Table Of ContentAMERICAN INDUSTRIAL HYGIENE ASSOCIATION
and
U. S. ATOMIC ENERGY COMMISSION
(Office of Information Services)
MONOGRAPH SERIES ON
INDUSTRIAL HYGIENE
WILLIAM E. MCCORMICK, Managing Editor
AMERICAN INDUSTRIAL HYGIENE ASSOCIATION
ADVISORY COMMITTEE
EDGAR C. BARNES, Pittsburgh, Pennsylvania (Retired)
GEORGE D. CLAYTON, George D. Clayton and Assoc. Inc.
WILLIAM G. HAZARD, Owens-Illinois Inc.
NORTON NELSON, Ph.D., New York University Medical Center
REVIEWERS OF THIS MONOGRAPH
DAVID A. FRASER, SC.D., University of North Carolina
THEODORE F. HATCH, SC.D., Fitzwilliam, New Hampshire (Retired)
HARRY F. SCHULTE, LOS Alamos Scientific Laboratory
MONOGRAPH TITLES
Pulmonary Deposition and Retention of Inhaled Aerosols
Beryllium—Its Industrial Hygiene Aspects
Thorium—Its Industrial Hygiene Aspects
Particle Size Analysis in Industrial Hygiene
Aerosol Technology in Hazard Evaluation
(In Preparation)
Radioiodine—Its Industrial Hygiene Aspects
Uranium—Its Industrial Hygiene Aspects
Aerosol Technology
in Hazard Evaluation
THOMAS T. MERCER
Department of Radiation Biology and Biophysics
The University of Rochester
Rochester, New York
Prepared under the direction of the American Industrial Hygiene Association for
The Office of Information Services, United States Atomic Energy Commission
ACADEMIC PRESS New York and London 1973
A Subsidiary of Harcourt Brace Jovanovich, Publishers
COPYRIGHT © 1973, BY ACADEMIC PRESS, INC.
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ACADEMIC PRESS, INC.
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United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1
Library of Congress Cataloging in Publication Data
Mercer, Thomas T
Aerosol technology in hazard evaluation.
Includes bibliographical references.
1. Air-Pollution-Measurement. 2. Aerosols.
3. Particle size determination. 4. Respiratory
organs-Foreign bodies. I. Title. [DNLM: 1. Aero-
sols-Poisoning. 2. Air pollution. QV310 M554c 1973]
TD890.M47 614.7ΊΌ28 72-12189
ISBN 0-12-491150-1
PRINTED IN THE UNITED STATES OF AMERICA
Foreword
The tremendous technologic achievements in the past few decades have
intensified the need for a comprehensive treatment of information on a single
specific topic in occupational disease detection and control. In the early 1960's
the American Industrial Hygiene Association inaugurated a program of
preparation of a series of monographs under the auspices of the Division of
Technical Information (now the Office of Information Services) of the Atomic
Energy Commission on subjects pertaining to industrial hygiene.
The author of this monograph, the fifth in the series, has consolidated
information with references to original reports appearing in the literature
supplemented by a considerable amount of unreported work on the subject of
aerosol technology. It is hoped that this treatise will not only provide a
convenient source of information to those concerned in industrial hygiene but
also will stimulate the interest of those involved in all phases of environmental
health.
We are indeed grateful to the author for his dedicated efforts in developing
the information in this publication, to the competent reviewers, to the mem-
bers of the AIHA Monograph Committee who assisted in the mechanics of
the manuscript preparation, and to the Atomic Energy Commission for its
support of the AIHA Monograph Program.
PAUL D. HALLEY, President
American Industrial Hygiene Association
ix
Preface
A scientist of some eminence once pointed out to me that my interpretation
of something he had said was limited in its validity by the level of competence
I had reached in the subject under discussion—a level, his tone implied, that
was not very high. He had reason to be annoyed with me at the time and
spoke, no doubt, out of pique ; nevertheless, I could discern an element of
truth in what he said. When I was asked to prepare a monograph on aerosol
technology, the memory of his observation prompted me to add the qualifying
phrase "in hazard evaluation," thus limiting the scope of my discussion to
those areas of aerosol technology with which I am most familiar. With that
restriction, I have attempted to present, as quantitatively as I could, the
principles underlying the various techniques involved in the production and
characterization of aerosols.
A quantitative discussion requires equations and equations require symbols,
which are always a source of difficulty. Rather than trying to find enough
symbols to give a unique meaning to each, I have permitted certain symbols to
have more than one meaning, defining them each time they are used, if there
appeared to be any risk of ambiguity. Units, too, pose a problem, which I
have tried to avoid by adopting the following convention : if units are not
mentioned, the equation is valid for any consistent set; if units are given only
for certain quantities in an equation, all other quantities are in cgs units.
