Calculating And Understanding Particulate Contamination Risk

The author presents a method to calculate the relationship between supply air volume flow and airborne particle concentrations. These methods and approaches facilitate the overall understanding of airborne contaminants and provide valuable information when designing facilities and processes for sterile manufacturing.
Mar 07, 2011

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.
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 Reinm├╝ller 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.