in clean, semi-dirty, and dirty conditions. The top layer of each tested mat was removed using slow-peel and fast-peel variable speeds. The results are displayed in Figure 5.
Figure 5: Particle generation from mat peeling (0.5 µm).
Figure 5 shows that the level of particles increases in relation to the degree of dirt on the mat. The clean mat generated fewer particles than the semi-dirty mat; and the semi-dirty mat generated fewer particles than the dirty mat. In addition, the level of particles also rises if the mat is peeled away quickly compared with the mat being peeled away slowly.
The study of particles and cleanroom flooring revealed that the levels of particles removed from footwear and retained on the flooring were highest for the polymeric flooring when compared with a range of different adhesive mats.
A further risk arises from the use of disposable adhesive cleanroom mats in relation to the removal of the top layer of the mats, as the act of peeling an adhesive mat generates a relatively high number of particles.
The results are general trends and there are different variables to consider. Nevertheless, there appear to be advantages relating to the use of polymeric flooring as a means of ensuring tighter contamination control.
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Tim Sandle, PhD, is the head of microbiology at BPL and a visiting tutor with the University of Manchester (U.K.) School of Pharmacy. Contact: email@example.com
This article was published in the July/August 2012 issue of Controlled Environments magazine, pp. 10-13.