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MINML PROTECTION
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1OOR72008
TOXICOLOGY
OF
ATMOSPHERIC SULFUR DIOXIDE
DECAY PRODUCTS
Trent R. Lewis, Mary O. Amdur,
Martin D. Fritzhand,
and
Kirby I. Campbell
Division of Health Effects Research
ENVIRONMENTAL PROTECTION AGENCY
National Environmental Research Center
Research Triangle Park, North Carolina
July 1972
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The Aft series of reports is issued by the Environmental Protection
Agency to report the results of scientific and engineering studies, and
information of general interest in the field of air pollution. Information
presented in this series includes coverage of intramural activities
involving air pollution research and control technology and of coopera-
tive programs and studies conducted in conjunction with state and local
agencies, research institutes, and industrial organizations. Copies of
AP reports are available free of charge to Federal employees, current
contractors and grantees, and nonprofit organizations - as supplies
permit - from the Air Pollution Technical Information Center, Environ-
mental Protection Agency, Research Triangle Park, North Carolina
27711.
Publication Number AP-111
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CONTENTS
LIST OF FIGURES iv
LIST OF TABLES iv
ABSTRACT v
INTRODUCTION 1
TOXICITY IN EXPERIMENTAL ANIMALS 3
STUDIES OF MORTALITY AND PATHOLOGY 3
Sulfuric Acid Mist Particle Size 3
Exposure Duration 5
Conclusions 6
STUDIES OF PULMONARY FUNCTION 7
Sulfuric Acid Mist Particle Size 7
Sulfur-Containing Particulate Aerosols 10
Particulates in Combination with Sulfur Dioxide 13
Hygroscopic Properties of Sulfuric Acid 15
Conclusions 16
TOXICITY IN MAN 19
STUDIES OF SUBJECTIVE AND PHYSIOLOGICAL
RESPONSES TO IRRITATIVE PROPERTIES OF
SULFURIC ACID MIST 19
Detection by Subjects 19
Effects of Humidity, Acidity, and Particle Size on
Subject Response 20
Effects of Sulfuric Acid Mist on Respiratory
Retention 21
Conclusions 23
STUDIES OF SENSORY AND CENTRAL NERVOUS
SYSTEM RESPONSES TO SULFURIC ACID MIST 24
Optical Chronaxy 25
Dark Adaptation 26
Continuous Electroencephalography 28
Conditioned Reflexes 28
Conclusions 29
ill
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SUMMARY 31
TOXICITY IN ANIMALS 31
TOXICITY IN MAN 33
REFERENCES 35
BIBLIOGRAPHY: SELECTED TOPICAL REFERENCES 39
LIST OF FIGURES
Figure Page
1 Effect of Particle Size on Airway Response to
1 mg/m Zinc Ammonium Sulfate Aerosol 11
2 Dose-Response Curve of Zinc Ammonium Sulfate
Aerosol for Different Particle Sizes 12
3 Comparison of Irritant Potency of Sulfur Dioxide
and Sulfur-Containing Particulates 13
LIST OF TABLES
Table Page
1 Optical Chronaxy on Ninth Minute of Exposure of
One Subject to Test Atmospheres .......... 26
2 Eye Sensitivity to Light in Three Subjects During
Twentieth Minute of Dark Adaptation ........ 27
Number of Trials with H^SO^. Plus Light Required
for Appearance of Conditioned Reflex ........ 30
Threshold Aerosol Doses Required For Sensory Response . 30
IV
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ABSTRACT
Studies of atmospheric chemistry have shown that under certain
conditions sulfur dioxide can be converted to sulfuric acid and particu-
late sulfates. Additional aerometric and toxicologic research is needed
to supplement what little is known of the actual atmospheric concentra-
tions and chemical forms present under varying meteorological condi-
tions and of the toxicologic significance of this group of compounds
under different ecological conditions. Most available animal data,
exclusive of data from mortality studies, relate to alterations of pul-
monary function during and following exposure to sulfuric acid and
particulate sulfates. Although results of pulmonary function studies in
animals exposed to these substances cannot be extrapolated simply and
directly to predict human responses, two significant toxicologic princi-
ples related to the assessment of health hazards presented by such air-
borne contaminants have been demonstrated. First, data on mass con-
centration alone are an insufficient basis on which to predict irritant
potency because particle size plays an important role in determining
the irritant potency of sulfur oxide particles (submicron particles being
more irritant). Second, particulate oxidation products of sulfur dioxide
are generally much more potent irritants than the sulfur dioxide gas
per se.
Although sulfuric acid is known to be a much greater irritant than
sulfur dioxide to man, the combined effect of particle size and concen-
tration of sulfuric acid mist on exposed human subjects is still undeter-
mined. Industrial- and episode-exposure studies have implicated long-
and short-term exposure to sulfuric acid in the etiology of acute and/or
chronic respiratory disease. Russian scientists have studied the thres-
hold concentrations of oxides of sulfur that elicit certain sensory
responses.
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Other factors that affect the toxicity of particulate oxidation pro-
ducts of sulfur dioxide, such as retention, site of deposition, hygro-
scopic nature of the products, and the subject's breathing pattern are
discussed.
Key Words: toxicology, air pollution, sulfuric acid, sulfates, human,
animal, respiratory, neurosensory
Vi
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TOXICOLOGY
OF
ATMOSPHERIC SULFUR DIOXIDE
DECAY PRODUCTS
INTRODUCTION
Studies of atmospheric chemistry have shown that a portion of the
sulfur dioxide (302) emitted into the atmosphere undergoes oxidation,
leading, eventually, to the formation of sulfuric acid (H SO ) and sul-
fate particulates. Studies in experimental toxicology demonstrate that
some of these particulate oxidation products have a greater irritant
potential than sulfur dioxide itself. Clearly then, air quality standards
for sulfur dioxide should be set with due consideration for the potential
sulfur dioxide has, under certain atmospheric conditions, for forming
its more hazardous derivatives.
The acute air pollution episodes that occurred in Donora, Penn-
sylvania, and London, England, precipitated intensive research into
the toxicological and physiological effects of sulfur dioxide and certain
of its decay products. In spite of this research, most of which was
conducted between,1950 and 1963, many questions regarding health
problems related to atmospheric oxides of sulfur remain unanswered.
To consolidate the information that is available, this report presents a
review of the literature on the toxicology of some of the sulfur dioxide
decay products, and, in one section, presents recent data on the com-
parative toxicity of those decay products.
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TOXICITY IN EXPERIMENTAL ANIMALS
STUDIES OF MORTALITY AND PATHOLOGY
Experimental studies that use mortality and pathology as the
criteria of response by necessity involve the exposure of animals to
pollutant concentrations far above those occurring in the atmosphere
in order to produce measurable effects. Data from such studies cannot,
without some misgivings, be extrapolated to give practical guidance in
setting safe levels for those air pollutants. Such studies, however, are
usually the initial step in investigating the toxicity of a substance.
Considerable species differences have been observed in the
response of small laboratory animals exposed to sulfuric acid mist.
Treon et al. exposed small numbers of several species to H2SO4 mist,
95 percent of the particles of -which were smaller than 2 micrometers
(jim) in diameter. Using concentrations ranging from 87 to 1600 milli-
grams per cubic meter (mg/m3), they found increasing sensitivity as
shown: rabbits < rats < mice < guinea pigs. Guinea pigs died of expo-
sures not lethal for other species; when exposed to concentrations sub-
lethal to them, they had proportionately greater respiratory irritation
and lung pathology than the other species studied. The results of
Mathur^ also corroborated that guinea pigs are less resistant than mice
to sulfuric acid mist.
