Respirable Fraction (RF) cont'd

DOE-HDBK-3010-94

1.0 Introduction

(particles penetrating the upper respiratory airway and entering the thorax) with a 50% cut-

off at 15-m AED. The International Standards Organization - Europe defines "inhalable

dust" as particles entering the nasal or oral passages with a 50% cut-size of 10-m AED.

Accordingly, use of a 10-m AED cut-size for respirable particles is considered

conservative, and may even be overly conservative since the mass is a cube function of

particle diameter.

The size of a particle is a function of the measurement technique used. If the method used is

optical/electron microscopy or spectrometry, particle size is a projected diameter measured

by the plane that intercepts the light/electron beam or reflection from light scattered by the

particle. The size represents the two-dimensional area intercepting the beam and, as with all

projections of three dimensions into two, can result in considerable distortion. Projected

diameter approximates the Geometric Diameter (Dg). Dg is also approximated by sieving

where the size measurement is termed geometric/linear/least linear diameter. The

measurement represents the smallest dimension of the particle that will pass through the

openings in the sieve.

Liquid and air sedimentation techniques of inertial impaction by a cascade impactor measure

the settling velocity of a particle and report size as an aerodynamic characteristic. Size is

reported as an equivalent sphere with an equivalent settling velocity, or Stoke Diameter

(DStk). The Aerodynamic Diameter (DAED) specifically refers to an equivalent sphere with a

density of 1 g/cm3. DAED is the parameter of interest for defining respirable particles

(i.e., ≤ 10-m AED) as it normalizes materials of differing density. Other size units

include Ferret's diameter (the relationship between the maximum and minimum diameters) or

Sauter's mean diameter (the surface to volume ratio most representative of the distribution of

a group of liquid drops), but these are of little use for the purposes of this document.

Dg is related to DAED by the equation:

DAED = (Dg[ρp]0.5[CC,e/CC,a]0.5)/α

(1-2)

where:

ρp

= Particle density (g/cm3),

CC,e

= Cunningham slip factor corresponding to the volume equivalent

diameter,

CC,a

= Cunningham slip factor corresponding to the aerodynamic equivalent

diameter, and

= Aerodynamic shape factor.

The Cunningham slip factor is related to the potential for particle impact with the mean free

path of air molecules. Above the sub-micron size range, all particles impact with air

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