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Environmental
Health Perspectives

Volume 107, Number 11
November 1999

Introduction

Materials
and Methods

Results

Discussion

Related
EHP Articles

Citation
in Pub Med

Table
of Contents

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This Issue

PM10
Exposure, Gaseous Pollutants, and Daily Mortality in Inchon,
South Korea

Yun-Chul Hong,1
Jong-Han Leem,1 Eun-Hee Ha,2 and David
C. Christiani3
1Department of Preventive
Medicine, Inha University College of Medicine, Inchon, South
Korea
2Department of Preventive Medicine, Ewha
Women's University College of Medicine, Seoul, South
Korea
3Department of Environmental Health,
Harvard School of Public Health, Boston, Massachusetts,
USA

Abstract

To evaluate the relative
importance of various measures of particulate and gaseous air
pollution as predictors of daily mortality in Inchon, South
Korea, the association between total daily mortality and air
pollution was investigated for a 20-month period (January 1995
through August 1996). Poisson regression was used to regress
daily death counts on each air pollutant, controlling for time
trends, season, and meteorologic influences such as
temperature and relative humidity. Regression coefficients of
a 5-day moving average of particulate matter 10 µm in aerodynamic diameter (PM10) on
total mortality were positively significant when considered
separately and simultaneously with other pollutants in the
model. PM10 remained significant when the models
were confined to cardiovascular or respiratory mortality.
Sulfur dioxide (SO2) and carbon monoxide (CO) were
significantly related to respiratory mortality in the
single-pollutant model. Ozone exposure was not statistically
significant with regard to mortality in the above models, and
graphic analysis showed that the relationship was nonlinear. A
combined index of PM10, nitrogen dioxide,
SO2, and CO seemed to better explain the
exposure-response relationship with total mortality than an
individual air pollutant. Pollutants should be considered
together in the risk assessment of air pollution, as opposed
to measuring the risk of individual pollutants. Key
words: air pollution, carbon monoxide, cardiovascular,
mortality, nitrogen dioxide, ozone, respiratory, sulfur
dioxide, PM10. Environ Health Perspect
107:873-878 (1999). [Online 5 October 1999]

http://ehis.niehs.nih.gov/docs/1999/107p873-878hong/abstract.html

Address correspondence to Y-C. Hong, Department
of Preventive Medicine, Inha University College of Medicine,
253 Yonghyun-Dong, Nam-Gu, Inchon, 402-751, South Korea.
Telephone: 82 32 890 2860. Fax: 82 32 890 2859. E-mail: [email-masked]

This work was supported by the Inha University Research
Fund, 1998.

Received 20 April 1999; accepted 22 June 1999.

Introduction
The most acute air pollution
episode in history occurred in London in 1952, when 4,000 excess
deaths were attributed to air inversion. Other acute episodes have
been reported in Donora, Pennsylvania, in 1948 and Los Angeles,
California, in 1963. Since these episodes, investigations into the
relationship between air pollution and mortality have demonstrated a
positive association between air pollution levels and mortality,
even at lower air pollution levels. A number of daily time-series
studies have demonstrated an association between short-term exposure
and increased mortality in air pollution areas when confounders were
controlled (1,2). Daily mortality has been associated with
total suspended particulates in Steubenville, Ohio (3);
Philadelphia, Pennsylvania (4); and Mexico City, Mexico
(5); and with particulate matter of 10
µm (PM10) in Utah Valley, Utah (2); Birmingham,
Alabama (6); St. Louis, Missouri, and Kingston, New York
(7); and Amsterdam, The Netherlands (8). Most
published U.S. studies show that particulate matter is the pollutant
which exhibits this relationship most clearly (2,3,9-13),
even though most of these studies have not considered multiple
pollutants. The relationship between particulate air pollutants and
mortality seems to continue well below current ambient air quality
standards, and the exposure-response relationship is linear with no
evidence of a threshold (14-16). Investigations have been
carried out in Europe, although they yielded more diverse results in
terms of the pollutant with the strongest associations. In most of
the European studies there were significant relationships between
mortality and either sulfur dioxide (SO2) or particulate
matter (17-22). Different areas of the world have different
mixtures and levels of pollutants. Thus, it is unclear whether the
same pollutants and mortality relationships found in the United
States or Europe would apply to Inchon, an industrial city in South
Korea.

