The Role of Glasses in Aseptic Production: A Detail Often Ignored

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Pharmaceutical Technology, Pharmaceutical Technology-10-02-2006, Volume 30, Issue 10

Glasses are important when operating in a sterile environment, and it is necessary to ensure that they will stand up to repeated sterilization processes without introducing contaminants. The glasses were subjected to numerous steam sterilization cycles to assess durability and microbial reduction. Results showed that the glasses most widely available on the market have been refined by the manufacturer to satisfy pharmaceutical customer needs by withstanding repeated sterilization cycles and minimizing contaminating particle release.

Hood, suit, faceplate, cover shoes, gloves: these are the necessary items of clothing when operating in A- and B-grade areas. The principal purpose of protective clothing is to minimize the risk of microbiological contamination caused by personnel. Thus, protective garments should not release fibers and must be able to contain particles produced and released by the body.

But how can we ensure that protective garments are not themselves vehicles of contamination? And how can we ensure that cleaning and sterilization processes are effective and do not alter the characteristics of the garments? We attempted to answer these questions, concentrating our attention mainly on glasses (in general, on individual protection devices usually referred to as masks).

Because glasses are not disposable, we must consider that stress conditions such as repeated sterilizations may compromise their use. The glasses may lose functionality and the components might be damaged, resulting in the release of contaminating material.

We prepared a study protocol to help verify the following aspects:

  • the glasses' ability to endure repeated sterilization processes without suffering alterations;

  • the ability of the sterilization process to obtain a 12-log reduction of the starting microbiological charge.

Figure 1: The type of glasses evaluated

We chose to verify only the steam-sterilization cycle because it is the process most commonly used in the pharmaceutical industry, although glasses also are sterilized using other methods (γ-rays, ethylene oxide, etc.).

For our tests, we used glasses (see Figure 1) with the characteristics outlined in Table I. Tests were conducted to verify that it was possible to subject glasses to repeated sterilization cycles without any alterations that could compromise their usefulness. Glasses in the trial were subjected to repeated steam sterilization cycles (temperature = 121 ± 1 °C, time = 30 min) according to the outline in Table II. At the end of the fixed sterilization cycles, the glasses were evaluated for adherence to the facial conformation, lens transmission, and particle release.

Table I: Characteristics of the glasses used.

The effectiveness of the sterilization process is a probabilistic function depending on the number of microorganisms present, the thermic resistance of these microorganisms, and the quantity of heat supplied. Therefore, determining the quantity of heat that is necessary to attain the 12-log reduction in the microorganism population to ensure sterility depends entirely on the thermic resistance of the present microorganisms.


The thermic resistance of the microorganisms was evaluated by verifying the D value as the time necessary to reduce 90% of the population of present microorganisms (1 log) in specific sterilization conditions. Even if the sterilization cycle recommended by the producer is a typical overkill cycle, it is necessary to evaluate the D value of the microorganism in a trial because this value strongly depends on the possible interactions between the microorganisms and the material on which they are found.

Table II: Glasses subjected to steam sterilization cycles.

An autoclave known as a BIER-vessel must be used to evaluate the D value. The most important characteristic of this autoclave is its ability to produce a sterilization graphic to wave quadrant (see Figure 2), which allowed the verification of the D value to one sterilization cycle's specific temperature.

Figure 2: Sterilization graphic to wave quadrant that enables us to verify the D value to one sterilization´s specific temperature.


Facial adherence and transmittance checks. The facial adherence did not change after 30 steam sterilization cycles. The goggles maintained their adherence without any shape modification caused by steam.

Even if the transmittance variations are minimal and can be attributed to measurement uncertainty, we verified that the transmission increased slightly with the increase in the number of sterilization cycles. The 84.1% transmittance value of the lenses increased to about 2 points after 20 cycles. After 30 cycles, transmittance decreased slightly under the starting value because of the appearance of superficial sediment and slight blurring of the lenses' surface. In any case, we concluded that after 30 sterilization cycles, the transmittance does not vary significantly. In fact, for all tested samples, transmittance was more than 75%, which linked with a check of the unchanging lenses' surface transparency, guaranteed that high visibility was maintained.

Particle-release check. Particle-release results for glasses subjected to repeated sterilization cycles were analyzed separately for the visor (lenses and support) and the elastic strip.

Check of the visor particle release. The particles' cumulative calculation, in the range between 0.2 and 1.0 μm, shows a proportional linear growth of the total particles compared with the autoclave cycles to which the glasses were submitted (see Figure 3).

Figure 3: Percentage of particles in the 0.2–1.0 mm range released by the frames during the autoclave cycles.

The contribution is given by the particles with diameters smaller than 0.2 µm that reach values greater than 90% of the total after 10 sterilization cycles, with exponential growth. The 0.4, 0.6, 0.8, and 1.0-μm diameter particles decrease with analogous progression.

The glasses' particle release, in the considered dimensional range, may be attributed to pollution of the sample obtained after repeated autoclave cycles. The polymer reticule of the glasses could favor the inclusion of the 0.2-μm dimension particles that are in the water used for the autoclave feeding and used as washing water before the analysis. The high temperatures to which the glasses were subjected favor plastic material expansion and the introduction of particles with smaller dimensions.

The 0.4–1.0-μm particles that quickly decrease to values lower than 10% of the total after 10 sterilization cycles were present on the sample's surface and not inside the polymer, because they would not be able to imbed themselves during the autoclave cycles and were washed away from the glasses' surface.

With the exception of what happens to the particles included in the 0.2–1.0-μm range, with bigger particles, we did not observe a significant variation in their percentage ratio compared with the total calculation. This probably means that the physical degradation of the material in analysis, which leads to particle release, does not vary qualitatively during the autoclave cycles, although there is acceleration after the twentieth cycle.

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We concluded that visors subjected to repeated autoclave cycles show an increase in particle release dependent on the number of cycles to which they are submitted. Particles of 0.2- and 0.4-μm diameters increase proportionally in greater measure; those particles that are present on the exterior surface of the sample and not incorporated in the polymer are washed away from the glasses' surface during the autoclaving.

Check of the elastic strip particle release. The treatment of elastic strips in autoclave cycles shows a remarkable increase in particle release after five autoclave cycles. The particles then settled until the end of 30 cycles of treatment. The biggest increase is that of the particles of about 0.2-μm diameter.

Figure 3 shows the percentage variation of particles of various sizes. There is an interesting absence of variation of particle composition after 30 autoclave cycles. In the 2.0–1.0 μm range, the increase in the particles released for strips follows a linear course proportionate to the number of autoclave cycles. The most important percentage is constituted by 2.0 μm-dimension particles.

The increase in the particle release from the elastic strip in the considered range is related to the number of autoclave cycles to which the strips are subjected. Based on these data, we concluded that elastic strips subjected to repeated autoclave cycles show an increase in the particles released dependent on the number of cycles to which they are submitted. Specifically, the number of 2.0-μm particles increased more than the others. Later, the number of 0.2-μm particles released decreased and the number of larger particles decreased, probably because of the second phase of material degradation.


Based on the data, the masks that are currently available have been further developed by the manufacturer, both in the frame color and in the elastic strip material, to better satisfy pharmaceutical customer' needs. These adjustments contribute to a reduction in the particles released, thus improving the masks' performance and usefulness in controlled-contamination areas.

Maurizio Battistini is the general manager of APP Pharmaceutical Partners, Switzerland, Via Cadepiano 24, CH-6917 Barbengo, Switzerland, Maurizio

Submitted: March 2, 2006\. Accepted: April 14, 2006

Keywords: aseptic processing, cleanroom, clothing systems, microbial sterilization