Distribution of Particle Surface Area by Particle Size

For inhalation toxicology, a commonly used aerosol particle size distribution measure is the mass of particulate material smaller than the size(s) of interest. The size of interest is often the aerodynamic diameter, which controls deposition by inertia and gravitation. Aerodynamic size discrimination can be obtained by using a cascade impactor and measuring the mass captured on each stage. Alternatively, a respirable sampler can be used, such as a cyclone or single-stage impactor designed to separate the aerosol into fractions larger and smaller than a specific size, such as 2.5um.

Although particle mass is clearly important, a case can also be made for the significance of particle surface area, as will be discussed here.

Consider a 1mg cube of material of the same density as water (1g/m3). This cube would measure 0.1cm on a side (L=1000um). It would have a surface area of S=6L*L or S=0.06cm2. Breaking it evenly into cubes of L/10 or L'=100um would produce 1000 particles with the surface area of 0.0006cm2 each, a total surface area of S=0.6cm2. The next such division of the lengths by 10 would produce 1 million L"=10um particles with S=6cm2. The next such division would produce 1 billion 1um (respirable) particles with S=60cm2. Yet another such division would produce 1 trillion 0.1um (respirable) particles with S=600cm2. [The number increases as the reciprocal of the length cubed. The total surface area increases as the reciprocal of the length.] The 0.1um particles would now have a total surface area 10,000 times that of the original 1mg cube, without any increase (obviously) in the mass.

The surface of the airborne particles would tend to adsorb materials from the air. One implication of the enlarged surface area is that far more adsorbed material would then be carried by inhaled particles to their deposition sites. The adsorbed material could be toxic even if the particles are not. Alternatively, the adsorbed material could interfere with the normal chemical activity of the particles.

A second implication of the enlarged surface area is that the dissolution of the particles will be accelerated. The time for dissolution is approximately proportional to the square of the particle size, which means much more rapid dissolution for these smaller particles. The concentration at the site of deposition will be determined by fluid flow in the vicinity of the particle and by chemical reactions at the site. The faster the dissolution, the higher the concentration. The faster the dissolution or reaction, the shorter the time before the material has been totally dissolved or removed.

In designing and in interpreting inhalation toxicology experiments, consideration may need to be paid to the surface area distribution as well as to the mass distribution. Such concerns are part of the discussions underway concerning the revision of the threshold limit values (TLVs) for beryllium, where details of the particle size distribution (thus the mass and surface area distributions) are key.

By: Douglas W. Cooper, PhD