Particulate matter levels in Inchon are much higher than in most
developed countries; however, SO2 levels have recently
been decreasing rapidly. The combined effects of dust particles and
other pollutants may play more important roles in the damaging
process than dust particles alone. Combination indices of
PM10 and gaseous pollutants can be useful tools to
evaluate the relationship of pollutants with mortality; single or
multiple pollutant models cannot analyze pollutant effects
accurately when the pollutants are highly correlated with each
other. Few studies have described the association between present
levels of air pollution in East Asia and mortality. The objective of
our study was to report on the association between daily mortality
and ambient air pollution in Inchon, South Korea.

Materials and Methods

Daily mortality, weather, and air pollution data.
Daily deaths for the Inchon area were read from the annual
detail mortality tapes of the National Statistical Office (Taejon,
Korea) for the period from 1 January 1995 to 31 August 1996.
Cause-specific categories included in our analysis were total
deaths, respiratory deaths, and cardiovascular deaths. The deaths
due to accidents or violence were excluded from the total counts.
Codes from the International Classification of Diseases, 10th
revision (World Health Organization, Geneva) were used to define
these categories.

The city of Inchon is located on the western side of the Korean
peninsula, with a sea to the west and metropolitan Seoul to the
east. It is the third largest city in the country. The 1996 census
indicated that the population of Inchon was 2.4 million. Inchon has
a four-season climate and low-level temperature inversions are
common during the winter months. Data on 24-hr mean temperature and
relative humidity were obtained from a centrally located Inchon
weather station.

Air pollution data were obtained from the Department of the
Environment (Seoul, Korea). Two monitoring sites for 24-hr
measurements of PM10 and gaseous
pollutants--SO2, nitrogen dioxide (NO2), ozone
(O3), and carbon monoxide (CO)--were established in
residential areas. Concentrations of these pollutants were measured
hourly, and 24-hr averages were constructed between measurement
sites. For O3, a daytime 8-hr average was used instead of
a 24-hr average.

Statistical analysis. The associations between
daily mortality and air pollutants, as well as weather variables,
were analyzed with a generalized additive model (GAM) estimating
Poisson distribution. The relationship between pollution and
mortality was complicated by the fact that periods with the highest
pollution levels occurred during the winter months (with the
exception of O3), and the higher incidence of death may
be at least partially due to cold winter weather. Seasonal change or
time trends also could confound the pollutant effects. Daily
mortality was fit to the GAM, which included loess function of
temperature, relative humidity, time trends, and indicator variables
of the season. Once these models were established, the association
of each air pollutant and multiple pollutants in predicting daily
mortality was evaluated separately. Air pollutants may affect
mortality with some time lags, and the appropriate averaging time
for exposure may exceed 24 hr. Therefore, single-day exposure and
multiple-day moving averages of up to 6 days were examined. To
consider the correlation between the pollutants, and to evaluate the
combined effects of pollutants, we introduced indices of combination
of primary gaseous pollutants with PM10 using a 5-day
moving average of pollutant levels.

Index of all pollutants = PM10 m5/mean
(PM10) + NO2 m5/mean
(NO2) + SO2 m5/mean
(SO2) + CO m5/mean (CO)

Index of PM10 + NO2 = PM10
m5/mean (PM10) + NO2
m5/mean (NO2)

Index of PM10 + SO2 = PM10
m5/mean (PM10) +
SO2m5/mean (SO2)

Index of PM10 + CO = PM10
m5/mean (PM10) + CO m5/mean (CO)

A pollutant divided by the mean value reflects the
relative variations and may be used when comparing it to other
pollutants. Using these indices, we can evaluate the combined
effects of pollutants that reflect real environments, and thus
better predict the mortality risk. To graphically analyze the
dose-response relationships, the relative risks of mortality for
pollutants and the combined indices were plotted using the GAM.

Results

Table 1 shows the distribution of mortality, meteorologic
measurements, and air pollution between 1 January 1995 and 31 August
1996 for Inchon. An average of 22 persons died in the city each day.
The Pearson correlations between the variables and total mortality
are presented in Table 2. Correlations between mortality and primary
pollutants were small but significantly positive: They ranged from
0.05 to 0.19. A negative correlation between the daily mortality and
O3 was noticeable. There was also a negative correlation
between the daily mortality and temperature or relative humidity.
Correlations between exposure variables showed that there was a
problem of collinearity among pollutants. PM10 was
correlated significantly with other air pollutants, positively with
NO2, SO2, and CO, and negatively with
O3.

