Carsten Moschner of Dastex asks whether they are an extravagance or an effective contamination reducer
Often, at times, only little importance is attached to the clothing worn under cleanroom garments à. In fact the operating authorities of cleanrooms realise that people are one of the biggest sources of contamination but many users work under the assumption that by the introduction of cleanroom garments they have contained this source. Unfortunately this assumption ignores the considerable risk of contamination from the cotton clothing worn under the cleanroom garments. This is especially true if the employees are allowed to wear their outdoor clothes under the cleanroom garments as this gives the operational authorities of the cleanroom no influence on changing habits, cleanliness, washing procedures or general condition of the undergarments. Apart from realising the importance of defined cleanroom undergarments there are two other considerations hindering the introduction of cleanroom undergarments: firstly the increase in costs and secondly, take up by the employees which can be difficult to obtain. Introduction of new materials during the last two years has greatly assuaged the latter of these considerations by improvement of both the physiological wear attributes and look. The former of these has been countered by argument that there is an increased risk of product contamination caused by non cleanroom compatible undergarments. Therefore in the spring of 2002 Dastex decided to carry out a new study of this topic in co-operation with ITV Denkendorf (Institut für Textil- und Verfahrenstechnik) and with the company Labor L + S. In fact the ITV already proved the efficiency of cleanroom compatible undergarments at the beginning of the nineties [2-4], but with this new study we were looking to investigate the connection between particle and microbiological contamination with variation in the undergarments. In addition a new material for undergarments, Light Tech, would be tested within the study. In many semiconductor facilities, defined cleanroom compatible undergarments have already gained acceptance. In microbiological controlled sectors however only a few companies have carried out this step (mainly large concerns that are controlled by FDA), even in 1992 Hecker explicitly referred in his paper to "undergarments" as a possible particle source ä. The results of this new study provide further evidence for the importance and efficiency of cleanroom compatible undergarments.
Basic conditions/experiment set-up The tests were carried out under practical conditions in a cleanroom at ITV Denkendorf (comparable to a cleanroom class ISO-4 / ISO-5 or to an A/B sector) with vertical airflow (0.45ms-1 at 21 - 22°C and approximately 45% relative humidity. It was our aim to detect different contamination degrees and quantify them in relation to the selected undergarments whilst taking into consideration different motion sequences. The uniform cleanroom garments were manufactured by ION-NOSTAT VI.2 and consisted of four pieces, coverall with knitted cuffs at arms and ankles, one hood providing full cover with attachable face cover (also made from ION-NOSTAT VI.2) and knee-high overboots. Micronclean Germany decontaminated the cleanroom garments twice and autoclaved them afterwards. The undergarments were made of 100% cotton or 100% polyester and consisted of trousers and a long-sleeved top/t-shirt, respectively. These were washed in a commercial washing machine before the tests were started. Initially the measuring cleanroom at ITV Denkendorf received an intensive cleaning and all surfaces were wiped off with a sterile alcohol mixture (Premier Klercide from Shield Medicare, 70 % alcohol, 30 % WFI) using sterile cleanroom wipes. This procedure was repeated daily and cross contamination was thus excluded to the greatest possible extend. The measuring instruments were two particle counters from Met One of identical construction and two air germ accumulators from Biotest. For determination of the germ numbers we used Rodac plates and RCS stripes. Two pairs of probes each consisting of a particle counter and an air germ accumulator were placed at workstation height (approximately 80 - 90cm) and at neck height (approximately 150 - 160cm). An employee of ITV operated the measuring instruments and carried out the respective contacts tests. Every time she passed through the hatch she received a newly decontaminated and autoclaved four-piece set of cleanroom garments (as described above) as well as two pairs of sterile powder free nitrile gloves (double gloving). The ITV employee always passed through the hatch before the test subject did.
