This manuscript was prepared as part of the Environmental
Epidemiology Planning Project of the Health Effects Institute, September
1990 - September 1992.
This paper is based, in part, on research supported by
Cooperative Agreement CR 811563 between New York University Medical Center
and the U.S. EPA and is part of a Center Program supported by ES 00260
from the National Institute of Environmental Health Sciences.
Introduction
While it is well established that short-term exposures of humans to ozone
(O3) produce a plethora of transient responses such as reduced
ventilatory function; increased symptoms, permeability, and reactivity (1);
and an influx of inflammatory cells and mediators (2), there is relatively
little known about the roles of repetitive transient exposures and the responses
they induce in the development of cumulative lung damage and/or disease.
Many of the transient responses produced by exposures to O3 are
similar to those produced by cigarette smoke, a known causal factor for
chronic lung disease. Since about half of the U.S. population lives in communities
having O3 concentrations that exceed the current National Ambient
Air Quality Standard, there is an ample basis for research on the effects
of chronic O3 exposure.
While past research studies on the chronic effects of O3 have
not been definitive, there are some provocative indications that there may
be substantial adverse effects. The indications include: greater rate of
loss of lung function in nonsmoking men and women in both Glendora, California,
(high oxidant) and Long Beach, California (moderate oxidant and moderate
SOx), than in Lancaster, California (moderate oxidant and low
SOx) (3,4); reduced baseline lung function when
annual average O3 concentration is greater than 40 ppb, based
on a national population sample (5); and an unexpectedly high incidence
of centriacinar region disease in the lungs of adolescents and young adults
examined post-mortem in Los Angeles County (6).
There are a variety of ways in which epidemiologic research can provide
evidence of adverse chronic health effects in humans resulting from long-term
exposure to O3 and/or the other ambient air pollutants that coexist
with it. Prospective cohort studies in well-defined populations of interest
could be performed with suitable and careful measurements of exposure, activity
patterns, symptoms, lung function, etc. However, it may be hard to justify
such a study at this time for several reasons, including lack of firmer
evidence that chronic effects are occurring, the very high costs of properly
performed prospective studies, and the long time frame for results (i.e.,
at least 7-10 years).
For the above reasons, retrospective human lung studies may be most appropriate
at this stage of inquiry, despite the great difficulties in adequate characterization
of past exposure to O3 and copollutants and adequate evaluation
of confounding and modifying factors. Such studies have their own inherent
advantages (i.e., the existence and extent of early chronic lesions in the
peripheral lung tissues can be quantified), and no other kind of human studies
can provide such information. Thus, quantitative comparisons of the extent
of lesions in the lungs of well-matched individual cases that have lived
in areas with different pollutant exposures can indicate whether there is
an association between pollution and chronic lung damage. Furthermore, if
there are differences in lung structure associated with chronic exposure,
the nature and extent of such differences would provide an extremely valuable
resource for designing follow-up studies of function and symptoms in living
populations and chronic inhalation exposure studies in laboratory animals.
In other words, knowledge of the structural changes that occur in humans
should guide the selection of end points and measurement methods that are
likely to produce significant results in more conventional studies.
This brief paper outlines the rationale for a study of chronic effects
of O3 exposure based on postmortem lung tissue as well as the
opportunities and problems that face an investigator in conducting such
a study.
Rationale
There are a series of specific factors that help establish the appropriateness
of a study of postmortem lung tissue of individuals having definable variations
in chronic exposure to oxidant air pollutants. First, predictive lung uptake
models (7,8) indicate that delivered ozone (O3)
dose is greatest in respiratory acinus of humans, rats, and other species.
This region of the lung is inaccessible for studies based on direct in vivo
examination, except that cells from this region can be recovered by bronchoalveolar
lavage along with cells from adjacent regions. Second, chronic and subchronic
exposures of rats (9) and monkeys (10) at near ambient levels
of exposure produce changes in epithelial cell size and distribution in
terminal bronchioles and immediately distal airways. These exposures also
produce evidence of lung inflammation. All of these results are consistent
with predicted uptake sites for O3. Third, intermittent exposures
of monkeys (4 weeks on, 4 weeks off) produce changes that are similar to
or greater than those seen in monkeys exposed continuously (and, therefore,
having twice the total exposure) (10). These results have implications
for both seasonal and daily patterns of human exposure. Fourth, the structural
changes seen in the chronic and subchronic exposures in rats and monkeys
are associated with the functional changes consistent with emphysema and
a stiffening of the lungs, both of which correspond to premature aging of
the lungs (11). Fifth, an autopsy study of 107 lungs from 14- to
25-year-old fatal injury victims in Los Angeles County by Sherwin and Richters
(6) showed that 27% had what the authors judged to be severe degrees
of structural abnormalities and bronchiolitis not expected for such young
subjects, and another 48% of them had similar, but less severe, abnormalities.