Sulfuric Acid Mist Particle Size
Questioning the influence of aerosol particle size on response,
Amdur et al. ^ began by determining the 8-hour LCso* of solfuric acid
particles with a mass median diameter (MMD) of 1 (im. The LC5Q was
18 mg/m3 for 1- to 2-month-old guinea pigs and 50 mg/m3 for 18-month-
old animals. In subsequent studies, sulfuric acid mists of 2.7-(j.m-MMD
particles proved more toxic than those of 0.8-fi.m-MMD particles and
*LC50 is the concentration lethal for 50 percent of the experimental
population.
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the smaller material was more toxic at 0° C than at room temperature. *
These data indicate that the age of the animal, particle size, and tem-
perature all influence lethality.
Other variables have been found to influence the toxicity of sulfuric
acid: the presence of smoke^ resulted in increased mortality and the
presence of aerosolized ammonium carbonate-* diminished the effects of
the acid. The increase in mortality was attributed to the larger size of
the toxic particles, which resulted from the adsorption of the acid, upon
the smoke particles. The decrease in effects was attributed to chemical
neutralization of the sulfuric acid.
Amdur et al. and Pattle et al. both examined the gross and
microscopic pathology of the lungs of exposed guinea pigs and agreed
in their findings. Animals that died after exposures to H2SO4 of less
than 2 hours had grossly distended and emphysematous lungs, but no
other serious lesions. The cause of death appeared to be asphyxia-
tion caused by bronchoconstriction and laryngeal spasm. The lungs of
animals that died after longer exposures had gross pathological lesions,
including capillary engorgement and hemorrhage. ' It has been sug-
gested that these changes represent sequelae to the combined effects of
anoxia and increased intrathoracic pressure caused by bronchoconstric-
tion and laryngeal spasm. The lungs of animals that survived the
longer exposures showed spotty areas of old hemorrhage and some
areas of consolidation, especially around the hilar regions. Although
such exposures do not give rise to death, during or afterwards, the
damage is repaired only very slowly.
Amdur et al. 3 reported that, when the H2SO4 concentration was
8 mg/m3, extending the exposure time to 72 hours did not increase
mortality beyond the amount observed in 8-hour exposures. Pathologi-
cal examination of the lungs, however, revealed that the longer expo-
sures greatly increased the damage. In the 8-hour exposures, damage
was limited to thickening of the alveolar walls and consolidation of lung
tissue (CT*=64 mg/m3-hr). In animals exposed for 72 hours (CT=576
mg/m3-hr), damage was greatly enhanced. These workers postulated
*CT is the product of time and concentration.
4 TOXICOLOGY - S02 DECAY PRODUCTS
-------
that sulfuric acid mist has two distinct toxic actions. It promotes
laryngeal and bronchial spasms, causing immediate death by an action
related primarily to concentration and individual sensitivity, and it
also produces parenchymal lung damage, an effect that is more depend-
ent upon total dosage. Pattle et al. 5 pointed out that the lung damage
probably results from the direct irritant action of acid mist deposited
on the lung tissue and that such deposition and irritation may be the
mechanism by which the mist causes death in animals more resistant
to bronchial-laryngeal spasm than the guinea pig. This mechanism
may account, as well, for many of the marked differences in mortality
observed by early investigators who used different exposure concen-
trations.
Exposure Duration
Thomas et al. " exposed guinea pigs to sulfuric acid mist for
periods of 18 to 140 days. They used three particle sizes (0. 6, 0. 9,
and 9. 0 fim) in concentrations of 4 mg/m3 or less, except for occasional
accidental higher exposures. ,The coarse particles produced lesions in
the upper respiratory tract and slight edema in the larynx and trachea,
with less mucus in the major bronchi. The mists composed of 0. 9-^m
particles produced greater lung damage than either the 0.6- or 9. 0-|o.m
particles. Exposure of young animals to 0. 9-nrn mists produced
slightly edematous lungs and, rarely, capillary hemorrhage. Some
increase in desquamated epithelial cells was observed in the minor
but not in the major bronchi; the epithelium appeared intact. Major and
minor bronchi both had less mucus than controls. The authors con-
cluded that the guinea pig could tolerate continuous exposure to levels
of about 2 mg/m sulfuric acid mist for more than 3 months with only
minor effects.
Bushtueva ' exposed guinea pigs to 2 mg/m sulfuric acid mist
of unspecified particle size for 5 days. The animals developed edema
and thickening of alveolar walls. Exposures to 1 mg/m3 for 1 to 2
•weeks produced slight catarrh in the mucous membranes of the trachea
and bronchi, and slightly defined but widely distributed interstitial pro-
liferation accompanied by round-lymphoid-cell infiltration around the
Toxicity in Experimental Animals 5
-------
blood vessels and bronchi. These latter changes seemed to be progres-
sive, for they were more highly developed 2 to 3 months after exposure.
A concentration of 0. 5 mg/m produced only a slight "lung irritation. "
Conclusions
Sulfuric acid is a respiratory irritant, and, when present in
sufficient concentration, it will cause the death of exposed animals.
The guinea pig is the most sensitive species among the common small
laboratory animals, followed in decreasing order by mice, rats, and
rabbits. The guinea pig's greater sensitivity probably results from its
greater susceptibility to bronchial and laryngeal spasm. Although the
enhanced sensitivity makes this animal useful for some studies of
the irritant potential of low concentrations of sulfuric acid, the results
must be applied judiciously when extrapolation is made to the effects
of sulfuric acid air pollution on human health. In all cases, con-
centration has been shown to be a more important factor in sulfuric
acid-produced death than duration of exposure.
*
Sulfuric acid also causes pathological lesions in the lungs after
much less than lethal exposures. Effective, short exposures (up to
72 hours) produce a degree of lung damage related to the arithmetic
product of concentration and time (CT) rather than to the concentration
alone. Such damage is slow to repair. The fact that concentrations
below 2 mg/m^ produce only minor lung damage after extended expo-
sures (about 3 months) indicates that the CT-damage relationship
observed in 8- and 72-hour exposures cannot be extrapolated directly
to long-term effects of low concentrations.
Data clearly indicate that the particle size of sulfuric acid mist
is also an important factor in determining its irritant potency. When
the criterion was mortality resulting from an 8-hour exposure, a mist
of larger particles (2. 7 (im versus 0. 8 (im) was more toxic. When
pathological lesions resulting from long-term exposures were the
criterion of response, the effect of 0. 9-fj.m particles was greater than
that of either 0. 6- or 9. 0-|j.m particles. The effect of particle size
will be discussed further in the next section.
TOXICOLOGY - S02 DECAY PRODUCTS
-------
STUDIES OF PULMONARY FUNCTION
Methods like the ones discussed in this section can be used to
evaluate the effects of low concentrations of irritants that do not
usually produce lung damage or cause death. In other words, they can
be used to examine the response of experimental animals to air pollu-
tants at concentrations known to occur in urban atmospheres.
Sulfuric Acid Mist Particle Size
Employing unanesthetized and spontaneously breathing guinea
9-11
pigs, Amdur measured the effect of sulfuric acid mist on pulmo-
nary function by the following parameters: flow resistance of the lungs
and airways; compliance (a measure of the elasticity of the lungs); tidal
volume (the volume of each breath); respiratory frequency; and minute
volume (tidal volume times respiratory rate). These data also per-
mitted her to make rough estimates of elastic-resistive work and the
total work of breathing.
Amdur used concentrations ranging from 2 to 40 mg/m and
particles of 0.8, 2.5, and 7.0 ^m (MMD). Experimental results
indicated that the size of particles in the aerosol is critical to both the
nature and degree of the irritant response elicited. Results of these
experiments made clear that data obtained from exposure to high con-
centrations could not have been extrapolated to indicate, in a valid
manner, the reaction to lower concentrations , since irritant potency
varied with both concentration and particle size of the sulfuric acid
mist. This fact underlines the need for, and the importance of,
methods that can directly assess responses to low concentrations of
irritants.