Test performance The test subject passed through the hatch wearing the undergarments to be tested and put on one pair of sterile powder-free latex gloves. This was followed by the full cover hood, the textile face mask was buttoned in and the coverall and overboots were put on. Finally a second pair of sterile powder-free latex gloves was put on over the first pair. On entering the cleanroom, contact tests on the cleanroom garments were carried out at three defined points (right thigh, right forearm and stomach). Prior to the test program starting, the test subject slowly moved in the cleanroom for an hour to progress the cleanroom garments to a 'used' condition. Following on from this the defined movement program was carried out directly in front of the measuring instruments. Following an IES recommendation å there were three simulated movements, walking, grasping and bending down. Altogether the movement program (with breaks) took 15 minutes. The particle concentration in the cleanroom was continuously determined during the movements and air germ readings were taken at several stages of the program. Afterwards, surface contact tests on the cleanroom garments were carried out at six defined points (right thigh, right forearm, right armpit, stomach, right shoulder and at the top of the hood). For comparison we also carried out surface contact tests on the undergarments, once the cleanroom garments had been removed, at four defined points (right forearm, right armpit, stomach and right shoulder). The test subject then left the cleanroom for a few minutes, re-dressed in the cleanroom garment used in the previous test and then re-entered the cleanroom as previously described. It should be noted that the test subject used new gloves. This test procedure was used to simulate the repeated wearing of cleanroom garments. In microbiologically controlled sectors, especially in A/B sectors of small and medium-sized companies (that generally are not controlled by FDA) the sterile cleanroom garments are often put on and taken off several times a day. After a short "acclimatisation period" of approximately 15 minutes, the movement program was started again and the cleanroom garments and undergarments were microbiologically examined again by surface contact tests as described above. Two completely new sets of cleanroom garments (this means decontaminated and sterilised) were tested backwards and forwards per test subject.
Results There was proven a clear contamination reduction for both particles as well as microbiologically when wearing cleanroom compatible undergarments instead of simple cotton undergarments. Germ reduction on the surface of the cleanroom garments was better than 50%. This reduction was demonstrated on initial testing as well as after the movement program and after the second wearing. It was also the case that cleanroom garments that are worn a second time show a higher germ population than garments after the first wearing cycle. The particle reduction, especially at work place height, is the best evidence for the importance of cleanroom compatible undergarments. The reduction in contamination was over 50% on average. On occasion the cleanroom compatible undergarments reached as low as 5% of the contamination found with cotton undergarments. These statements are also valid for the particle sizes >3, >5 and >10 µm. This is not obvious at first glance because of the high scaling that is necessary for the chosen display format. Surprisingly the repeated wearing of the cleanroom garments did not have a negative influence on the particle counts at neck and workplace height with these tests.
Discussion As there does not exist any other literature showing the relationship between contamination of cleanroom garments and the type of clothing worn underneath it is not possible to compare the results of these tests with others. The proven germ reduction makes it reasonable to assume, however, that defined cleanroom compatible undergarments (e.g. from Light Tech material) are preferable for microbiologically controlled sectors. The direct connection between microbiological contamination and particle contamination also became quite obvious with this test series. Based on these test results we can confirm that it is not recommended to wear sterile cleanroom garments repeatedly (even with cleanroom compatible undergarments). The proven reductions in airborne particles in these tests show the same clear tendencies as the ITV studies at the beginning of the nineties [1-3]. This is regardless of the fact that the tests carried out in the older studies checked the contamination level of the cleanroom garments and not the number of particles arising at workplace height in the cleanroom as in our study. A further difference between the examinations is the material of the undergarments. For our tests we chose a 100 % polyester cloth instead of knitwear made from 100 % polyester, the advantage with this variation being better employee acceptance as the polyester cloth is far more comfortable. Both variants (100% polyester woven fabric and 100% polyester knitted fabric) produce considerably fewer particles and fibres under the cleanroom garments in comparison with cotton because of the better abrasion resistance. This minimises the risk that the abrasion product will leave the cleanroom garment system unintentionally thus also reducing the risk of product contamination. The fact that considerably more particles were found at workplace height compared to neck height can be explained by the airflow in the cleanroom. After the particles have left the cleanroom garment system (especially at head/neck region) they were caught up by the air currents and taken to the front/down in the form of an ellipsoidal curve. This form of air conditioning can be found in many controlled sectors and so all the more importance should be attached to the efficiency of all garment components including the undergarments.
Conclusion The introduction of cleanroom compatible undergarments will increase the maintenance costs (decontamination, supply, etc) but it will have the significant benefit of a clear reduction in particles and germ count within the controlled sector.
Acknowledgements Thank you to all persons and companies having been directly involved in these examinations and thank you to CCI von Kahlden, MT Messtechnik and Micronclean Deutschland for their support.
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