In the absence of corresponding analyses of lungs of comparable subjects
from communities having much lower levels of air pollution, the possible
association of the observed abnormalities with chronic O3 exposure
remains speculative. Some of the abnormalities observed could have been
due to smoking and/or drug abuse, although the authors noted that published
work on the association between smoking and small airway effects showed
lesser degrees of abnormality (12).
Hypothesis
The lung abnormalities produced by subchronic and chronic O3
exposures in rats and monkeys at near peak ambient levels are sufficiently
similar to those seen in 14- to 25-year-old residents of Los Angeles to
suggest that long-term ambient exposures to O3 contributed to
these effects. Furthermore, the data suggest that such exposures, if continued
over a greater proportion of normal life span, could lead to reduced ventilatory
capacity later in life and perhaps to chronic lung diseases such as chronic
obstructive lung disease and emphysema.
Discussion
The kinds and degrees of abnormalities seen in the studies involving
analyses of animal and human lung tissues discussed above would be largely
subclinical and poorly related to conventional lung function indices. Measurement
of spirometry and pulmonary flow resistance are generally controlled by
airway calibre in the large and midsized conductive airways, whereas the
locus of damage associated with O3 is in the small airways, which
normally contribute little to overall flow restriction at early stages of
disease progression.
The lungs of rats, monkeys, and humans were all examined at relatively
young ages. Thus, there is concern that continued chronic O3
exposure could lead to further progression of the structural and functional
changes and thereby accelerate the normal rate of loss of lung function
with age in a manner analogous to the accelerated loss of function associated
with chronic cigarette smoke exposure.
Research Opportunities
To test the hypothesis that O3 exposure can cause or facilitate
an accelerated loss of lung function with age in human adults, it is necessary
to show that there are significant differences in age-adjusted lung abnormalities
in appropriately matched populations living in areas of relatively high
and relatively low ambient O3 concentrations.
Additional requirements, aside from appropriate matching or adjustment
for smoking, age, gender, ethnicity, etc., would include climate and lifestyle.
High oxidant, low acidic aerosol California communities would best be matched
by other Pacific Coast communities that have relatively low levels of both
types of secondary pollution, such as Santa Barbara, California; Portland,
Oregon; Seattle, Washington; Victoria and Vancouver, British Columbia; etc.
Cities in the midwest with moderately high oxidant and acid aerosol concentrations,
such as Chicago, Illinois; Cleveland, Ohio; Detroit, Michigan; Buffalo,
New York; and Toronto, Canada, might be matched with more westerly cities
that have lower concentrations of such secondary pollutants such as Minneapolis,
Minnesota; Milwaukee, Wisconsin; Kansas City, Missouri; and St. Louis, Missouri.
For hot, humid cities, Houston, Texas, with high oxidant concentrations,
could be matched with lower oxidant communities in Florida such as Tampa
Bay, Orlando, Miami, and Fort Lauderdale to minimize possible confounding
by differences in ambient temperature and humidity.
Specific Exposure-Related
Research Needs
Of all the criteria pollutants, O3 probably has the most extensive
data base for ambient community levels. Quality-assured federal and state
network data are readily available, and exposure modeling for locations
within a monitored area is relatively straightforward. Temporal variations
on a daily and seasonal level are largely predictable, and as a secondary
pollutant, O3 concentration variations within local regions are
less extreme than those for primary pollutants such as carbon monoxide and
lead. Some of the same considerations apply to acidic sulfate particles,
another class of secondary pollutant that also deposits preferentially in
small conducting airways.
The health effects associated with sulfates are most likely due to the
associated hydrogen ion rather than the ammonium ion or sulfate itself (13).
The H+/SO4-2 ratio is highly variable,
and SO4-2 concentration data are usually available
only on the basis of 24-hr averages every sixth day. For chronic effects
studies, the available data on SO4-2, SO2,
O3, temperature, and humidity are thought to be sufficient to
permit good estimates of long-term average exposure to SO4-2
and H+, at least for an examination of potential interaction
of acidic sulfates and ozone in the production of accelerated aging of the
human lung.