The 7. 0-fim-MMD particles, at a concentration of 30 mg/m ,
produced a statistically significant increase in flow resistance but no
other detectable changes in respiration. Few particles of this size
would be expected to penetrate beyond the nasal passages, so that the
observed increase in flow resistance was probably restricted to the
upper respiratory tract, especially since the other parameters measured
were not altered.
Toxicity in Experimental Animals
-------
The 0. 8-nm-MMD particles produced an increase in resistance,
accompanied by a proportionally lesser decrease in compliance, an
increase in elastic and resistive work, and, hence, an increase in the
total work of breathing. These changes were statistically significant
at the lowest concentration tested (1. 9 mg/m^). Onset of response was
prompt and resembled the pattern of changes observed after inhalation
of sulfur dioxide. Changes in pulmonary function such as these are
consistent with narrow air passages caused by bronchoconstriction,
mucosal swelling, or increased mucosal secretion.
The 2. 5-^m-MMD particles also increased flow resistance at all
concentrations examined (2. 3 to 44 mg/m-). The relative toxicity of
the 0. 8- and 2. 5-|j.m-MMD particles, evaluated by increased pulmonary
flow resistance, was highly dependent upon concentration; however, at
high concentrations, the larger particles elicited a much greater re-
sponse, whereas at low concentrations the smaller particles gave the
greater response, again demonstrating that data obtained from expo-
sures to high concentrations could not be used to predict accurately the
responses to low concentrations. The increased toxicity of 2.5-^m-
as compared with 0. 8-(j.m-MM-D particles at high concentrations con-
A
firmed the earlier finding of Pattle et al.
Further comparison of data for the two particle sizes suggested
a difference in physiological action, presumably related to the site
where the particles were deposited in the respiratory system. Response
to the 2. 5-jim-MMD particles, but not to the 0. 8-|J.m-MMD particles,
was slow to develop, occurring only after 40 to 45 minutes of exposure.
This delay suggested the possibility that the different responses were
mediated by different mechanisms, a possibility that was confirmed
both by the nature of the changes in the mechanical properties and by
the post-mortem appearance of the lungs.
The alterations in mechanical properties that occurred indicated
that large sulfuric acid particles produced a complete obstruction of
some air passages,- whereas small particles caused a narrowing of the
airways but not complete obstruction. The gross pathology of the lungs
TOXICOLOGY - SO, DECAY PRODUCTS
-------
was consistent with data derived from pulmonary function tests. Ani-
mals exposed to the high concentration of 2. 5 -|J.m-MMD particles devel-
oped extensive areas of atelectasis, frequently involving whole lobes.
In addition, their lungs were edematous, as demonstrated by a signifi-
cant increase in the ratio of lung weight to body weight. These findings
suggested that high concentrations of larger particles, which probably
are deposited in the vicinity of the major bronchi, produce severe
local irritation of the larger air passages and cause swelling and
increased exudation of fluid, which eventually lead to complete
obstruction.
The pulmonary flow-resistance values were slow to return to
pre-exposure levels after the end of a 1-hour exposure. Animals
exposed to 15 mg/m or more still showed elevated resistance 2 hours
after exposure had ended. This slow return to normal has been found
consistently after exposure to irritant aerosols or to the combination
of irritant gas and inert particulate material. '•'•' ^' The reasonable
explanation for the slower recovery from exposure to an aerosol as
opposed to an irritant gas is apparently the fact that the particles,
having been deposited on the respiratory tract, continue to exert their
irritant effect. The irritant gases, on the other hand, are cleared
from the lung when exposure ceases, and the respiratory system
returns to normal functioning rather rapidly unless the exposure con-
centration has been extremely high. The enhancement that results from
combining inert aerosols and irritant gases appears to be mediated
through the formation of an irritant aerosol, ' which could account
for the prolonged response to such exposures.
Lewis et al. ^> ^ studied the effects of chronic H2SO4-aerosol
exposure on canine pulmonary function. Animals were exposed for
621 days to 0.5-(xm-MMD HzSO4 particles, at a concentration of 0. 8
mg/m^, and pulmonary function tests were performed 225 days and
621 days following the onset of exposure. At 225 days, diffusion
capacity had decreased. After 621 days of exposure, average carbon
monoxide diffusion capacity and residual lung volume were reduced and
Toxicity in Experimental Animals
-------
total pulmonary resistance was increased. Post-mortem examination
showed that the net lung volume was reduced.
Sulfur-Containing Particulate Aerosols
Amdur and Corn'- studied the irritant potency of zinc ammonium
sulfate, zinc sulfate, and ammonium sulfate, and found a difference in
the potencies as judged by increases in flow resistance resulting from
exposure to 1 mg/m concentrations having an MMD particle size of
0.29 him.
Zinc sulfate exhibited about half the irritant potency of zinc
ammonium sulfate. Ammonium sulfate was the least irritant of the
three compounds, producing a response about one-third to one-fourth
that observed from exposure to an equal concentration of zinc ammonium
sulfate.
In further studies of the effect of particle size on the irritant
potency of zinc ammonium sulfate, four MMD sizes were used: 0.29,
0.51, 0.74, and 1. 40 (J,m. Concentrations ranged from 0. 25 to 3. 60
mg/m . Figure 1 shows the results obtained when the concentration
of aerosol was held constant at about 1 mg/m and the particle size was
varied. Particle size by weight is indicated in micrometers. This
method for calculating mass-size is explained by Amdur and Corn.
From the curve it is evident that the irritant potency, as judged by the
increase in flow resistance produced by the 1-hour exposure, increased
as the particle size decreased over the range of sizes examined.
Amdur and Corn1" concluded that the greater irritant potency of the
smaller particles could represent the reaction to an increased number
of pinpoint stimuli produced by the greater number of particles present
in the more finely dispersed material; to the differing depth of penetra-
tion into the peripheral areas of the lung; or to a combination of these
factors.
From data available for several concentrations of zinc ammonium
sulfate at various particle sizes, Amdur and Corn prepared a family of
dose-response curves. As Figure 2 shows, not only was the response
10
TOXICOLOGY - S02 DECAY PRODUCTS
-------
200
100 —
20
J LJ
0.01
0.05 0.10 0.50 1.0
AEROSOL MEAN SIZE BY WEIGHT, micrometers
5.0
10.0
Figure 1. Effect of particle size on airway response to 1 mg/m^ zinc ammonium sulfate
aerosol. Numerals beside each point indicate number of animals. Data and calculation
of mean size by weight from Amdur and Corn.16
to a given concentration greater as the particle size decreased, but the
slope of the dose-response curves was also steeper. This means that
a small increase in mass concentration produced a larger increment in
biological response as the particle size decreased, re-emphasizing
the fact that if a compound is dispersed as particulate material a
measure of mass concentration alone is not sufficient to permit useful
assessment of its irritant potency.
Nadel et al.l'' studied the effect of aerosols of histamine and of
zinc ammonium sulfate on anesthetized, artificially ventilated cats.
The aerosols were submicrometer in size and the concentrations of
zinc ammonium sulfate were 40 to 50 mg/m . The sulfate aerosol
produced a physiological response similar to that produced by hista-
mine, but lesser in degree. The response to a 3-minute inhalation
included increased pulmonary resistance, decreased pulmonary com-
pliance, and increased end-expiratory transpulmonary pressure. As
Toxicity in Experimental Animals
11
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a 60
0.4
1.2 1.6 2.0 2.4
ZINC AMMONIUM SULFATE, mg/m3
2.8
3.2
3.6 4.0
Figure 2. Dose-response curve of zinc ammonium sulfate aerosol for different particle
sizes. Numerals beside each point indicate number of animals. Data from Amdur and
Corn.16
correlation with the physiological studies, the authors made anatomical
studies after rapidly freezing the lungs in the open chest. These showed
that bronchioles up to 400 (im were narrower in exposed than in control
animals and that the mucosa was longitudinally furrowed. Bronchi and
bronchioles larger than 400 ^im were not significantly different from
controls. The authors noted the similarity between the action of hista-
mine and of zinc ammonium sulfate aerosol. In regard to extrapolating
the results they concluded, "It remains to be demonstrated that acute
changes reported here at aerosol concentrations of 40 to 50 mg/m are
exaggerated but similar in nature to those produced, if any, at lower
concentrations1'. Follow-up work is clearly needed in this field.
