  The particle filtration efficiency (PFE) of a respirator or face mask is one
of its key properties. While the physics of particle filtration results in the
PFE being size-dependent, measurement standards are specified using a single,
integrated PFE, for simplicity. This integrated PFE is commonly defined with
respect to either the number (NBFE) or mass (MBFE) distribution of particles as
a function of size. This relationship is non-trivial; it is influenced by both
the shape of the particle distribution and the fact that multiple practical
definitions of particle size are used. This manuscript discusses the
relationship between NBFE and MBFE in detail, providing a guide to
practitioners. Our discussion begins with a theoretical discussion of the
underlying principles. We then present experimental results for a database of
size-resolved PFE (SPFE) measurements for over 900 candidate respirators and
filter media, including filter media with systematically varied properties and
commercial samples that span the 20%-99.8% MBFE. The observed relationships
between NBFE and MBFE are discussed in terms of the most-penetrating particle
size (MPPS) and charge state of the media. For the NaCl particles used here, we
observed that MBFEs were greater than NBFEs for charged materials and vice
versa for uncharged materials. This relationship is observed because a shift
from NBFE to MBFE weights the distribution towards larger sizes, while charged
materials shift the MPPS to smaller sizes. Results are validated by comparing
the output of a pair of TSI 8130As, a common instrument used in gauging
standards compliance, to that of MBFEs computed from a system capable of
measuring SPFE.
