Relationship between air pollution and daily mortality in a subtropical city: Taipei, Taiwan

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Abstract

Air pollution has been associated with daily mortality in numerous studies over the past decade. However most of these studies were conducted in the United States and Europe with relatively few done in Asia. In the current study, the association between ambient air pollution and daily mortality in Taipei, Taiwan's largest city which has a subtropical climate was undertaken, for the period 1994–1998 using a case-crossover analysis. This design is an alternative to Poisson time series regression for studying the short-term adverse health effects of air pollution. The air pollutants examined included particulate matter (PM10), sulfur dioxide (SO2), ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO). The largest observed effect, which was without statistical significance, was seen for NO2 and CO levels on deaths due to respiratory diseases (ORs=1.013 and 1.014, respectively). The well established link between air pollution levels and daily mortality may not be as strong in cities in subtropical areas, although other factors such as differences in pollutant mix or the underlying health of the population may explain the lack of a strong association in this study. Further studies of this type in cities with varying climates and cultures are needed.

Introduction

Over the past decade, many epidemiologic studies have found associations between ambient air pollutant levels and non-accidental daily total mortality. Most evidence comes from studies in North American and European cities Schwartz, 1994a, Katsouyanni et al., 1997, Holgate et al., 1999, and these studies found that the elevated risk of death associated with short-term increases in air pollution is primarily due to respiratory and cardiovascular mortality Schwartz, 1994b, Zmirou et al., 1998. However, only a small number of studies has been conducted in Asia Xu et al., 1994, Hong et al., 1999, Lee et al., 1999, Wong et al., 2002, Tsai et al., 2003.

Time series analysis has been the analytic method of choice for studying the effects of air pollution on daily counts of mortality since 1990 (Schwartz and Marcus, 1990). Recently, the general additive model (GAM) approach using Poisson time series regression models has been used to adjust for the confounding effects of trend and seasonality (Schwartz, 1993). However, the methods used to model the covariates relating to environmental exposures and health counts over time have been criticized because of the complexity of the models and the potential for confounding due to temporal and weather variables Li and Roth, 1995, Moolgavkar et al., 1995, Styer et al., 1995. In addition, the findings produced by the use of this methodology lack a robustness that would allow them to be generalizable across locations Li and Roth, 1995, Moolgavkar et al., 1995, Bateson and Schwartz, 1999.

The case-crossover study design was proposed by Maclure (1991) to study the effects of transient, intermittent exposures on the subsequent risk of rare acute-onset events in close temporal proximity to exposure. This design can be regarded as a special type of case-control study in which each case serves as his or her own referent. A subject's characteristics and exposures at the time of a health event (case period) are compared with another time period when that subject was a noncase (control period). Each risk set consists of one individual as that individual crosses over between different exposure levels in the intervals between the control and case period Neas et al., 1999, Sunyer et al., 2000. This design can be an alternative to Poisson time series regression models for studying the short-term effects of air pollution (Levy et al., 2001a). In general, the case-crossover design and the GAM approach have produced almost identical results Neas et al., 1999, Lee and Schwartz, 1999. A few recent studies have adopted this design to analyze the short-term effects of air pollution Neas et al., 1999, Lee and Schwartz, 1999, Sunyer et al., 2000, Sunyer and Basagana, 2001, Kwon et al., 2001, Levy et al., 2001b.

To investigate the association between air pollution and mortality, and to compare the results with those reported elsewhere, we performed a case-crossover study relating concentrations of air pollutants and daily mortality in Taipei, the biggest metropolitan city of Taiwan, over a 5-year period from 1994 to 1998.

Section snippets

Materials and methods

This study looked at daily mortality in relation to air pollution in Taipei for the 5-year period from 1994 through 1998. Taipei is the largest metropolitan city in Taiwan with a population of about 2.64 million and is located in northern Taiwan. The major air pollution source is automobile exhaust emissions. Taipei has a subtropical climate, with an annual average temperature of 22 °C.

Results

The distribution of air pollution, meteorologic measurements, and daily number of deaths in Taipei during the period from 1994 to 1998 are shown in Table 1. An average of 25 persons died of nonaccidental causes each day in the city over the study period.

Table 2 shows the estimated ORs for daily mortality attributable to each interquartile change in pollutant concentration after controlling for meteorological variables (temperature and humidity). There is no indication of an association between

Discussion

Public health measures have led to a substantial reduction in air pollution levels, particularly in countries where the problem has been severe for years. However, recently there has been widespread discussion about the short-term health effects of lower levels of air pollution (i.e. lower than the national or international air quality standards).

This paper implements a study design that overcomes many of the common limitations of the widely used time-series approach. Studies of air pollution

Acknowledgements

This study was partly supported by a grant from the National Science Council, Executive Yuan, Taiwan (NSC-92-2320-B-037-046).

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