Tuesday 24 November 2020

Unidirectional airflow - what does the science tell us?


Particles in well run cleanrooms can only come from two sources: people and machinery. People are the main sources of microorganisms.

We design cleanrooms to control particles in the air. As almost all microorganisms in cleanrooms are attached to particles, controlling particles is key to cleanroom contamination control.

We are trying to avoid people entering Grade A, but it remains a necessity. We rely on unidirectional air. A UDAF produces what is known as in the cleanroom world as ‘safety ventilation’.

What do we know about unidirectional air, its advantages and limitations? And what can this tell us about interventions and making connections?

1. First, unidirectional air needs to be fast if it is to stand any chance of pushing particles away from the point of concern. When air slows, it is of a greater risk. Although we assess air velocity across the range 0.36 to 0.54 metres per second, we want to ideally have the air towards the upper end.

2. Second, is that we disrupt the air whatever we do. Standing still we produce personal vortices. These are fairly stable and are less of a risk as the dispersion away from us follows the path of the unidirectional air.


 However, when we move, we produce unstable wakes, and this challenges the safety ventilation we have in place. This is especially so when moving hands and arms. Here the vortices become unstable and they change the direction of the air. How big a challenge this represents depends on the degree of movement, speed of movement, and the number of times the movement is repeated.

Scientist have shown this using a technique called hot film anemometry.

This means no matter how many times you perform an intervention, there’s no such thing as a safe intervention. Every intervention carries a risk. We can de-risk to an extent by running airflow visualisation patterns and practicing interventions in media fills and looking at the particle profile.

What we can’t do is undertake interventions that we haven’t done before and practiced in a media fill or carry out interventions more times than we would in a media fill.

3. Third, unidirectional air is not at the same velocity at every point and it becomes slower the closer it becomes to a surface. As someone who works in cleanrooms, the air velocity is greater over your head, shoulders and main body. It slows as it reaches your waist and becomes really slow at your knees and below. In fact, at this level it isn’t unidirectional at all.

This has been shown using real live simulations by scientists in Sweden.

What this means is that no connection operation, working with stoppers, unloading freeze dryers and soon, should be done close to floor level - ever. Ideally, all operations are at waist height to neck height.

Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (http://www.pharmamicroresources.com/)

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