The development of protocols for obtaining residential and personal risk
factors information on fatal injury victims whose lungs are analyzed is
a specific research need. For those for whom such information can be obtained
reliably and who have no complications of smoking or occupational exposures
to lung irritants, cumulative O3 exposure based on ambient concentrations
at pollution monitoring sites adjacent to or surrounding the residence or
work sites can be calculated. To assess the cumulative exposures of individuals
whose lungs are studied, these data should be obtained: a) residential
histories--inclusive years at each address; b) distances from nearest
continuous quality-assured monitoring sites for O3 and other
pollutants at each residential address; c) participation in outdoor
activities, sports, and regular exercise (including intensity, duration,
location, and time of day); d) history of acute or chronic lung diseases;
e) cigarette smoking as well as occupational and hobby exposures;
f) residential exposures to confounding factors environmental tobacco
smoke (ETS), unvented gas and kerosene cookers or space heaters, wood smoke,
molds, mildew, etc.); and g) commuting patterns resulting in different
levels and types of air pollution exposures.
The development of alternate indices of cumulative pollutant exposure
for correlation with activity patterns to yield individual exposure metrics
is another research need. The exposure indices would then be correlated
with the extent of observed lung abnormalities. Pollutant data resources
of reasonably reliable quality include a) EPA and local monitoring
data for O3, NOx, SO2, SO4-2,
and PM10; b) weather bureau data such as temperature,
humidity, wind speed and direction; and c) airport visibility data
(which can serve as surrogates for fine particle concentrations).
With regard to research needed to develop methodologies for retrospective
exposure assessment, some preliminary research of this type has been performed
at New York University (14) and Harvard (15); further research
in this area is continuing. It involves improving and validating predictive
models by using available pollutant concentrations and meteorological data
bases. While the preliminary work is encouraging, much more needs to be
done.
Retrospective exposure assessment research needs for chronic O3
epidemiology studies include delineation, investigation, and development.
Delineation of the influence of various factors on local outdoor O3
concentrations and indoor/outdoor (I/O) O3 ratios is necessary
for this type of research. Outdoor factors include sources of O3
scavengers such as NO from motor vehicles, elevation, and local micrometerology.
Factors affecting the I/O ratio include air exchange ratios and the nature
of indoor surface sinks for O3. Investigation of the reliability
of models for estimating ambient concentrations of H+ from intermittent
(every sixth day) measurements of SO4-2 or continuous
measurements of fine particle mass or light-scatter coefficient is another
research need. Also necessary is the delineation of the influence of various
factors on local outdoor aerosol H+ concentrations and I/O ratios.
Major factors here are the strengths of the indoor and outdoor sources of
ammonia (NH3) and the rates of neutralization of H+
by NH3 from outdoor and indoor sources. A final retrospective
exposure assessment research need is development and validation of exposure
models that combine air concentration data and activity data to yield personal
estimates of total hourly or daily inhalation rates.
The design and evaluation of a personal history questionnaire about residential
and occupational histories and personal risk factors of the accident victims
whose lungs are to be analyzed is also needed in exposure-related research.
Among the data that should be acquired, as possible, for each individual
are: a) analysis of blood for COHb and/or cotinine as well as for
evidence of substance abuse that might produce lung abnormalities; b)
residential history from next of kin, to be verified to the extent possible
by information from driver's licenses, school records, etc.; c) occupational
history, if any, from next of kin, to be supplemented by employers' records;
d) patterns of outdoor activity from next of kin, supplemented by
records of team sports, running clubs, etc.; and e) records on location
of nearest air pollution monitoring site(s) for residences lived in for
the past 10 years or longer, if appropriate, along with a listing of the
pollutants monitored at each site.
A final research need is the identification of suitable cities and collaborating
pathologists for maximizing the range of chronic ozone exposures and access
to suitable lungs in those communities. Consider possibilities for matching
for climate, socioeconomic levels, and other potentially confounding factors.
In terms of the selection of the communities that may provide the best opportunities
for the collection of lungs having the greatest range of air pollutant exposures
of interest, it would be ideal to have a) high ozone with low H+,
b) high H+ with low ozone, c) high ozone with high
H+, and d) low ozone with low H+.
