United States
Environmental Protection
Agency
Health Effects Research
Laboratory
Research Triangle Park NC 27711
vvEPA
Research and Development
EPA-600/S1-81-036 July 1981
Project Summary
Interaction of Acid Sulfates
and the Respiratory System
D. Covert, M. Morgan, T. Larson, N. Frank, N. Horike, D Holub
Acidic sulfate compounds constitute
a large fraction of the mass of sub-
micrometric aerosol particles in pol-
luted air. This has prompted extensive
research into the health effects of
sulfuric acid (H2SC>4) and its neutrali-
zation products with ammonia (NH3).
The research summarized here covers
three aspects of the interaction of
acidic sulfate aerosols with the respi-
ratory system.
The extent of neutralization of in-
haled H2SO4 aerosol by endogenous
NH3 has been measured in the surgi-
cally isolated upper airways of anes-
thetized dogs. Neutralization was
observed to be inversely proportional
to particle size. H2SO4 particles with
initial dry diameters of 0.5 [im and
1.0/ym underwent 0.28 (0.08) and
O.06 (0.06)% neutralization per ppbof
laryngeal NH3 respectively during pas-
sage through the mouth and out the
larynx at a flow of 0.1 I/sec. At a given
particle size neutralization is related to
both the route of entry and the flow
rate. It is greater for entry via the
mouth than the nose and greater fora
flow of 0.11/sec than 0.2 I/sec. These
results are consistent with a reaction
that is limited by the rate of IMH3
diffusion to the particle's surface.
The respiratory response of the
guinea pig when exposed to sulfate or
histamine aerosols was measured.
The response of the guinea pig to
histamine aerosol is described as a
positive control for future animal
exposure studies. Dynamic lung com-
pliance exhibited a progressive de-
crease during ten-minute exposure to
an aerosol mass concentration of 885
/ug/m3; those guinea pigs that re-
sponded to a lower concentration of
565 /jg/m3 exhibited the same pro-
gressive decrease in compliance. This
observation is consistent with the
hypothesis that a fraction of any
guinea pig population is hypersensi-
tive. Comparisons with the findings of
other investigators is made. These
data will be used as an index of sensi-
tivity of our biological preparations in
the study of pulmonary functional
response of guinea pigs to sulfur com-
pounds in atmospheric aerosols.
The potential for ammonia produced
naturally in the mouth to modify the
effects of inhaled sulfuric acid aerosol
particles was investigated in healthy
adults. If significant chemical neutrali-
zation were to occur before deposition
the irritant potency of the aerosol
might be significantly reduced. For
acid particles with a mass median
diameter of 0.65 fjm, at the concen-
tration of 500 fjg/m3, neutralization
should have been nearly complete
during quiet mouth breathing. This
situation was compared to moderate
exercise where neutralization should
be greatly reduced. Using standard
measurements of lung mechanical
function and a measure of density
dependence of expiratory flow, there
was evidence for mild constriction of
small airways when exposure occurred
during intermittent exercise. The ef-
fect of acid aerosol, whether neutral-
ized or not, was minor, however, com-
pared to the independent influence of
exercise in briefly constricting large
airways.
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This Project Summary was devel-
oped by EPA's Health Effects Research
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
The occurrence of oxidized sulfur
compounds in the atmosphere is a well-
known consequence of fossil fuel com-
bustion and other industrial operations.
Sulfuric acid (H2SO4) and its neutraliza-
tion products [NH4HS04 - (NhUlzSO*]
with ammonia (NHs) are significant
components of the submicrometer size
particles in the atmosphere over large
areas of North America and Europe.
Inhalation of H2S04 aerosol at 1 mg/m3
for one hour in controlled human expo-
sures affects mucocilliary clearance
rate. Inhalation of (NhUbSCU at the
same concentration has no effect.
Similar exposures to HaS04 have shown
no changes in lung mechanical function,
however.
