Sterile air and vent filter

01

Place the filter and choose the membrane

FDA's guidance on sterile drug products produced by aseptic processing (September 2004) says compressed gases such as air, nitrogen and carbon dioxide used in cleanrooms should be of appropriate purity, with microbiological and particle quality after filtration equal to or better than the room air they enter. It says membrane filters can produce a sterile compressed gas, and recommends sterile membrane filters for autoclave air lines, lyophiliser vacuum breaks and tanks containing sterilised materials, with sterilised holding tanks kept under positive pressure or sealed against back-flow of non-sterile air. [1]

It states that gas filters, including vent filters, should be dry, because condensate can block them or let microorganisms grow, and that hydrophobic filters and heating where appropriate prevent moisture residues. EU GMP Annex 1 (2022) likewise says hydrophobic bacteria-retentive vent filters on water-for-injection storage tanks should not be a source of contamination and that condensation on them should be prevented, for example by heating. [1] [2]

For sterilising filtration of liquids, the same FDA guidance describes sterilising-grade filters with a rated pore size of 0.2 µm or smaller, validated with Brevundimonas diminuta (ATCC 19146, mean diameter 0.3 µm) as the challenge organism; it treats 0.22 µm and 0.2 µm as interchangeable ratings. Ask the supplier how the gas filter's retention rating was validated. [1]

Give the supplier the application (tank vent, autoclave air, lyophiliser vacuum break, process gas), gas, flow and pressure, temperature, sterilisation method and number of cycles, membrane material and rating, housing material, surface finish and connections, heating jacket if needed, integrity test method and values, documentation (validation guide, certificates) and quantity.

02

Set integrity testing and service life

FDA recommends that filters forming sterile boundaries or supplying sterile gases that can affect product be integrity tested upon installation and periodically afterwards (for example at end of use), and after any activity that may damage them; failures should be investigated and filters replaced at defined intervals. [1]

Annex 1 requires the integrity of critical sterile gas and air vent filters — those directly linked to product sterility — to be verified by testing after use with the filter still in its housing. For non-critical air or gas vent filters, integrity is confirmed and recorded at appropriate intervals; where filters stay in place for extended periods, they are tested at installation and before replacement, with maximum duration of use specified and monitored by risk, for example the number of uses and heat-treatment or sterilisation cycles permitted. WFI tank vent filters are tested before installation and after use. [2]

Editorial note: classify each filter as critical or non-critical in the order, because the testing obligation differs, and ask the supplier for the integrity test method and limit values for the exact cartridge. [2]

03

Hiross's stainless-steel sterilising filters

Hiross lists stainless-steel sterilising filters in its filter range; ask it for the membrane, retention validation, integrity test values and steam-cycle limits that Sartorius states for its Aerosart cartridge below.

Ask any supplier for the validation guide, integrity test values and permitted sterilisation cycles for the cartridge quoted. [2] [1]

04

Check the cartridge's retention, pressure limits and steam cycles

Sartorius' specification page for its Aerosart 0.2 µm 10" vent cartridge shows the figures a maker states for a sterilising-grade gas filter. It has a hydrophobic PTFE membrane on a polypropylene support, 0.75 m² of filtration area (1.5 m² at 20" and 2.25 m² at 30"), and quantitative retention of 10⁷ CFU/cm² Brevundimonas diminuta ATCC 19146 per ASTM methodology. Its maximum differential pressure is 5 bar at 20 °C and 0.5 bar at 134 °C; maximum back pressure is 3 bar at 20 °C and 0.5 bar at 134 °C; the minimum required bubble point is 1.0 bar. [3]

Steam limits are stated per method: autoclaving at 134 °C, 2 bar, 30 min, minimum 25 cycles; in-line steam sterilisation at 134 °C for 20 min at a maximum differential pressure of 0.5 bar, maximum 150 cycles. The cartridge is delivered non-sterile, is not gamma stable, and lists bacterial endotoxins below 0.25 EU/mL by LAL test and USP <88> Class VI materials. [3]

Editorial note: give the supplier your sterilisation method, temperature, cycle time and planned cycle count, and ask for the cycle limit and pressure limits of the exact cartridge quoted, so they can be written into the maximum duration of use that Annex 1 asks you to specify. [2]

05

Know what the bacterial challenge test does and does not prove

ASTM F838-20 "determines the bacterial retention characteristics of membrane filters for liquid filtration using Brevundimonas diminuta as the challenge organism". It uses a challenge of 10⁷ bacteria per cm² of effective filtration area, chosen to give "a high degree of assurance" that the method can detect oversized pores, and describes B. diminuta as "widely considered to be a small bacterium" and an industry standard for qualifying sterilizing filters. It also states that other species may be a worst case and that the test "does not provide assurance that filters can completely retain such bacteria", and that it is not intended for product- and process-specific validation. [4]

Sartorius quotes quantitative retention of 10⁷ CFU/cm² Brevundimonas diminuta ATCC 19146 for its Aerosart PTFE vent cartridge "per ASTM methodology" [3], which matches F838's challenge level [4]. F838 is written for liquid filtration, while a vent or compressed-gas filter runs dry; ask the supplier how the gas filter's retention was established (liquid challenge, aerosol challenge or both) and for the validation guide that correlates its bubble point or integrity test values with that retention.

FDA recommends integrity testing of filters forming sterile boundaries at installation and periodically, for example at end of use [1], and EU GMP Annex 1 requires integrity verification of critical sterile gas and vent filters [2]. F838's own caution that it does not cover process-specific validation [4] is the reason to ask for both the standard challenge data and the integrity test limits for the exact cartridge quoted.

Editorial calculation: at 10⁷ organisms per cm², Sartorius' 0.75 m² (7,500 cm²) 10" cartridge would see a total challenge of 7.5 × 10¹⁰ organisms if tested at full area; the 30" size at 2.25 m² would see 2.25 × 10¹¹. [3] [4]

Values are from ASTM's scope page for F838-20; the test procedure and acceptance criteria in the full method were not reviewed.

06

Membrane, cartridge, housing, heating jacket, test port and service-life record

The membrane, heating jacket, test port and service-life rows rest on FDA or Annex 1 text; the cartridge and housing rows carry no reviewed source and are for the supplier to fill.

07

Pressure, retention rating, temperatures, flow and materials for a vent filter order

Sartorius' Aerosart limits above — 5 bar differential pressure at 20 °C, 0.5 bar at 134 °C — are the kind of figure to obtain for the blank pressure, flow and pressure-drop rows. [3]

08

Send the application, gas and testing data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Application: [tank vent; autoclave; lyophiliser; gas line]
Critical?: [yes/no]
Gas/flow: [type; m³/h; pressure]
Sterilisation: [method; cycles]
Membrane: [hydrophobic material; rating]
Housing: [material; finish; connections; heating]
Integrity: [method; values]
Quantity: [cartridges; housings]

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