The manuscript for this monograph was reviewed in whole or in part by
reviewers for the AIHA, who are identified elsewhere; by Jess Thomas, David
Sinclair, Myron Robinson, and Lyle Schwendiman for the AEC; and by
Marvin Tillery and Randall Mercer for me. I am indebted to all of them for
their comments and suggestions. I am also indebted to my wife, who did the
lioness's share of the work of preparing the manuscript.
T. T. Mercer
xi
1
Introduction
1-1 The Nature of an Inhalation Hazard 1
1-1.1 Deposition of Particles in the Respiratory Tract . . .. 2
1-1.2 Criteria of Particle Deposition 5
1-1.3 Factors in Lung Clearance 7
1-1.4 The Definition of Hazards 8
1-2 Sampling Procedures 9
1-2.1 Sampling to Estimate Exposures 9
1-2.2 Sampling for Hazard Control 13
1-2.3 Sample Variability 13
1-2.4 Sampling Duration 17
References 19
1-1 The Nature of an Inhalation Hazard
Airborne toxic particles, swept into the respiratory tract during inhalation,
may be deposited there and subsequently give rise to undesirable biological
effects. Since the complete removal of contamination from the air is sometimes
impossible and is always unattractive economically, it is rarely attempted.
Instead, the hazards associated with the inhalation of toxic particles are
minimized by maintaining the concentration of the contaminant below some
level which has been deemed unlikely to cause detectable biological damage in
people exposed over a long period. The two quite distinct processes of estab-
lishing the criteria that define that level and of monitoring atmospheres of
questionable character to see if they conform to the established criteria con-
stitute hazard evaluation.
Many factors that are not directly related to the chemical or radioactive
properties that make a substance toxic play important roles in determining the
1
2 Introduction 1
probability that a toxic agent carried by an airborne particle will reach a site
appropriate for causing biological damage. An appreciation of what these
factors are and how they contribute to the problem of hazard evaluation can
be obtained from a consideration of the processes of deposition in, and clear-
ance from, the respiratory tract.
1-1.1 DEPOSITION OF PARTICLES IN THE RESPIRATORY TRACT
The conducting airways of the human lung extend to the end of the terminal
bronchioles. Beginning with the trachea, they split into increasing numbers
of tubes of ever-diminishing cross section. The inside surfaces of these airways
are lined with a ciliated epithelium and the rhythmic beating of the hair-like
cilia continually moves a thin sheet of mucus toward the entrance to the
trachea. Insoluble particles deposited in these tubes are rapidly swept up
through the trachea and swallowed. In the nomenclature of the International
Commission on Radiological Protection (ICRP) Task Group on Lung
Dynamics [1], these airways collectively comprise the tracheobronchial (T-B)
compartment. Beyond them, the tubes are not ciliated and they lead through
respiratory bronchioles and alveolar ducts to the alveoli. Gas exchange takes
place throughout this volume, which is designated the pulmonary (P) com-
partment. Some of the insoluble particles deposited in this region remain for
very long periods.
Figure 1.1 is a stylized portrayal of a portion of the lung structure. It is
obviously an oversimplification, which is necessary for modeling purposes
because the actual lung arrangement is much too complex for theoretical
calculations. Moreover, despite its many shortcomings, it has permitted cal-
culations that yield deposition values in quite respectable agreement with
experimental results. The calculations are based on a regular system of branch-
ing at a fixed angle into two tubes of equal diameter, as shown in Fig. 1.1. It is
FIG. 1.1. Schematic diagram of the lung conducting airway.
The Nature of an Inhalation Hazard 1-1 3
assumed that the tubes have inflexible walls and lead ultimately into spherical
alveoli. Considering only particles that are too insoluble to exhibit any hygro-
scopic action in the humid atmosphere of the lung, four processes may be
effective in bringing about deposition :
(1) Particles may be intercepted as they pass from one tube into a smaller one
because the streamline along which they move enters the second tube at a distance
from its wall that is less than the radius of the particle.
(2) Particles having sufficient inertia may deviate far enough from the air stream-
lines to impact near the tube bifurcation.
(3) Particles moving through tubes that are not perfectly vertical have a component
of their sedimentation velocity that is normal to the tube surface and some of them
fall onto the surface.
(4) Very small particles are buffeted about sufficiently by the random thermal
motion of the air molecules that they are deposited on the tube walls by the process
of diffusion.
The effect of turbulent flow, which occurs in the larger airways, usually is not
taken into consideration.