In the course of examining the effect of aerosols on response to
sulfur dioxide, Amdur and Underbill18 examined the irritant potency
of several other sulfates. They reported that 1 mg/m3 ferric sulfate
TOXICOLOGY - S02 DECAY PRODUCTS
-------
[^62(304)3] produced a 77-percent increase in flow resistance during
a 1-hour exposure, which would definitely indicate that this substance
is an irritant. On the other hand, ferrous sulfate (FeSO4) and manganese
sulfate (MnSO/j) at the same concentration produced no detectable
alteration in resistance. Not all sulfates, then, are irritant in nature,
and the sulfate ion per se is not known to have irritant potency.
Particulates in Combination with Sulfur Dioxide
Because data from atmospheric chemistry studies have indicated
that sulfuric acid and particulate sulfates are formed from sulfur
dioxide emitted to the atmosphere, it is essential that the comparative
irritant potency of sulfur dioxide gas and its chemical reaction pro-
ducts be examined. A long-term research program undertaken by
Amdur et al. has provided data on the effects of sulfur dioxide gas and
several particulate oxidation products on the guinea pig. The authors
are using the same physiological techniques for all the pollutants, which
permits direct comparison of the irritant potency of these compounds.
Their data are summarized in Figure 3.
150
100
I
! 50
UJ »,
^t 20
UJ
7 -
18
0.01 0,02 0.05 0.1 0.2 0.5
SULFUR EQUIVALENTS,
i.o
2.0
5.0
1.0
0.01
0.1
SULFUR DIOXIDE, ppm
1.0
Figure 3 . Comparison of irritant potency of sulfur dioxide and sulfur-containing particulates.
Numerals beside each point indicate number of animals. Data from Amdur.19
Toxicity in Experimental Animals
13
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Even a casual glance indicates clearly that an equivalent amount
of sulfur present as sulfur dioxide gas produces a lesser irritant
response than either the particulate sulfates or sulfuric acid. One
part per million of SO is the equivalent of 1. 3 mg S/m3. When pre-
sent as sulfur dioxide gas, this amount of sulfur increases flow
resistance by about 15 percent. If, through reaction in the atmosphere,
this sulfur equivalent were converted to 0. 7-|j.m-MMD sulfuric acid
mist, it could produce an increase in resistance of about 60 percent,
a fourfold potentiation of irritant response. If, on the other hand, that
same amount of sulfur were converted to zinc ammonium sulfate parti-
cles of 0. 3-|j.m MMD, resistance would increase about 300 percent, a
twentyfold potentiation. Such complete conversion in the atmosphere
is of course highly unlikely, but an oxidation of 10 percent can occur
readily. Ten percent of 1 ppm sulfur dioxide has an equivalent sulfur
content of 0. 13 mg/mj, and if this were present as an irritant sulfate
with a particle size of 0. 3 |am, it would increase flow resistance about
45 percent. If the response to the particulate sulfate and the response
to the remaining sulfur dioxide (which increased resistance about 15
percent) were purely additive, one could estimate a total increase in
pulmonary flow resistance of approximately 60 percent, an important
fourfold potentiation in irritant response. The response to such com-
binations is more than additive, 1">'-(J however, so that the overall irri-
tant effect •would be even greater than these figures indicate. These
data clearly suggest that a toxicological response to sulfur dioxide gas
alone is not a sufficient basis for assessing its role in air pollution.
The overall problem of the effect of inert aerosols on physiolog-
ical responses to irritant gases will not be discussed in detail here.
It has been reviewed in the air quality criteria documents for both sul-
fur oxides and particulate material, and is discussed in a recent
23 18
review by Amdur. Amdur and Underhill reported the results of a
study on the effects of various aerosols on the response to sulfur dioxide
gas. Their findings indicated that the degree to which the response to
sulfur dioxide was potentiated by an aerosol was determined by the
solubility of sulfur dioxide in the liquid droplet and by the extent to
14 TOXICOLOGY - S02 DECAY PRODUCTS
-------
which it was oxidized to sulfuric acid. The importance of solubility
was suggested by the fact that the potentiating ability of aerosols of
sodium chloride, potassium chloride, and ammonium thiocyanate could
be related, in a reasonable manner, to the solubility of sulfur dioxide
in solutions of these salts. Further evidence for the role of solubility
came from the observation that none of the various solid, dry aeroools
that were tested potentiated the response to sulfur dioxide. These
included spectrographic carbon, activated coconut charcoal, iron oxide
fume, triphenyl phosphate, fly ash, and manganese dioxide. None of
these substances, at concentrations of 8 mg/m or above, produced a
detectable effect on pulmonary flow resistance or potentiated response
to levels of sulfur dioxide ranging from 1 to 100 ppm. In contrast,
aerosols of soluble salts of ferrous iron, manganese, or vanadium at
concentrations of 0. 7 to 1 mg/m potentiated response to sulfur dioxide
about threefold. At significant humidity levels like those in the respi-
ratory tract, these substances would not only form droplets, but would
also absorb sulfur dioxide; they are known to be agents that favor the
oxidation of sulfur dioxide to the more potent sulfuric acid. The pro-
perties of these and other sulfur compounds have been studied thoroughly
by many researchers, who have found that the toxicity of the compounds
is directly related to the metal ion itself rather than to the attached
sulfur radical. These compounds will not be reviewed here, but a
relevant bibliography is appended.
Hygroscopic Properties of Sulfuric Acid
Another toxicologic aspect of sulfuric acid aerosol inhalation is
related to its hygroscopic nature. Even when sulfuric acid particles
are inhaled in relatively dry air, they will expand within the respiratory
system by water absorption because inspired ambient air becomes satu-
rated with water drawn through the nose, mouth, and pharynx. Since
large particles are retained with greater efficiency, more hygroscopic
particles, such as sulfuric acid and water-soluble sulfate aerosols, are
deposited than non-hygroscopic materials of the same size. Depending
on other factors, physiologic response may, but need not, be enhanced
Toxicity in Experimental Animals 15
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in proportion to the increase in percentage retained because of expan-
sion in situ.
In the case of hygroscopic aerosols, the original size of the
parent particle will also determine the subsequent size of the hydrated
particle, and, hence, the site of its retention along the respiratory
system. Larger particles will have a tendency to be trapped along the
upper respiratory airways (nasopharnygeal), where their relative
toxicity is usually lessened and from which removal is more rapid.
Increases in particulate size from the nares to the alveoli are difficult
to quantitate in vivo; therefore, studies of the effects of hygroscopic
particles have been limited largely to empirical models, in which
alterations in, or potentiation of, physiologic responses are related to
variations in physicochemical properties of the particulate matter; for
example, size, chemical composition, solubility, catalytic converti-
bility, and moisture absorbability.
When interpreting physiologic responses to aerosols of a sulfurous
nature, assessing their toxic potential, and relating aerosols to models
of respiratory deposition, one must not lose sight of the fact that these
phenomena are influenced by the hygroscopic nature of the particles.
Conclusions
The studies discussed above cannot be extrapolated simply and
directly to predict the human response to particular substances. They
do provide useful information regarding the nature of mechanisms by
which effects are produced; however, they may lend more insight into
responses of the sensitive segment rather than of the robust healthy
segment of the population.