A greater association with mortality was seen with the 5-day
moving averages and the previous day's exposure than with other
exposure variables. Tables 3-5 show the Poisson regression model,
which includes time trends, season, and weather variables for
temperature and relative humidity. In the models that included a
5-day moving average of one pollutant or multiple pollutants,
PM10 was a significant predictor of total mortality,
whereas gaseous pollutants remained insignificant. An increase in
the 5-day moving average of PM10, equal to 10
µg/m3, was associated with an increase in the relative
risk of mortality to 0.8%. When the previous day's concentrations
were considered, PM10 and NO2 were significant
pollutants in the single-pollutant models. PM10 remained
significant when the models were confined to cardiovascular or
respiratory mortality. The association with PM10, as
measured by regression coefficients, was greatest for respiratory
mortality. SO2 and CO were significantly related to
respiratory mortality in the single-pollutant model using the
previous day's concentrations. However, O3 was not
statistically significant when related to mortality except for the
previous day's concentrations in the single-pollutant model of total
mortality. The direction of the coefficient was negative in its
relationship with total and cardiovascular mortality and positive
with respiratory mortality.

A loess function of 5-day moving averages of PM10,
NO2, SO2, and CO levels was used in the GAM to
graphically analyze the dose-response relationship between
pollutants and daily mortality. Figures 1 and 2 show the relative
risks of total mortality by 5-day moving averages of pollutants and
the combination indices of PM10 and primary gaseous
pollutants. Excess mortality risk is clearly evident in the higher
range of PM10 levels and the increase is dose responsive.
When using combined indices of the pollutants, the exposure-response
relationship for the index of overall pollutants was better than
that of single pollutants or other combination indices. The index of
PM10 + SO2 also showed a near-linear increase
of relative risk, which suggests that the joint effect model of
PM10 and SO2 is a better predictor of
mortality risk than effect models of individual pollutants. Contour
analysis illustrated this joint effect well, showing that when
concentrations of the two pollutants were high, the mortality rate
peaked (Figure 3).

Figure 1. Graphic analysis of relationship of
the primary pollutants [(A) PM10, (B)
nitrogen dioxide, (C) sulfur dioxide, and (D) carbon
monoxide] with daily mortality by generalized additive model using
loess function after controlling time trends, season, and weather
variables. PM10, particulate matter
10 µm in aerodynamic diameter. The 5-day moving average of
pollutant concentrations is along the x-axis.

Figure 2. Graphic analysis of relationship of
the pollutant indices [(A) OP, (B) PNP, (C)
PSP, and (D) PCP] with daily mortality by generalized
additive model using loess function after controlling time trends,
season, and weather variables. Abbreviations: OP, index of overall
pollutants; PCP, index of PM10 + CO; PM10,
particulate matter
10 µm in aerodynamic diameter; PNP, index of PM10 +
NO2; PSP, index of PM 10 + SO2.

Figure 3. Contour analysis of PM10
and SO2 with daily mortality shows interaction of the
two pollutants. PM10, particulate matter
10 µm in aerodynamic diameter. When concentrations of both
pollutants are in the high range, mortality is higher than in any
other combination.

Outputs from GAMs using a 5-day moving average and the previous
day's concentrations were analyzed graphically to clarify the
contradictory relationship between O3 and daily mortality
(Figure 4). There was a clear change of direction in the
relationship at approximately 23 ppb O3 concentration,
suggesting that there is a threshold for the effects of
O3 on mortality.

Figure 4. Graphic analysis of relationship of
ozone concentrations for (A) the 5-day moving average of
ozone concentrations (O3) and (B) the previous
day's ozone concentrations (O31) with daily mortality
by generalized additive model using loess function after
controlling time trends, season, and weather variables.