There are many factors, physical and
physiological, which when integrated
contribute to the effects that inhaled
acid sulfate compounds in paniculate
form will have on the respiratory system.
Initial physical aerosol parameters such
as chemical composition and particle
size distribution are relatively easily
controlled. These properties and the
resultant magnitude and location of
dose may be altered, however, by physi-
ological parameters such as gaseous
ammonia in the airways, elevated rela-
tive humidity (RH) and breathing param-
eters (e.g. frequency flow rate and tidal
volume).
In animal exposures to sulfate com-
pound aerosols, the sensitivity of lung
mechanical functional measurements
is critical to the assessment of effects.
The full report addressed three aspects
that are of importance to the determina-
tion of health effects arising from the
breathing of acid sulfate compounds in
particulate form.
Measurements of Respiratory
Ammonia and the Chemical
Neutralization of Inhaled
Sulfuric Acid Aerosol in
Anesthetized Dogs
Sulfuric acid (H2S04) and its neutrali-
zation products [NhUHSCMNHUkSCU]
with ammonia (NHs) are important
components of the submicrometer-
diameter particles suspended in the
ambient atmosphere. Inhalation of
H2S04 aerosol at concentrations 1
mg/m3 for one hour has been shown to
affect mucocilliary clearance rates in
humans, whereas inhalation of (NhU)
280)4 aerosol at the same concentration
has no effect. It was hypothesized that
during inhalation acid particles undergo
neutralization by respiratory NH3 as
they course the upper airways. To the
extent that it is the acidity of these
particles that is irritating, acid neutrali-
zation by endogenous NH3 during inha-
lation is important.
Also examined was the relationship
between respiratory NH3 and the chemi-
cal neutralization of inhaled (-(2804
aerosol in surgically isolated upper
airways of anesthetized dogs.
It was shown that H2S04 particles can
undergo significant neutralization during
inhalation via the nose or mouth by the
time the particles pass the larynx. For a
given flow rate, the percent neutraliza-
tion depends both on NH3 concentration
and particle size, with 0.5 ^m particles
being neutralized more rapidly than 1.0
//m particles. Measurements of the rate
of neutralization were in reasonable
agreement with simple diffusion theory.
On the basis of chemical transforma-
tion occurring in the airways, the results
suggest that smaller acid particles may
be less irritating than larger ones and
that inhalation via the nose is not
necessarily more protective than via the
mouth. These conclusions are tempered
by the fact that the irritant potential of
an acid particle is determined not only
by its chemistry but also by its site of
deposition. Nevertheless, these results
can partially explain the results of Wolff
and co-workers, who found that 0.9/urn
H2S04 particles are more irritating to
dogs than 0.3 prr\ particles, but cannot
help to explain the results of Amdur and
co-workers, who found that 0.3 /urn
H2S04 particles were more irritating to
guinea pigs than the same concentration
of 1.0 //m particles. In conclusion,
neutralization of H2S04 aerosol by
airway NH3 appears to be an important
phenomena that can be studied by
relatively simple techniques.
The Respiratory Response of
Guinea Pigs to Aerosols. I
Histamine Aerosol Control
Study
The investigation of the potentially
mild pulmonary functional responses in
guinea pigs to components of atmos-
pheric aerosols requires an accurate
description of both the aerosol under
investigation (chemical species, mass
concentration and size distribution of
the droplet of particle, temperature,
relative humidity) and of the animal
preparation. The use of guinea pigs is
confounded by intra- and inter-animal
variability and small but important dif-
ferences in protocols between labora-
tories which make comparisons difficult.
The full report describes the time
course of pulmonary functional response
in guinea pigs exposed to moderate
concentrations of histamine aerosol, a
known bronchoconstrictor. The aerosol
parameters and the sensitivity of the
guinea pig preparation are sufficiently
well described to permit interlaboratory
comparison. These results will provide a
basis for a forthcoming paper on the
analysis of pulmonary functional re-
sponse data of guinea pigs that were
exposed to aerosols of 802 and particu-
late sulfate compounds.