For practical considerations in using these experiments as indi-
rect guidance in air pollution research, two critical points emerge.
The first is the extreme importance of the particle size to the irritant
potency of particulate products of sulfur dioxide. The most important
size is in the submicron range. Data on mass concentration alone pro-
vide an insufficient basis for predicting irritant potency of particulate
products of sulfur dioxide.
16 TOXICOLOGY - S02 DECAY PRODUCTS
-------
The second important point is that the particulate oxidation pro-
ducts of sulfur dioxide generally have much greater irritant potency
than the parent sulfur dioxide gas, as indicated by the comparative
data in Figure 3 as well as by increased response to sulfur dioxide
in the presence of particulate material known to dissolve the gas and/
or catalyze its conversion to sulfuric acid or sulfates. Particle sizes
of oxidative products of sulfur dioxide are, in part, affected by the
hygroscopic properties of the products; consequently, physiologic
responses and pulmonary deposition of inhaled particulates may be
significantly altered by moisture in the atmosphere and within the
respiratory tract. Atmospheric levels of sulfur dioxide and their
health implications should therefore be assessed in terms of the
potential for formation of more irritant substances. To do otherwise
is to miss the toxicological point and seriously becloud important
Toxicity in Experimental Animals 17
-------
TOXICITY IN MAN
Only limited research has been done on the effects of sulfuric acid
on human pulmonary function, and none on the effects on man of the
various sulfates that have irritant potency for the guinea pig. Studies
that regard both physiological methodology and characterization of the
aerosol particle size with meticulous attention are badly needed. The
exposure of human volunteers to low concentrations of H2SC>4 mist, to
define its toxicological effects, has involved two divergent, though
complementary, branches of research. One branch has concerned
itself with the effects of H^SCJj aerosols upon the normal physiological
responses of the central nervous system: the role of H2SO4 mist and
its interrelationship with sensory modalities and cerebral cortical
function. These studies are reported exclusively in the Russian litera-
ture. The other branch of research has been concerned primarily with
the irritative effects of H2SO4 aerosols and the changes in respiratory
function that occur subsequent to exposure to concentrations that
approximate the allowable industrial standards set by the governments
of the United States24'25 and the Union of Soviet Socialist Republics. 26
STUDIES OF SUBJECTIVE AND PHYSIOLOGICAL RESPONSES
TO IRRITATIVE PROPERTIES OF SULFURIC ACID MIST
Detection by Subjects
27
In 1913, Dorsch studied workers in the battery room of a
German telephone exchange. He found that H2SO4 mist was only
rarely detected by the employees when the concentration was below
0.5 mg/m3. They complained of slight nose and throat irritation at
2. 0 mg/m3, distinct discomfort at 3 to 4 mg/m3, and marked discom-
? Q
fort when the concentration rose to 6. 8 mg H2SO4/m3 . Bushtueva
exposed ten volunteers to H2SO4 aerosols with the following results.
When exposed to concentrations less than 1. 1 mg/m3, the volunteers
compained of ticklish, scratchy throats; when exposed to levels rang-
ing from 1. 1 to 2. 4 mg/m3, all subjects noticed considerable
19
-------
irritation at the base of the esophagus and 40 percent of them experi-
enced some irritation of the eye mucosa. When the aerosol level was
raised to 2. 4 to 6. 0 mg/m3, all subjects experienced acute irritation
of the mucous membranes, a pronounced reflex cough, and irritation
of the eye mucosa. The threshold concentration was approximately
0. 7 mg/m3 (0. 60 to 0. 85 mg/m3). In that study, the particle size of
the acid mist was not stated. Amdur2° was unable to detect responses
in subjects exposed to aerosol levels less than 1 mg/m3, using odor,
taste, or irritancy as parameters, although all 15 of her subjects
detected the aerosol when the level was raised to 3 mg/m , noting
that it "felt like breathing dusty air. " Some subjects found 5 mg/m3
very objectionable, but others less so, and, for all subjects, a deep
breath of this concentration usually produced coughing. In another
study, Bushtueva3^ found that the presence of 0. 4 mg H2SO4/m3. even
when combined with 1 mg SO2/m , was below the sensitivity threshold
of all subjects. The fact that the results reported in each of these
papers are remarkably similar to Dorsch's^^ original description of
H2SO4 mist irritancy, indicates that the dose-response curve for
human beings has remained quite stable during the past 60 years of
experimentation and, in contrast to animal data, appears to be
physiologically predictable in almost all instances.
Effects of Humidity, Acidity, and Particle Size on Subject Response
Sim studied the response of healthy males, aged 18 to 46, to
r^SO,^ mist. He noted that at a given concentration a mist of larger
particle size and lower acidity, produced under conditions of high
humidity, was more irritant than a mist produced under lower humid-
ity at the same concentration, but with smaller particles and higher
acidity. Pulmonary air-flow resistance was 35 to 100 percent greater
than normal when the volunteers were exposed to a ION H2SC>4 aerosol
of 1-p.m-MMD particles at a concentration of 39 mg H2SC>4/m , a
temperature of 18.4° C, and humidity of 61 percent. Altering the
aerosol to 25° C, 91 percent humidity, acidity of 4N H2SO4, and
particle size of 1. 5 (xm evoked intense coughing, lacrimation, and
20 TOXICOLOGY - SO, DECAY PRODUCTS
-------
rhinorrhea, which continued throughout the exposure. Coughing
decreased somewhat after the first 10 minutes of exposure and airway
resistance ranged from 43 to 150 percent greater than normal. Adding
magnesium oxide particulate to the exposure air did not alter the irri-
tancy, although addition of ammonia toward the end of the H2SC>4
exposure resulted in rapid and complete disappearance of all symptoms,
thus corroborating Pattle's5 observations on guinea pigs. Toyama33
also correlated airway resistance with particle size. In an aerosol of
4. 6-(im particles, a. mist concentration of 0. 8 to 1.4 mg/m3 increased
airway resistance 36. 5 percent in 24 healthy males; as an aerosol of
1. 8-|_im particles, a mist concentration of 0. 01 to 0. 1 mg/m3 increased
airway resistance 17. 9 percent. The latter concentrations appear
questionable, since 0.01 to 0. 1 mg H^SO^/m are concentrations far
below the reported thresholds for any physiological response.
Hamilton32 stated in 1948 that the site of action for H2SO4 fumes
is the upper respiratory tract, "that this action is not severe, and that
no instance of bronchitis [is]. . .traceable to such fumes. " Sim3-!
described, however, how he, himself, became increasingly sensitive
to the mist during 10 months of exposure, developing a moderately
severe but extremely persistent bronchitis, which immediately
exacerbated into uncomfortable periods of coughing and wheezing that
occurred whenever he was exposed to H2SC>4 mist.
Effects of Sulfuric Acid Mist on Respiratory Retention
Apparent increases in the respiratory rate of human subjects
exposed to H2SO4 irritant were described in reports by Bushtueva, 2°
Sim, 31 and Amdur. The increase is not suprising, for Wilson34
in 1948 showed that the retention of inhaled aerosol particles decreased
as the respiratory rate increased. Morando3^ briefly (for 15 minutes)
exposed normal human subjects not accustomed to inhaling HzSO4
fumes to concentrations varying from 0. 35 to 5. 0 mg/m . Even at
concentrations not subjectively detected by the volunteers (less than
0.5 to 0. 7 mg/m3), the respiratory rate increased rapidly and remained
Toxicity in Man 21
-------
elevated for several minutes after exposure. Amdur^9 exposed fifteen
subjects to 0.35 to 0. 5 mg/m3 and noted that the respiratory rate
increased about 30 percent above control values; the maximum inspira-
tory and expiratory flow rates decreased by about 20 percent; and tidal
volume increased by about 28 percent. These changes occurred during
the first 3 minutes of exposure and were maintained throughout the 15-
minute exposure period. Values returned to pre -exposure levels within
15 minutes after the exposure ended. During the first minute after expo-
sure, the tidal volume rose above and then declined to control levels.