Discussion

This study documents a strong association between mortality and
particulate air pollution in Inchon during the period of January
1995 to August 1996. This association was statistically significant
in a model controlling other variables, and exhibited an
exposure-response relationship. The increase in mortality risk with
a 10-µg/m3 increase in PM10 was 0.8%. Results
of most of the previous studies have suggested that a
10-µg/m3 increase in PM10 is associated with
an increase in daily mortality equal to 0.5-1.5% (1). The
mortality studies in Utah Valley (2) and Philadelphia
(4) show that deaths due to respiratory disease were most
strongly associated with particulate pollution levels, and
statistical associations were also observed for deaths due to
cardiovascular disease. In our study the highest regression
coefficients were in the model for respiratory mortality. However,
they did not reach statistical significance except when using
multiple models with a 5-day moving average because of the low
frequency of nonmalignant respiratory death. We also found
PM10 to be significantly related to cardiovascular
mortality, but the regression coefficients were lower than those for
respiratory mortality.

The sources and levels of ambient air pollution, as well as
population characteristics and habits, vary widely among the United
States, Europe, and Korea. However, the results of this study
suggest that these differences do not exhibit a large influence on
the relationship with mortality. It is highly unlikely that
concordant results across so many locations could have occurred
because of confounding or by chance. The highest particulate
concentrations in Inchon occur during cold winter weather. There are
substantial differences in the coincident weather patterns that
accompany particulate exposure in different locations in the world.
However, observed associations between particulate pollution and
mortality are similar in many locations around the world. It is also
unlikely that confounding weather effects are the source of the
apparent pollution effects.

The most likely confounder that could be responsible for the
effects observed is another pollutant or combination of pollutants
which are highly correlated with particulate pollution (1).
The high degree of correlation between air pollutants makes it
difficult to analyze the single or multiple pollutant models
appropriately. The fact that SO2 and NO2 are
converted to sulfates and nitrates, thus contributing to the fine
particles, makes the individual effects of the gaseous pollutants
more difficult to explain (23). Therefore, indices of
pollutants can more accurately represent the effects of complex air
pollutants in spite of the different modes of action of the
pollutants and can also be better predictors of mortality risk. When
using combined indices of pollutants, the dose-response relationship
of the index of overall pollutants is better than that of a single
pollutant alone or of other combination indices. This index of
overall pollutants can be useful to predict mortality risk and to
manage air pollution policies.

Our analysis shows no specific particulate pollution threshold.
Mortality risk increased with PM10 levels in an
exposure-response fashion. However, there seems to be a threshold
for mortality for O3. The negative direction of the
O3 relationship in the Poisson regression model was due
to low O3 concentrations; therefore, results from higher
O3 areas may differ.

The mechanism of the relationship between daily mortality and
particulate air pollution is not obvious. Some studies have provided
evidence that PM10 has free radical activity and causes
lung inflammation (24,25). Heavy metals, transition metals,
and polycyclic aromatic hydrocarbons in particulate pollutants may
also cause harmful effects (26,27). However, specific
mechanisms remain unclear. This epidemiologic study cannot explain
the toxicologic mechanism for particulate pollution-induced
mortality. However, the relationship of the combined indices
suggests that there is a mutual interaction between particles and
gaseous pollutants.

This study is limited by its use of environmental monitoring data
alone to represent ambient concentrations, which do not necessarily
represent individual exposures. This can result in nondirectional
misclassification of exposure and bias toward the null. Therefore,
it is likely that our positive results underestimate the magnitude
of the association. Some studies have found that PM10
levels are highly correlated between indoor and outdoor air
(28). Therefore, outdoor measurements of pollutants can also
represent indoor environments, and thus be used as a proxy in the
analysis.

In summary, the association of particulate air pollution with
daily mortality in Inchon is similar in magnitude to the results in
other communities in the United States and in Europe. These
associations are still observed when weather variables and other
pollutants are accounted for. The association between particulate
air pollution and mortality was largest for respiratory deaths.
Increased cardiovascular mortality associated with particulate air
pollution was also observed. Further, there is no evidence to
support a threshold for particulate air pollution. However, there
appears to be an O3 threshold level for adverse health
effects and mortality. Additional studies will be needed to confirm
this threshold in communities with varying concentrations of
O3. A combined index of overall pollutants, a variable
more accurately reflecting real environments, can be a better
predictor of acute mortality. Future studies should be directed at
evaluating the mutual interaction of pollutants and their chemical
composition.

References and Notes

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Last Updated: October 5, 1999