As the air moves deeper into the lung, where the tubes are more numerous
and have smaller diameters but have a greater total cross-sectional area, the
average velocity through a tube decreases. This favors deposition due to
sedimentation and diffusion, but diminishes the effect of impaction. The
smaller diameters of the tubes also favor deposition due to interception and
sedimentation.
Figure 1.2 brings out the manner in which these processes cooperate to bring
about deposition of particles from air moving through the airways. The
divisions along the top indicate the level of the lung that is under consideration.
They are based on the original Findeisen [2] model of the lung structure. The
ordinate is the ratio between the number of particles remaining in a given
volume element of air at the indicated segment and the number it contained
when it entered the trachea. For an upright man, deposition in the trachea is
negligible except for particles of very high diffusivity. At each bifurcation
there is a considerable loss of larger particles due to impaction. As the air
flows through a tube, there is a continuous loss of particles due to sedimenta-
tion and diffusion, an effect that becomes more pronounced as the air moves
into the smaller airways. Air that penetrates all the way to the alveoli is
cleared of the larger particles and of most of the very small particles. During
exhalation, additional deposition occurs. The percentage of deposition,
relative to the particles remaining airborne at the start of exhalation, is very
nearly the same for the processes of sedimentation and diffusion, but deposition
due to impaction is greatly reduced.
It must be emphasized that this refers to deposition of particles after they
enter the trachea. During nasal breathing, unit density particles of about
10 μιη and larger are completely removed from the incoming air as it passes
4 Introduction 1
SITE OF DEPOSITION
FIG. 1.2. Lung deposition, during inspiration, of particles that enter the trachea. Abrupt
changes represent impaction at airway entrance. Other changes represent diffusion and
sedimentation losses. Losses in the alveolar sacs represent changes with time rather than
distance traversed.
through the nose. Calculations of nasal deposition, which is primarily an
impaction phenomenon, have not been entirely satisfactory, but there are
quite good empirical relationships available to describe it. Pattle [3] studied
the nasal deposition TV of monodisperse méthylène blue particles and obtained
the following relationship :
N = -0.62 + 0.475 \ogD 2F (1.1)
A
where D is the aerodynamic diameter of the particle in micrometers and F
A
is the inspiratory flow rate in liters/minute. Hounam et al. [4] felt the cor-
relation could be improved by the use of pressure drop in place of flow rate,
but Lippmann's [5] data were in excellent agreement with Pattle's equation.
Deposition in the mouth can be correlated with D 2Fin a similar manner [1].
A
Figure 1.3 shows an example of the overall deposition patterns derived
from calculations of the type described above. It is apparent at once that if it is
necessary for a particle to get into the pulmonary regions to cause biological
damage, then a great many particles are completely irrelevant to hazard
evaluation. Of those particles that are relevant, some are more important than
others. If we are to have a definition of hazard that takes account of this fact,
our criteria must be based on the factors that determine relevance to hazard.
Since the regional deposition apparent in Fig. 1.3 is the result of the action of
The Nature of an Inhalation Hazard 1-1 5
1,0-
.8-
6
I · :
15 RESPIRATIONS/MIN
1450 CM^ TIDAL VOLUME
2 A-
LU
a
.2-
I ' i ~i 1 1 r^ 1 1 1
0 1 2 3 H 5 6 7 8 9 10
AERODYNAMIC DIAMETER,ym
FIG. 1.3. Calculated deposition of particles in the nasopharyngeal (N-P), tracheobronchial
(T-B), and pulmonary (P) compartments, relative to number inhaled.
the four processes described above, we must look for our criteria in the physical
characteristics of the inhaled particles.
1-1.2 CRITERIA OF PARTICLE DEPOSITION
The probability that a particle will be removed by interception upon entering
a tube is related to the ratio of the particle diameter to the tube diameter.
Except for particles of extreme shapes, such as long fibers, or of very low
density, this effect can be ignored because particles large enough to show
significant interception effects are certain to be caught by impaction or
sedimentation.
The probability that a particle of diameter D moving with an air stream of
average velocity U in a tube of diameter W will impact at a bifurcation is
related to a parameter, Stk, called the Stokes number:
Stk = ρΚ Ό2ϋΙ9ηΐν, (1.2)
Β
where η is the viscosity of air, p is the particle's density, and K is its slip factor,
s
a quantity that is very nearly equal to unity for spherical particles if D is
greater than 1 μτη but increases rapidly when the diameter is comparable
to the mean free path of the air molecules. As far as particulate characteristics
are concerned, the significant quantities are p, K , and D2.
s
The probability that a particle will settle out in a tube is related to the
particle's terminal settling velocity U given by
G
U = pKD2g/\ty, (1.3)
G s
where g is the acceleration due to gravity. Again the significant particulate
quantities are p, K, and D2.
s