These experiments do not provide the physiological evidence
necessary to indicate conclusively that bronchoconstriction was the
response to H2SC>4 mist, although, the data suggest that this was the case.
2R
Bushtueva found clear-cut reflex changes in respiratory rhythm and
amplitude when she gave volunteers multiple exposures to an
of 1.8 to 2. 0 mg/m3. At concentrations of 1. 0 to 1. 1 mg/m , changes
in respiration were also noted, though rhythm and amplitude were
decidely lower. She was unable to demonstrate respiratory changes
when the mist concentration was below 1. 0 mg/m3, but this inability
may have reflected differences in the degree of "aerosol dispersion. "
As the respiratory rate increases, the percentage of H2SO4 mist
retained by the respiratory tract is definitely reduced. The change in
respiratory rhythm and amplitude may be a reflex protective mechanism
that prevents the retention of the irritant, ^9, 34 thereby decreasing its
toxic effects. By measuring the total volume of air breathed, and the
concentration of the H2SO4 aerosol in the mixing chamber and in ex-
haled air, Amdur^° was able to show that average retention of inhaled
H2S<-)4 varied from 50 to 87 percent with exposure concentrations of
0.4 to 1.0 mg/m . She gave two possible reasons for such relatively
high retention. First, the aerosol levels used in the experiment in-
creased the respiratory rate by only 35 percent (5 to 7 breaths per
minute), certainly not enough to reduce retention far below baseline
values; and, second, the strong affinity of H2SC>4 droplets for moist
surfaces, such as the mucous membranes of the respiratory tract,
may have enhanced retention.
22 TOXICOLOGY - SO? DECAY PRODUCTS
-------
As noted earlier, the respiratory rate and pulmonary air-flow
resistance of human volunteers rose rapidly as exposure concentrations
of H2SO4 aerosol were increased. Symptomatically, bronchospasm
was also evident in the healthy subjects, presenting a kaleidoscopic
picture of wheezing, increased secretions, dyspnea, spasmodic cough-
ing, and occasional chest pains. The intensity and combinations of the
symptoms depended upon the aerosol concentration and length of
exposure. Two observer-subjects reported occurrence of chronic
bronchitis during long-term intermittent exposure to H2SC>4 aerosol.
Bronchospasm, increased upper-respiratory-tract secretions,
and a rapid respiratory rate were consistently found, along with eleva-
ted pulmonary air-flow resistance, in volunteers exposed to mist levels
greater than 0. 35 to 0. 5 mg/m . All of the subjects involved in pre-
vious experiments were young healthy adults who could easily compen-
sate for the increased resistance imposed on their breathing. Elevated
airway resistance severely embarrasses breathing in individuals already
burdened by cardiorespiratory deficiencies, however, making it more
difficult to obtain needed oxygen. Moreover, bronchospasms and
increases in secretory and respiratory rates impair gaseous exchange
in the lungs, and carbon dioxide accumulates, which causes respiratory
acidosis. An acidotic condition superimposed on a restricted oxygen
intake explains many of the effects seen in pulmonary cripples when
they are exposed to rapidly rising concentrations of t^SO^ aerosols,
above threshold levels.
Conclusions
Research data concerning the effects of aerosol particle size,
temperature, or humidity on the human response to H2SC>4 mist
exposure are scarce. The few long-term human-exposure studies have
been limited, essentially, to epidemiological surveys. Ethical con-
siderations preclude achieving the requirements of controlled long-
term, experimental, human exposure as such. Intensive industrial-
exposure investigations, rather than controlled laboratory experiments,
may then be the only acceptable source of needed information. Chronic
Toxicity in Man 23
-------
bronchitis did develop over the course of at least one long-term study,
however, and various case histories have reported the development of
acute and/or chronic respiratory disease after patients endured short-
or long-term exposure to H2SO4 aeros.ols. ^°> '
Definitive data are lacking, but exposure to low concentrations of
sulfuric acid appears to increase pulmonary flow resistance in human
subjects. Determining the minimum concentration at which such
changes might be expected to occur, which is possible for SO2. is
impossible for H2SO4- Unfortunately, most experiments on the general
complex of sulfur -containing pollutants have dealt with the less toxic
sulfur dioxide. Additional work, to be most meaningful, must be con-
ducted by a design that incorporates the refinements of both physiolog-
ical methodology and aerosol technology. A failure in either of these
areas would severely limit the value of the data.
STUDIES OF SENSORY AND CENTRAL NERVOUS SYSTEM
RESPONSES TO SULFURIC ACID MIST
In the U. S. S. R. , air pollutant limits have been rigidly set by the
standard, "that the ambient air should not contain odors to be imposed
on the population against its wishes". ..." With this standard as a
guideline, the Russians in the middle fifties conducted studies to limit
SC>2 and H2SC>4 in the air to acceptable levels. The limits finally
selected were based on the subjective reactions of human volunteers
exposed to either SC>2 or H2SO4- The perception threshold for odor
and mucosal irritation was about 1. 5 mg SO2/m3 and about 0. 5 mg
. Subsequent to these studies, in 1957, the Russian govern-
ment established the allowable single concentrations of SC>2 and
at 0. 5 and 0. 3 mg/m3, respectively levels one-half to one-third the
threshold values. *26, 40
#The term threshold concentration applies to that level of H SO
aerosol or SC>2 gas that is just perceptible as either odor or irritation.
This concentration lies between 0. 6 to 0. 85 mg/m for the aerosol,
and between 1. 6 to 2. 8 mg/m3 for the gas. 26> 28> 30> 40> 41 Subthres-
hold implies that the level of exposure cannot be detected either by
odor or by irritation.
24 TOXICOLOGY - S02 DECAY PRODUCTS
-------
Between 1956 and 1962, Russian scientists attempted to define
that level of SO2 and/or H2SO4 that could trigger responses in specific
receptors of the respiratory system (including the nose). They exposed
subjects simultaneously to subthreshold concentrations of SO2 and H2SC>4
in an effort to detect responses in the central nervous system (brain).
The respiratory receptors, particularly those in the olfactory area,
have a well-recognized sensitivity to the action of minimal concentra-
tions of chemical substances that is partially explained by the fact that
they are located where the outside air initially comes in direct contact
with the internal organism.
The varied responses to H2SC>4 exposure discussed in this section,
although not strictly within the realm of classical toxicology, are
included because the data are closely related to the biological considera-
tions of this review. The practical ramifications and toxicological
implications of the subsequent neurophysiological responses have not
been adequately explored. In addition, one must exercise caution in
interpreting such data, since no replicative studies have been made to
confirm the reported neurophysiological responses, and informative
descriptions of experimental conditions have been less than adequate.
Optical Chronaxy
Alterations in optical chronaxy* following stimulation of the
respiratory receptors were examined extensively in hopes of finding
the allowable limits of atmospheric pollutants. No change in optical
chronaxy was noted when the legal maximum concentrations of SO2
and/or H2SC>4 aerosol were administered to volunteers (for H2SO4,
*Weak electrical current applied to the eyeball that produces the sensa-
tion of a flash of light. Each subject has a maximum intensity of stim-
ulation below which he does not perceive light. The intensity thres-
hold, expressed in units of electrical potential (i. e. , volts) is called
the rheobase. Chronaxy is the electrical current times twice the
intensity that produces the rheobase. A stimulating current of two
rheobases causes light sensation only when it is sufficiently prolonged.