Female guinea pigs were monitored
for dynamic lung compliance (Cdyn),
pulmonary flow resistence (Ri_), breath-
ing frequency (f) and minute volume
(Vmin).
The only persistent change observed
in any guinea pig exposure mode was
the decrease in Cdyn at an aerosol
concentration of 885 /ug/m3 during the
first exposure. Since all guinea pigs of
that group displayed a decrease in Cdyn
(statistically significant in nine often
animals), the time course or response to
this aerosol was examined. A progressive
decrease in Cdyn which began within
one minute, accompanied exposure to
histamine. After 5-6 min, dispersion in
the data increased. This can be explained,
at least in part, by the large number of
deep breaths observed. The average
intra-animal coefficient of variation of
Cdyn during control periods was 9% and
the average inter-animal coefficient of
variation of Cdyn was 30%. The large
inter-animal variation addsdispersion
to the data not related to observations
on the time-course effect of histamine
aerosol; this variation was eliminated by
normalizing the data for each guinea
pig-
The Influence of Exercise and
Endogenous Ammonia on
Pulmonary Irritation by
Sulfuric Acid Aerosol
Recently a factor has been identified
which may be of considerable impor-
-------
tance in determining human response
to inhaled acid particles. It has been
shown that ammonia (NHa) generated
within the normal airways, and espe-
cially in the mouth, is capable of con-
verting inhaled sulfuric acid to (NH«)
2SC>4 or to NHUHSO* and presumably
reducing its irritant potency. Ammonia
produced in the mouth is sufficient to
convert up to 1500 /ug/m3 H2SC>4 to
(NH4) 2804, a nearly neutral salt, provided
that the acid particles remained in the
mouth for a sufficient time. That this
occurs while the acid particles traverse
the mouth before entering the lower
airways was not confirmed, but full
conversion was found in exhaled par-
ticles. It was proposed that inhaled acid
particles may not be fully neutralized
before entering the lower airways under
certam conditions, namely: during nose
breathing because of lower NHs con-
centrations, and during rapid mouth
breathing when particles encounter'
high NH3 levels only briefly. Under these
conditions, evidence of lower airways
irritation might be expected, while
during exposure with quiet mouth-
breathing, no effect was anticipated.
To test a part of this hypothesis,
human subjects were exposed to sulfuric
acid particles during mouth breathing
while at rest and then while exercising.
The particle concentration, particle size
and exercise level were chosen so that
neutralization in the mouth should not
be complete during exercise, while at
rest the conversion was expected to be
more nearly complete. Lung mechanical
function was determined before and
after exposure both at rest and with
exercise.
The acute irritant potency of H2S04
aerosol, in concentrations of 500/ug/m3
or more, is virtually undetectable in
normal adults by usual measures of
lung mechanical function. A more site-
specific test, density dependence of
maximum flow, provided evidence of
very mild small airways constriction, but
the work of others shows that measure-
ment of tracheobronchial clearance is a
much more sensitive indicator of re-
sponse to H2S04. Mechanical changes
were slight or absent even under condi-
tions strongly unfavorable for neutrali-
zation of H2S04 by endogenous NHs.
Thus the influence of NHs conversion on
lung function, if any, must be assessed
by a method which is more sensitive
han the ones used here. Subjects with
known airway hypersensitivity, such as
asthmatics, may provide a better means
for testing the hypothesis, as might the
use of a functional test such as clear-
ance rate.
D. Covert, M. Morgan, T. Larson, N. Frank, N. Horike, andD. Holubare with the
University of Washington, Seattle. WA 98195.
Milan Hazucha is the EPA Project Officer (see below).
The complete report, entitled "Interaction of Acid Sulfates and the Respiratory
System," (Order No. PB 81-190 498; Cost: $6.50, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
1 US GOVERNMENT PRINTING OFFICE 1961-757-012/7156
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