Pass-through chambers and airlocks

An airlock is not a small room between two larger ones. It is the device that lets a pressure cascade survive the fact that people and materials have to cross it, and its whole function rests on two things the cascade cannot supply by itself: a closed door and elapsed time.

The three configurations, and what each one declares

Air leaks from higher pressure to lower, so the pressure at which an airlock is held states what is being protected.

A cascade airlock sits at a pressure between the two rooms, so air flows continuously from the cleaner side, through the lock, to the dirtier one. This protects the clean room from the corridor and is the standard arrangement for ordinary aseptic manufacturing.

A bubble airlock is held above both adjacent rooms, so air flows outward in both directions. Nothing enters the lock from either side. This is used where two classified spaces must be kept from cross-contaminating each other, and where neither is a containment hazard.

A sink airlock is held below both rooms, so air flows inward from both sides and is extracted from the lock. This protects the corridor from what is inside — the correct choice for containment, potent compounds and biosafety work, and the exact inversion of the aseptic case. Choosing the wrong one produces a room that looks compliant and does the opposite of what it is for. EU GMP Annex 1 treats airlock design and its pressure regime as part of the contamination control strategy rather than as detailing.

Interlocks are the mechanism

Every configuration above assumes that at most one door is open at a time. Open both and the pressure differential vanishes, the two rooms are directly connected, and the lock becomes a short corridor — the cascade does not degrade gracefully, it simply stops existing. A pass-through hatch makes this failure easy, because it is small, convenient and used constantly. The interlock, mechanical or electrical, is therefore not a safety accessory but the thing that makes the airlock an airlock.

Time is the second requirement. After a door closes, the lock is contaminated by whatever the opening admitted, and the differential and particle concentration need time to return. A dynamic pass-through — one with its own HEPA-filtered air supply — is specified by that recovery: the second door stays locked until a purge of defined duration has run. The recovery test described in ISO 14644-3 is the same argument applied to a room. A pass-through operated faster than its purge time is being used as a hole in the wall.

Decontamination is a dose, not a switch

Where surface bioburden on incoming materials matters, the lock decontaminates as well as separates. Vaporised hydrogen peroxide works as a cycle with distinct phases — dehumidification, gassing, dwell and aeration — and the kill is delivered during dwell, verified with biological indicators, conventionally spores of Geobacillus stearothermophilus, at a defined log reduction. Aeration cannot be skipped: residual peroxide is toxic to cells and to operators, and materials that absorb it, such as many plastics, extend that phase.

Ultraviolet at 254 nm is the weaker option and for a specific reason: it acts by line of sight only. Dose is irradiance multiplied by time, so any shadowed surface, crevice, label underside or stacked item receives effectively nothing. It is a surface treatment for exposed, uncluttered geometry, and it should not be described as sterilisation of a load.