This value, that is, the time threshold necessary for the appearance
of light sensation, is called optical chronaxy.2°
Toxicity in Man 25
-------
0.3 mg/m ; for SC>2, 0.5 mg/m3); nor when subthreshold concentra-
tions were administered (for t^SC^, 0. 6 mg/m3; for SC>2, 1.2 mg/m3)
(Table 1). 30 Exposure to levels of SC>2 or H2SO4 beyond threshold
consistently elicited increases in the optical chronaxy. When the two
irritants were combined, such that the concentration of both was equal
to or greater than the subthreshold concentration, the optical chronaxy
value indicated an additive or synergistic depression of central nervous
system excitation. These data were not sufficient to determine whether
the primary response came from the optic cortex itself, or whether a
response in the olfactory zone caused a secondary inhibitory effect in
the visual cortex, for no changes were detected until concentrations
exceeded the odor threshold. Ryazanov has postulated the latter
mechanism as a possible explanation for the increased optical chronaxy
effect.
Table 1. OPTICAL CHRONAXY ON NINTH MINUTE OF EXPOSURE OF ONE SUBJECT
TO TEST ATMOSPHERES3' 3°
Test gas
Fresh air
so2
so2
H2S04
H2S04
S02 and H2S04
SO- and H2S04
S02 and H2S04
S09 and H0SO,,
2 24
Concentration, mg/m
-
1.5 to 1.8
1.0 to 1.2
0.73 to 0.75
0.60 to 0.63
1.2 and 0.6
0.8 and 0.6
1.2 and 0.4
1.5 and 0.75
Average optical chronaxy, yF
0.61 ± 0.0037
0.80 t 0.0015
0.62 ± 0.0070
0.70 + 0.0090
0.62 + 0.0058
0.71 ± 0.0120
0.71 1 0.0120 (sic)
0.61 t 0.0100
0.98 + 0.0280
Results of tests on one subject, Madame B.
Dark Adaptation
Information based on optical chronaxy was substantiated by studies
of dark adaptation. The Hagel adaptometer, an instrument that meas-
ures changes in light sensitivity of the visual organ while the subject is
in darkness, was used, and test subjects were exposed to several con-
centrations of SC>2 and/or H2SO4 aerosol. The average sensitivity of
subjects to light was computed from a curve illustrating changes in dark
26
TOXICOLOGY - SO? DECAY PRODUCTS
-------
adaptation with time. The raw data (in relative units) in Table 23°
show a marked similarity to the data obtained with optical chronaxy
techniques. Subthreshold concentrations had no effect on dark adapta-
tion, but levels exceeding threshold reduced adaptation activity in the
visual cortex. ' ' According to Ryazanov, 2 this effect can also
be explained as the result of a primary reflexive response in the
olfactory zone that leads to a short period of optical cortex inhibition,
which is followed by another, prolonged period of depressed activity in
the optical cortex secondary to the influence of reticular formation.26'43
The second component of this diphasic response was noted in some
subjects when their light sensitivity was found markedly depressed 50
to 60 minutes after the dark-adaptation curves had shown post-exposure
return toward normal.
Table 2. EYE SENSITIVITY TO LIGHT IN THREE SUBJECTS
DURING TWENTIETH MINUTE OF DARK ADAPTATION30
Test
no.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Concentration of
gas added, mg/m3
S02
H2S04
Madame D
0
0.65
3.0
0
3.0
0
0.3
0
0.7
0.7
Madame P
0
0.65
3.0
0
3.0
0
0.3
0
0.7
0.7
Madame V
0
0.65
3.0
0
3.0
0
0.3
0
0.7
0.7
Average light
relative
33,400 + 1,
33,200 ± 1,
41,100 + 1,
42,400 t
54,600 t 1,
54,300 1 2,
55,500 ± 1,
69,300 + 5,
62,850 + 3,
82,750 t 1,
42,133 +
41,767 +
56,200 t 3,
54,700 t 1,
68,967 t 3,
sensitivity,
units
153
841
570
0 (sic)
200
857
385
100
268
432
972
546
377
570
050
Toxicity in Man
27
-------
Continuous Electroencephalography
The Russian scientists next turned to continuous monitoring of
the cerebral cortex by electroencephalography (EEC), directly record-
ing the state of central nervous system function during exposure to SO2
40 43 44
and/or H2SO4 aerosols. ' ' Mucous membrane irritation,
secondary to the sulfur irritation, innervates the trigeminal nerve,
while odor perception results from direct excitation of the olfactory
receptors. Stimulation of any receptor organs is followed by alpha-
rhythm (low-frequency, high-amplitude EEC signal) suppression or
desynchronization. Individuals with well-defined alpha rhythms were
selected as subjects and the criterion of response was defined as the
onset of electrical desynchronization, or alpha-rhythm suppression.
Exposing subjects to subthreshold concentrations elicited no electro-
encephalographic changes, nor did application of both SC>2 and H2SO4
at maximal allowable concentrations elicit alpha-rhythm desynchroni-
zation, at either beginning or end of exposure. Desynchronization was
observed only when threshold doses of H2SC>4 mist and/or SC>2 were
used; then it lasted for 1 to 2 seconds. The H2SO4 aerosol changed
alpha rhythms only when the concentration was sufficient to be noted
subjectively by the volunteers. Subthreshold concentrations, not
subjectively perceived, did not appear to alter the central nervous
system signals, and, at higher concentrations, the crude subjective
indices - odor perception and mucosal irritation - apparently correlated
well with the onset of alterations in the central nervous system. Sub-
jective evidence of cortical response was found only when the concen-
trations were sufficient to affect the course of the other reflex effects
(i. e. , optical chronaxy and dark adaptation), so that subjective
responses apparently were valid as objective indices of the cortical
effects of the irritants.
Conditioned Reflexes
An additional physiological concept is referred to in the Russian
literature, although apparently not discussed at all in American toxico-
45 4.1
logical studies. According to Gershuni and Dubrovskaya, cortical
28 TOXICOLOGY - SO? DECAY PRODUCTS
-------
reactions may occur upon exposure to certain subthreshold (subsensory)
excitations. This may play an essential role in toxicological and
health effects. If cortical reactions to subthreshold levels do occur,
one must conclude that to be acceptable ambient pollutant levels must
not only be below the level of odor perception and irritancy but also
below the threshold of subconscious reflex reactions, particularly
those capable of altering the functional state of the cerebrum. The
electrocortical conditioned reflex is a central nervous system phenom-
enon elicited only after a succession of repeated, conditioned reflex
trials. As indicated previously, temporary desynchronization of the
alpha rhythm occurs each time a subject is given a strong light signal.
When light is excluded, the alpha rhythm is restored. Experimentally,
a. system is established by which a subject inhales, for 10 seconds, a
known subthreshold concentration of SO2 gas and/or H2SO4 aerosol
that by itself does not cause the alpha rhythm to desynchronize. He
then receives a light over a 5- to 10-second interval. After repeating
this combination (irritant plus light) several or many times, desynchro-
nization begins to appear before the light; that is, apparently in
response to the unperceived odor. The unperceived odor thus appears
to become the conditioning stimulus, generating the so-called condi-
tioned electrocortical reflex. " Through this reflex, EEG alpha-wave
desynchronization occurred when individuals were exposed to an H£SO4
aerosol concentration of 0. 4 mg/m , an SC>2 concentration of 0. 6
mg/m , or to a combination of either 0. 15 mg H2SC>4/m plus 0. 5 mg
SO2/m3, or 0. 3 mg H2SO4/m3 plus 0. 25 mg SC>2/m3. 40' 43> 44 All of
these concentrations are below the subjective sensory threshold. With
respect to H2SO4, the conditioned reflex was usually developed in the
course of 19 to 23 trials with H2SO4 aerosol plus light (Table 3).
