Bioaerosols and Bioaerosol Dynamics
Bioaerosols are defined by the ACGIH as airborne particles, large molecules or volatile compounds that are living, contain living organisms or were released from living organisms. The size of a bioaerosol particle may vary from 100 micons to 0.01 micron. The behaviour of bioaerosols is governed by the principles of gravitation, electromagnetism, turbulence and diffusion.
The above diagram is a scale representation of the relative size of pollen, pollen spores, bacteria and viruses. The scale of this diagram is roughly 8000:1. Each of the dots on this screen version represent 15 viruses, or virions. In this diagram, approximately 100,000 of these virions fit within the 100 micron circle representing the pollen. In actuality, many millions of virions could fit within the cross-section of a pollen.
BIOAEROSOL DYNAMICS
Brownian Motion
Bioaerosol particles are subject to Brownian motion according to Einstein's equation
where X = root mean square particle displacement, cm
t = time, s
r = particle radius, cm
For particles greater then 1 micron, diffusion due to gravitational settling is dominant. For spherical particles the terminal velocity (in calm air) due to gravity is:
where rho = particle density, typically 1.1 g/cm3
d = particle diameter, cm
For particles less than 0.5 microns the terminal velocity is approximately zero.
Physical decay (not biological decay) is due to gravitational settling and is approximated by the first-order decay process
where N0 = number of particles at time t=0
Nt = number of particles at time t
k = first-order decay rate constant
And,
Even though the overall bioaerosol may have a neutral charge the bioaerosol particles themselves are invariably charged to a greater or lesser degree in accordance with Boltzman's distribution.
Thermal Gradients & Electromagnetic Radiation
Thermal gradients can induce aerosol movements. Aerosol particles interact with electromagnetic radiation primarilly through reflection, refraction, absorption and scattering. In both cases transparent particles move towards heat sources because they act as a lens thereby focusing energy on the distal side. Opaque particles move in the opposite direction. This phenomenon is known as thermophoresis (for thermal gradients) and photophoresis (for radiation).
In laminar flow particles are carried along the airstream with the air molecules, but on a change in direction the heavier bioaerosol particles will break the streamlines. As a result, the particles may deposit on curved or angled surfaces. Consider flow in a curved pipe, where the linear air velocity is given by
where Q = flow, cm3/s
D = pipe diameter, cm
For the airflow, the Reynolds Number is given by
where D = pipe diameter, cm
rho = air density, g/cm3
mu = air viscosity, g/cm s-1
Although this may be laminar, the Reynolds number for the particle, which is experiencing higher inertial forces, is
where r = particle radius, cm
Vp = particle velocity, cm/s
The particle velocity is given by Stokes Law. For spherical particles this is
where rho = particle density, gm/cm3
d = particle diameter, cm
R = radius of pipe bend, cm
For laminar flow in which the particle is turbulent, deposition is more likely to occur. For turbulent flow, deposition is less likely. This factor is important in the design of inertial samplers, or impactors, as described below.
The ideal impactor is depicted in Figure 4, and consists of a laminar airstream turning before a plate. The distance travelled by a particle is
where t = L / V
Vp = particle velocity, cm/s
t = travel time, s
L = length of curved trajectory, cm
V = air velocity, cm/s
Substituting and rearranging we have
combining with Stoke's Law above we have
and
which is called the Sinclair Stopping Distance. This defines the probability of a given size particle impacting the plate under a given airflow. The collection efficiency is affected by the adhesion of the surface, particle bounce, and particle shape.
Bioaerosol particles have a diversity of shapes, including spherical, dodecahedral, needle-like and flakes. Many viruses are pleomorphic and change their shapes. Most sampling methods rely on the definition of aerodynamic diameter, or the diameter of a spherical water droplet which settles at the same rate as the particle being sampled.
where rhow = density of water, gm/cm3
This is only one of several different equivalent diameters, the use of which is dictated by the sampling method. These may include the equivalent volume diameter, the equivalent surface diameter or the equivalent electrical diameter.
Particle shape is a fundamental property and is important in assessing health hazards and interpreting data from some sampling methods. The shape frequency distribution can be developed using Fourier analysis of the signature waveform, of which the first five harmonics represent the basic shape, and the higher harmonics describe the texture of the surface.
The surface texture is characterized by the fractal dimension, which is defined as 1.0 plus the absolute value of the slope of the profile perimeter estimate based on a specified increment. Typical fractal dimensions for 2 dimensional profiles are as follows:
Fractal Dimension ..... Surface Type
1 .......... smooth surfaces
1.15 .......... mild convolution
1.3 .......... severe convolution
- Bioaerosols Handbook, C.S.Cox & C.M.Wathes, CRC Press, 1995
- Atmospheric Microbial Aerosols, B.Lighthart, A.J.Mohr, Chapman & Hall, 1994
Measuring Indoor Air Quality, J.E.Yocom & S.M.McCarthy, Wiley Press 1991 - Architectural Design and Indoor Microbial Pollution, R.B.Kundsin, Oxford University Press, 1988
- Biological Contaminants In Indoor Environments, P.K.Morey, J.C.Feeley, J.A.Otten, editors, ASTM 1990
- Bacteriology Primer in Air Contamination Control, V.V.Kingsley, University of Toronto Press, 1967
- The Aerobiological Pathway of Microorganisms, C.S.Cox, Wiley Press, 1987
- Industrial Ventilation, R.J.Heinsohn, Wiley Press 1991.