Phoenix Journal · LEV Testing
In a cleanroom the ventilation is the primary control, and the airflow regime you pick decides how clean the space can ever be. Here is how the standards and the two airflow options actually fit together.
Cleanroom ventilation
In a cleanroom the ventilation system is not a comfort measure - it is the primary control, and the airflow regime you choose decides how clean the space can ever be.
Cleanrooms are built to hold particle counts, and sometimes microbial counts, below a defined ceiling. In the UK that ceiling is usually set two ways. ISO 14644-1 classifies air cleanliness from ISO Class 1, the most demanding, through to ISO Class 9, using the number of particles of a given size in a cubic metre of air. Alongside it, sterile medicines are made to EU GMP Annex 1, whose revised edition came into force in August 2023 and is enforced in Britain by the MHRA. Annex 1 sets grades A, B, C and D, where Grade A maps to ISO 5 conditions and is the critical zone for open aseptic work.
Both frameworks lean on the same physics. Clean air is filtered through HEPA or ULPA elements - graded under BS EN 1822, where an H13 filter is at least 99.95% efficient and an H14 at least 99.995% at the most penetrating particle size - then delivered to the room, made to sweep or dilute contamination, and taken away. The one decision that shapes cost, energy and achievable class more than any other is how that air moves through the space. The two answers are unidirectional (laminar) airflow and non-unidirectional (turbulent) airflow, and most real facilities use a considered mix of the two.
Unidirectional flow delivers filtered air from a fully filled ceiling in a single, even, downward sweep at a steady velocity, so contamination is carried straight to low-level or floor returns and out. There is no mixing to rely on - each particle is pushed out on one pass. This is the only way to reach the tightest classes, and it is what sits above an aseptic filling point or an exposed product.
Non-unidirectional flow, often called turbulent or mixed-flow, supplies filtered air through diffusers and lets it mix with room air, diluting contamination until the extract carries it away. It does not sweep in one direction - it works by dilution and displacement. For the majority of controlled spaces this is entirely sufficient and far more economical.
Whichever regime you choose, the room only performs if the supporting elements are right - the filter grade, the pressure regime and the class you are classifying to. A pressure cascade holds the whole scheme together - Annex 1 asks for a guidance value of at least 10 pascals between adjacent grades, so clean air always moves from cleaner to less clean, never the reverse. Balancing those differentials across a suite of interconnected rooms is a discipline of its own, and it shares its logic with balancing ventilation across a mixed-use building, where competing zones must each hold their design condition without stealing air from one another. The high fan powers involved also make noise a real design constraint, which is why the same care given to acoustic design in kitchen ventilation applies to cleanroom plant.
Read those together and the design almost writes itself. If a step exposes product or people to risk, you protect it with unidirectional flow and an H14 ceiling, then wrap it in a turbulent background room a grade or two lower. Everything else - gowning, storage, packing - runs on turbulent dilution at the lowest class that still meets its purpose. That layering is how you meet the standard without paying to run the whole building as if it were a filling suite.
None of it is a one-off exercise. ISO 14644 classification is confirmed on commissioning and reconfirmed on a defined schedule, filter integrity is scan-tested, pressure cascades are monitored continuously, and any local exhaust ventilation is examined and tested under COSHH at least every 14 months by a competent person. Get the airflow regime right for each zone, then keep proving it, and the cleanroom holds its class year after year rather than only on the day it was signed off.
Questions
The core standard is ISO 14644-1, which classifies air by particle count from ISO Class 1 down to ISO Class 9. Sterile medicines also follow EU GMP Annex 1, whose revised edition came into force in August 2023 and is enforced in Britain by the MHRA, using grades A to D. Filters are graded to BS EN 1822, with H13 and H14 elements individually tested at the most penetrating particle size.
It depends on the class you need to hold. Unidirectional flow is required for the demanding end of the scale - ISO 5 and cleaner, and Annex 1 Grade A critical zones such as aseptic filling. For ISO 6 to ISO 9 and Grades C and D, turbulent dilution flow is sufficient and much cheaper to run. Most facilities layer the two, using laminar flow only over exposed critical steps.
Cleanroom classification proves the room holds its particle count, but it does not cover process extraction. Where a cleanroom handles solvents, powders or fumes, that local exhaust ventilation is a separate control measure under COSHH and must undergo a thorough examination and test at least every 14 months by a competent person. The two are complementary checks, not the same certificate.
Phoenix Duct Clean · by the numbers
Phoenix examines and tests local exhaust ventilation to HSG258 and COSHH - measured, reported and certificated, UK-wide.