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Filtration efficiency of the packaging material for airborne micro-organisms

The average pore diameter of porous packaging material is limited according to International Standards. For example, the pore diameter of paper for paper bags or pouches shall be lower than or equal to 35 µm (EN 868-3 2009; 4.2.13). It has to be mentioned that the fraction of airborne microbial concentration with a particle size < 3.0 µm (bacteria and fungi) was in a long-term study in hospital wards within the range of 17.3-44.4% (Augustowska M, Dutkiewicz J. Variability of airborne microflora in a hospital ward within a period of one year. Ann Agric Environ Med 2006; 13:99-106).
The mechanisms of the particle-capture efficiency of the medical packaging material can not be addressed here in detail, but it should be emphasized that micro-organisms and other small particles with a size between 0.5 and 10 µm are not typically captured by a sieve-like function. These particles have to be captured by the fibrous structure of the packaging material. This filtration mechanism is referred to as impaction and interception.

Mobirise

 Air flow into the packaging during transport or storage challenges the gas-permeable components and requires effective filtration properties for the removal of airborne micro-organisms. The relationship between the required filtration efficiency and the airborne microbial challenge (number of colony forming units, CFU) is illustrated by this figure. Higher the microbial challenge, higher the filtration efficiency is required to maintain sterility at the level of 1:1,000,000. 

Consequently, specific test results are required to evaluate the barrier efficiency to retain airborne microbes. 

The International Standard ISO 11607-1  (subclause 5.2.3) states: “Porous materials shall provide an adequate microbial barrier to microorganisms in order to provide integrity of the sterile barrier system and product safety.” According to this standard (subclause 5.2), the term “impermeable” in relation to the microbial barrier properties should only be used for packaging material which has been tested according to “Test method for resistance of impermeable materials to the passage of air" (Annex C of ISO 11607-1).

There is a need, therefore, to ensure comprehensibly the compatibility of the barrier properties of the packaging with the environmental challenge by airborne micro-organisms. A quantitative data-based procedure should be used. The suitable method to evaluate the barrier properties of the packaging material with porous components is described by the ISO 11607-1:

"Evaluation of the microbial barrier properties of porous materials is typically conducted by challenging samples with an aerosol of bacterial spores or particulates under a set of test conditions …. The microbial barrier properties of the material, under these specified test conditions, are determined by comparing the extent of bacterial or particulate penetration through the material with the original challenge".  

Standardized test methods and procedures that may be used to demonstrate compliance of the packaging material with the requirements of the ISO 11607 are listed in Annex B.  But tests with airborne micro-organisms or airborne particles are not prescribed explicitly as mandatory. For example, the test according to DIN 58953-6 is listed in this Annex B, too.

Test according to DIN 58953-6: 
A mixture of quartz powder and bacterial spores is placed on the surface of the porous test sample which is fixed at the opening of a sterilized laboratory flask filled with 20 ml nutrient agar. Air passages are enforced by placing the flasks intermittently at 10 and 50 °C. A total of 15 microbes are accepted as limit for bacterial penetration of the test material, when 10 laboratory flasks have been used. This method may simulate the contamination with dust or settled microbes. A challenge with airborne micro-organisms is not given. 

Mobirise
Mobirise

Therefore, labels of conformance with ISO 11607-1 do not necessarily imply that the capacity of the packaging material to remove airborne micro-organisms was measured. 

An exposure chamber method was developed as a whole-package microbial aerosol challenge test (Patent-Nr. US 8,053,210 B2). The test packages are loaded with nutrient agar before sterilization and cultivated after exposure to monitor the bacteria passing the packaging. Bacteria are registered as colony forming units (CFU).

Mobirise

The exposure chamber comprises a volume of about 1 m³. Using a vacuum pump, the atmospheric pressure can be periodically reduced by 0-100 hPa which leads to an air flow through the gas-permeable component of the packaging. A microbial aerosol of Micrococcus luteus (mean particle diameter about 3 µm) is generated by a nebulizer. A glass impinger air sampler is used to determine the airborne bacterial concentration in the chamber. A gas meter measures the volume of air continuously drawn through the impinger. The periodic air pressure changes and the humidity are continuously monitored. The airborne microbial challenge of the package in the exposure chamber can be determined by calculating the total air volume passing the porous component of the packaging and by considering the average airborne microbial concentration in the chamber.

Using the ASTM 1608 test Sinclair et al measured the filtration efficiency of 16 commercial porous packaging materials against airborne microbial spores. The maximum spore penetration was, in most cases, between 1 to 10 % or above 10 % (Sinclair CS, Tallentire A. Definition of a correlation between microbiological and physical particulate barrier performances for porous medical packaging materials. PDA J Pharm Sci and Tech 2002;56:11-19).