Remarkable as it may seem, the complex neurological process result-
ing in disruption of cortical rhythm was initiated by a subjectively non-
perceivable odor. °
Conclusions
The Russian data are concerned wholly with the effects of H2SO4
aerosols upon the interrelationship between sensory modalities and
Toxicity in Man 29
-------
Table 3. NUMBER OF TRIALS WITH ^04 PLUS LIGHT
REQUIRED FOR APPEARANCE OF CONDITIONED REFLEX40
Subject
NN
GV
SN
ML
H2S04 concentration,
mg/m3
0.6 to 0.75
8
16
12
6
0.4
23
-
21
19
cerebral cortical function. Those concentrations of H2SO4 mist that
produce subjective sensations (odor and mucous membrane irritation)
were invariably associated with objective evidence of central nervous
system depression. Subjective, subthreshold, acid mist concentra-
tions were not associated with depression of either optical chronaxy,
dark adaptation, or electroencephalographic patterns. Only when the
electrocortical conditioned reflex technique was employed did sub-
threshold levels result in detectable central nervous system changes.
This is evidence that a chemical stimulator, unable to initiate an
electroencephalographic response when used in subthreshold doses by
itself, can stimulate the central nervous system and change the irri-
tability of cortical cells of the optical and other analytic or cognitive
40
centers when it is used as a conditioning stimulus. Table 4 sum-
marizes the sensory-neurologic dose-response data just described.
Table 4. THRESHOLD AEROSOL DOSES REQUIRED FOR SENSORY
RESPONSE44 (mg/m3)
44
Threshold dose
Response
Perception of odor and
irritation of mucosa
Suppression of dark
adaptation
Elevation of optical chronaxy
Disruption of alpha rhythms
Conditioning of electro-
cortical reflex
H2S04
0.6 to 0.85
0.63 to 0.73
0.73
0.63
0.4
S02
1.6 to 2.8
0.92
1.5
0.9
0.6
H2S04 + S02
0.3 + 0.5
0.3 + 0.5
0.6 + 1.2
0.3 + 0.5
0.15 + 0.5
or
0.3 + 0.25
30
TOXICOLOGY - S02 DECAY PRODUCTS
-------
SUMMARY
Studies of atmospheric chemistry have shown that under certain
conditions sulfur dioxide can be converted to sulfuric acid and particu-
late sulfates. Air quality criteria and standards for sulfur dioxide
should, therefore, take into account the irritant potency of these
oxidation products, recognizing that both sulfur dioxide and its
degradation products can react further with other air pollutant consti-
tuents. Additional aerometric and toxicologic research is needed to
supplement what little is known of the actual existence in the
atmosphere of this group of compounds and of their toxicology.
TOXICITY IN EXPERIMENTAL ANIMALS
Sulfuric acid is a respiratory irritant that in sufficient concentra-
tion will cause the death of exposed animals. The guinea pig is the
most sensitive species among the common small laboratory animals,
followed in decreasing order by mice, rats, and rabbits. The greater
sensitivity of the guinea pig results from its greater susceptibility to
bronchial and laryngeal spasm, a characteristic that makes the animal
useful for some studies of the irritant potential of low concentrations
of sulfuric acid, but that makes extrapolation of the results obtained of
questionable value in predicting the health effects of sulfuric acid air
pollution in man. Experiments to date have shown that concentration
is a more important factor than duration of exposure in producing
death in animals.
Sulfuric acid also produces pathological changes in lungs. The
degree of lung damage observed after effective, short exposures (up to
72 hours) is related to the mathematical product of concentration and
time (CT) rather than to concentration alone. Sulfuric acid damage is
repaired slowly. Concentrations below 2 mg/m3 produce only minor
luhg damage after extended exposures (about 3 months), which indicates
31
-------
that the CT relationship observed in 8- and 72-hour exposures cannot
be extrapolated directly to long-term effects of low concentrations.
The particle size of sulfuric acid mist seems to be another impor-
tant factor in determining its irritant potency. When the" potency crite-
rion was mortality resulting from an 8-hour exposure, a mist of larger
particles (2. 7 |J.m versus 0. 8 |J.m) was more toxic. When pathological
lung damage resulting from long-term exposures was the criterion, the
effect of 0. 9-(J.m particles was greater than that of either 0. 6- or 9-|J.m
particles.
Studies of pulmonary function in animals exposed to these sub-
stances cannot be extrapolated simply and directly to predict the human
response. They are, however, providing useful information regarding
the nature of mechanisms by which effects are produced, and may yet
lend insight into responses that are more analogous to the responses of
sensitive individuals in the population than.of healthy. For practical
considerations, these experiments present two principles for indirect
guidance in the area of air pollution research. First, because particle
size plays an important role in determining irritant potency of sulfur
oxide particulates (submicron particles being more irritant), data on
mass concentration alone are insufficient for predicting irritant potency.
The second important point is that the particulate oxidation products of
sulfur dioxide have a generally much greater irritant potency than
sulfur dioxide gas per se. This is indicated by comparative data as
well as by increases in response to sulfur dioxide in the presence of
particulate materials that are known to dissolve the gas and/or to cat-
alyze its conversion to sulfuric acid or sulfates. Along with factors
such as the original particle size, presence of other materials in the
inhaled air, the subject's breathing pattern, and the site of particle
deposition, the hygroscopic nature of sulfuric acid and certain sulfate
aerosols affects physiologic responses to their inhalation. Atmospheric
levels of sulfur dioxide and the implication .of these levels for health
should thus be assessed in terms of their potential for forming more
irritant substances. To do otherwise would be to miss the toxicological
point and seriously becloud important issues.
32 TOXICOLOGY - S02 DECAY PRODUCTS
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TOXICITY IN MAN
The combined effects of particle size and concentration of
mist on exposed human subjects have still not been determined. The
effects of temperature and humidity on the response in humans toH2SO4
have not been satisfactorily examined either. Long-term human expo-
sure information is, by necessity, limited to epidemiological surveys.
Since controlled long-term experimental exposure of humans cannot
ethically be performed, industrial and episodic exposures are the only
sources of needed information and should be investigated. One long-
term study and various case histories have reported the development of
acute and/or chronic respiratory disease following short- and long-term
exposure to 1^230^ aerosol.
Although definitive data are lacking, animal studies indicate that
sulfuric acid should increase pulmonary flow resistance in human
subjects. The concentration at which such changes might be expected to
occur is, however, impossible to determine. Regrettably, the bulk of
the experiments relating to the general complex of sulfur-containing
pollutants has dealt with the less irritating sulfur dioxide. More
research should now be directed to the other components of the complex.
Russian scientists have been almost entirely concerned with the
effects of H2SO4 aerosols upon sensory modalities, cerebral cortical
function, and their interrelationships. Concentrations of H2SO4 mist
that resulted in subjective sensory stimulation (odor and mucous mem-
brane irritation) were found to be associated invariably with objective
evidence of central nervous system depression. Subjectively deter-
mined subthreshold concentrations of acid mist were not associated
with depression of either optical chronaxy, dark adaptation, or electro-
encephalographic alterations. Only when the electrocortical conditioned
reflex was used did sensory subthreshold levels produce central nervous
system stimulation. This has been taken as evidence that, when used
as a conditioning stimulus, a chemical stimulator that is unable to
initiate an electroencephalographic response when used in subthreshold
doses by itself can stimulate the central nervous system and change the
irritability of cortical cells of the optical and other analytic or cognitive
centers.
Summary 33
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BIBLIOGRAPHY:
SELECTED TOPICAL REFERENCES
BARIUM SULFATE
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39
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MAGNESIUM SULFATE
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42 TOXICOLOGY - S02 DECAY PRODUCTS
•h U. S. G.P.O.: 1972- 746-753/3804, Region No. 4
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