Particulate contaminants are a very important factor in the pharma industry, and understanding particulate contamination sources
and their behaviour is critical to controlling their spread within production areas.
Ambient particles, potentially carrying microbial contaminants (viable particles), and nonviable particles can spread from
surfaces to the surrounding air volume of a proposed clean area through airborne contamination and/or via persons, contaminating both products and the manufacturing area. The main source of viable particles to the surroundings
is the operator. Other potential sources of microorganisms are pointed out in guidelines from regulatory agencies.1,2
One way of reducing contamination risk through airborne particles is to reduce the particle source strength. This can be achieved
by capturing contaminants at the source; however, it is difficult to capture all contaminants and different types of protection
are often required. In critical areas, for example, both barrier technology and controlled airflow directions are usually
employed.
High Efficiency Particulate Air (HEPA) filters are used to remove particles from incoming air and displace those that may
be spread into the controlled area. Large amounts of HEPAfiltered air are often required, but this solution is not always
reliable. The actual contamination risk can be quantified by better understanding the potential for contamination, as well
as how effective HEPA filtration is in mitigating contamination risks.
The model described in this article enables the contamination risk to be calculated.
Air disturbances
Large amounts of HEPAfiltered air can produce vortices because of the high energy levels created by the air movements. A vortex
can exacerbate a contamination source so it is important to reduce the occurrence of vortices to maintain a cleanroom or clean
zone.
Figure 1: Convective deposition of particles.
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A vortex can be described as a rotational motion of a fluid that is maintained by an energy source; for example, a cleanroom's
air supply. Investigation by Ljungqvist in 1979 showed that in ventilated rooms, vortex regions move as a rigid body or as
a free vortex.4 Ljungqvist and Reinmüller describe a vortex as having streamlines that are closed within a region.3 Vortices can accumulate high concentrations of particles that are released when the vortex is disturbed. Disturbances can
occur when people cross the vortex or when there is a major change in pressure or airflow routes, such as when a door is opened.
This is significant with both free and rigid vortex structures.
Another type of air disturbance phenomenon is turbulence. Turbulence can be described as a fluctuating, unsteady flow of fluid
with large velocity differences perpendicular to the net flow direction. The science regarding turbulence is complex and not
fully understood, but it can create difficulties regarding particle dispersions.