In reality, there are no large communities that meet any of these criteria,
except that large portions of the greater Los Angeles area fall into the
first category. Areas with complex terrain and variations in altitude, such
as Los Angeles, can include people with highly variable pollutant mixtures.
On the other hand, many large metropolitan areas in the midwest and along
the Atlantic coast have relatively uniform concentrations of secondary pollutants
such as O3 and H+. The tall stack SO2 emissions
that lead to H+ formation in the areas several hundred to several
thousand km downwind vary from west to east, and the most westerly of the
large midwestern cities such as Minneapolis, Minnesota; Kansas City, Missouri;
Dallas, Texas; and Fort Worth, Texas, should have much lower H+
exposures than other large cities on the plains or Great Lakes such as Detroit,
Michigan; Cleveland, Ohio; and Toronto, Canada. Ozone, formed closer to
ground level, should have lesser east to west gradients. Major cities having
low concentrations of both O3 and H+ include Seattle,
Washington; Portland, Oregon; and Vancouver, British Columbia. Smaller communities
include Santa Barbara and the Monterey Peninsula area in California.
The rate at which numbers of lungs that can be obtained from young accident
victims and processed in a uniform manner sufficient to permit sensitive
quantification of pathological abnormalities, localized morphometry, and
cell type distributions will be quite limited, especially for the smaller
cities or regions, it may not be feasible or desirable to look for differences
in means of responses by city or region. An alternative is to treat the
results on each individual case as an independent observation related to
that individual's cumulative exposure scores for O3. There should
be enough O3 monitoring data in the higher O3 areas
to devise a numerical value for each case with long residence in a high
or moderate O3 area. It may or may not be useful or desirable
to semiquantitatively rank them by H+ exposure based on measured
sulfate levels and background knowledge of atmospheric chemistry (14).
Specific Health Effects Assay Needs
The Sherwin and Richters (6) study, while a pioneering effort
of great interest, raised more critical issues than it settled. The distinctly
abnormal centriacinar lesions they found could have been due less to air
pollution than to cigarette smoking, drug abuse, or other stresses of the
lower socioeconomic status (SES) that affected many of the individuals studied.
Furthermore, the prospects of obtaining adequate background data on residence,
occupation, and other critical variables on these and similar individuals
are relatively poor. Another problem is that tissues were sampled for up
to 48 hr postmortem. Information from animal exposure studies suggests that
samples should be collected within one hour or less to obtain satisfactory
results. The animal studies also indicate that some additional analytic
protocols should be performed on human lungs in future studies, to improve
the prospects of seeing changes of interest in the tissues receiving the
highest doses of oxidant, and to provide complementary analyses for interspecies
comparisons.
It should be possible to overcome many of the limitations of the Sherwin
and Richters protocols in designing future studies of human lung tissue
from individuals chronically exposed to different levels of air pollution.
For example, it may be possible to obtain fresher tissues from transplant
donors. If such a source is used, the prospects of gathering adequate personal
history data from next of kin should be better than for medical examiner
cases. Selection of more optimal protocols for tissue analyses should be
based on the recommendations of an expert panel convened to design such
protocols.
The major developmental need for measuring the health impact of chronic
ozone exposure is to refine and standardize the pathological protocols used
in selecting target populations by age, location, background, etc. and used
for selecting, storing, processing, inflating, fixating, preparing, devising
analytical protocols for, managing data, etc., for human lungs. Consideration
must be given to matching end points to those that have been or can be used
in the chronic animal exposure studies. In terms of standardization of pathological
analyses, there is a need to convene one or more expert panels of pulmonary
pathologists with research backgrounds in irritant responses and let them
establish suitable analytic criteria that would be used for all specimens.
Summary and Conclusions
Parallel studies on quantitative methods for retrospective exposure assessment
need to be undertaken along with methods for quantitatively characterizing
lung pathology, morphometry, and cell distributions. Neither aspect is trivial
or easy, but the opportunities to do both are quite real and feasible.
Arrangements need to be made for coordination, standardization of protocols
and procedures, and quality assurance for each collaborating investigator
and group of investigators. Workshops should be conducted at the beginning
and intermediate stages, and the results and experiences among the different
groups should be used to develop and refine more optimal protocols. Furthermore,
consideration should be given to the need for and benefits from continuing
interchange of information with toxicologists performing chronic animal
inhalation studies.
