Gas molecules colliding with airborne particles give rise to diffusive spreading (Figure 2). The effect of molecular diffusion may be considered when airborne particles are small (microscopic), which may occur in
cleanrooms and clean areas.
Figure 2: Diffusion of particles.
Contamination risk — a model
Acknowledging the fact that cleanroom operators are often the main source of microorganisms, it can be useful to study the
probability that microorganisms will contaminate certain areas associated with operators. To calculate the gravitation deposition
of a concentration of airborne viable particles, Whyte describes an equation (Equation 1).5
The size of viable airborne particles from humans in a cleanroom environment is in the range of 5–20 µm. An assumption can
be made that the average viable particle size is 12 µm. Using Stokes Law, Whyte shows that the settling velocity for a 12
µm particle in air is 0.462 cm/s (Equation 2).
The knowledge of settling velocity is useful in volumes where the particles are only affected by gravity and not from major
airflow routes such as in cleanrooms. In a pharmaceutical environment, however, air is used to create barriers with air velocities
much higher than 0.462 cm/s. In critical areas using UDF units, common air velocities range between 0.3–0.5 m/s or 30–50 cm/s.
Compared with the settling time of a 12 µm particle (0.462 cm/s) affected only by gravity, the convective effect on the particle
in a critical area will be dominating and the effect of gravity can often be neglected.3